5 steps to calculate electricity bill savings from solar power generation

When calculating solar power generation, many practitioners ultimately do not only want to know how much will be generated. It is important to organize the results in a form that can be used for pre-installation evaluation and post-installation verification of how that generation will reduce electricity costs. Generation is expressed in kWh, but the amount of bill savings depends on self-consumption, the price of purchased electricity, basic charges, contracted demand, and time-of-day usage patterns. For that reason, simply multiplying generation by a unit price can differ from the actual savings.

This article explains the approach for calculating electricity bill savings from solar power generation, breaking the methodology into five practical steps that are easy to use in the field. To make it applicable not only to residential systems but also to commercial facilities such as factories, warehouses, shops, and offices, we sequentially describe organizing generation data, assessing self-consumption, handling unit price conditions, performing month-by-month verification, and conducting a post-operation review.

Table of Contents

• Organize the relationship between power generation and electricity bill savings

• Step 1 Understand power generation by month and by time of day

• Step 2 Separate the amount of generated electricity that can be self-consumed

• Step 3 Check the electricity purchase unit price and tariff structure

• Step 4 Calculate the reduction amount to avoid overestimation

• Step 5 Review calculation conditions based on actual data

• Practical considerations when calculating solar power generation

• Summary

Summarize the Relationship Between Power Generation and Electricity Cost Reductions

Before calculating the amount of electricity bill savings from solar power generation, the first thing to understand is that not all generated electricity has the same value in terms of reducing electricity costs. The effectiveness of solar power depends not on the generation volume itself but on how that generated electricity is used. If the generated electricity can be consumed within the building, it reduces the amount of electricity purchased from the utility. On the other hand, if power demand is low during the generation period, surplus electricity may be directed to storage or sold back to the grid, or—depending on contracts and equipment configuration—may be curtailed or left unused. These factors need to be evaluated separately from reductions in purchased electricity.

At the core of considering electricity bill savings is the amount of purchased electricity reduced through self-consumption. For example, if solar power is generating during a certain period and the building is using electricity at the same time, that amount can be regarded as electricity purchases avoided from the outside. By multiplying this reduced purchased electricity by the contracted energy unit price or time-of-use unit prices, you can calculate an approximate electricity bill reduction. However, depending on the contract, not only the energy charge but also the basic charge and the effect of revising contracted power may be relevant.

In practice, it is easy to assume that facilities with higher annual power generation will achieve greater savings. However, even with the same annual generation, the reduction effect differs between facilities with high daytime power usage and those whose usage is concentrated at night or in the early morning. Because solar power generates electricity during the day, facilities with steady daytime loads are more able to consume power on-site. Conversely, facilities that have low loads during holidays or lunch breaks may experience periods when, despite sufficient generation, on-site consumption is not possible.

Therefore, when calculating electricity cost savings, it is important not only to look at the annual total of generation but also to consider it broken down by month, by day, and by time of day. Looking by month allows you to confirm seasonal fluctuations in generation caused by solar irradiance conditions, temperature, and the equipment’s installation conditions. Looking by time of day lets you check whether there is electricity consumption during the times when generation occurs. By organizing the data this way, you can translate the calculated solar power generation into results that more closely reflect actual electricity cost savings.

Also, the amount of electricity cost savings is not necessarily constant over time. It fluctuates depending on power consumption, number of operating days, operating hours, additions of equipment, air-conditioning load, contract terms, aging of power generation equipment, and so on. Therefore, when estimating before implementation, you should set certain assumptions and, after implementation, review them using actual data. In practice, it is more important to clearly specify the calculation conditions and keep them in a form that can be verified later than to seek completely accurate figures from the outset.

Step 1: Assess power generation by month and time of day

The first step is to decide which units you will use to measure solar power generation. If your goal is to calculate electricity bill savings, annual generation alone does not provide enough information. Annual generation is useful for understanding the installation size and the rough effect, but electricity bill savings are actually tied to monthly billing and the amount of electricity purchased in each time period. Therefore, it is important to check monthly generation and generation by time of day as much as possible.

In pre-installation calculations, the expected generation is estimated by considering the system capacity, installation tilt, azimuth, local solar irradiation conditions, shading effects, temperature-related output degradation, and losses such as those from power conditioners and wiring. In a simplified method, annual generation can be obtained by multiplying the system capacity by a generation factor that reflects the region and installation conditions. However, when calculating electricity cost savings, it is more accurate to apportion the annual value by month or to use the monthly values from simulation results rather than using the annual figure as-is.

One reason to look at monthly power generation is that photovoltaic output varies with the seasons. Depending on the region and installation conditions, the generation profile changes—for example, during periods with long hours of sunlight, during times that are prone to the effects of the rainy season or snowfall, and during periods when rising temperatures make it harder for output to increase. Even if sufficient generation is expected over the year, if generation is low in months with high electricity consumption, the electricity bill reduction effect may be smaller than anticipated.

Generation by time of day is also important. Solar power typically begins generating in the morning, increases output around midday, and declines in the evening. Of course it varies greatly with the weather, but basically generation is concentrated during the daytime. For businesses, the amount that can be self-consumed depends on how much electricity is used before operations start, during lunch breaks, after work, on holidays, and so on. For households, how much of the generated power can be used depends on time spent at home, air-conditioning use, and the operating times of water heaters and appliances.

In practice, being able to compare generation and electricity consumption in 30-minute or 1-hour intervals improves the accuracy of estimated savings. During periods when generation is lower than consumption, it is more likely that the generated power can be self-consumed. Conversely, when generation exceeds consumption, the excess may not directly lead to reductions in purchased electricity. Calculating savings based only on annual generation without making this distinction can lead to overestimating the savings.

For installed systems, use generation monitoring data and electricity meter records to verify the actual power generation. When doing this, rather than looking only at daily totals, check the output curve on clear days, the fluctuations on cloudy days, monthly trends, and whether there were any abnormal shutdowns—this increases the reliability of the generation figures used in calculations. Judging annual reduction amounts based only on short-term performance is susceptible to weather and seasonal bias, so if possible you should review data for multiple months and correct for seasonal differences.

Also, you need to pay attention to the units of the generation data. Instantaneous output kW and the amount of energy generated over a certain period, kWh, have different meanings. For calculating reductions in electricity costs, you basically use kWh. Even if you look at instantaneous output and conclude that enough power is being generated, unless you know how long that state persisted you cannot convert it into cost savings. Before starting calculations, it is important to confirm whether the data you are handling is output (power) or energy.

Step 2 Separate the amount of generated power that can be self-consumed

The next step is to separate the portion of generated power that can actually be self-consumed. The main factor in electricity bill savings is the amount of electricity purchased from the grid that is reduced through self-consumption. Even if generation is high, if you are not using electricity at the same time, it will not directly reduce purchased electricity. Therefore, when calculating electricity bill savings from solar power generation, you need to consider total generation and self-consumption separately.

The basic way to determine self-consumption is to compare generation and electricity consumption for each time period. If the generation in a given period is less than or equal to the consumption, that generation can be considered fully used within the building. Conversely, if generation exceeds consumption, self-consumption is limited to the consumption and the excess generation is treated as surplus. By applying this approach to each time period and accumulating the results, you can obtain an estimate of self-consumption.

For example, factories and refrigerated warehouses with consistently high daytime electricity use tend to consume the power they generate on-site. This is because when air conditioning, ventilation, lighting, production equipment, pumps, compressors, and the like are operating during the day, solar generation hours are likely to overlap with the load. On the other hand, facilities with many holidays, limited daytime operation, or predominantly nighttime operations tend to have some surplus generation. Ignoring this difference and applying the same self-consumption rate will skew the evaluation of the amount of reduction.

Even when assuming a self-consumption rate, it is important to align it with the facility’s actual operations. If there is not enough time-of-day data prior to installation, set assumptions carefully based on monthly electricity consumption, the operating-day calendar, business hours, standby power on holidays, and whether there is a daytime load. Rather than simply assuming a fixed proportion of generation is self-consumed, it is more realistic to distinguish weekdays and holidays, summer and winter, and operating days and non-operating days.

For commercial facilities, it is also necessary to check the relationship with demand and contracted power. If solar power reduces purchased electricity during peak hours, it may lead to a reduction in contracted power. However, the effect on reducing the basic charge depends on whether the hours when generation occurs coincide with the hours when peak demand occurs, or whether peak demand is consistently reduced. Even if generation is high, if peak demand occurs during cloudy periods or in the evening, you cannot assume an overly large reduction in contracted power.

Also, clarify how to handle surplus power. Even if the surplus is sold to the grid, it is safer to calculate that amount separately from the reduction in purchased electricity. Revenue from selling power and reductions in purchased electricity are different in nature, and the applicable unit prices and contract terms also differ. Since this article focuses on electricity cost savings, a practical approach is to first clearly identify the amount of purchased electricity reduced by self-consumption and treat the surplus separately.

If there is energy storage equipment, the calculations become more complex. Storing surplus daytime generation and using it in the evening or at night can lead to reductions in purchased electricity. However, you need to consider charging/discharging losses, storage capacity, operational control, and the purchase price of electricity during the periods when discharge occurs. Even when including the storage portion in the calculations, it is important to treat it not as the generation itself but as the amount of electricity that ultimately reduced purchased electricity.

Step 3 Confirm the unit price of purchased electricity and the tariff structure

Once you have determined the amount of self-consumption, the next things to check are the unit price for purchased electricity and the tariff structure. As a basic rule, electricity cost savings are calculated by multiplying the reduction in purchased electricity due to self-consumption by the applicable unit price for purchased electricity. However, electricity charges are not always made up of a single simple unit price. They can consist of multiple elements such as contract type, time of day, season, consumption tiers, basic charges, items equivalent to fuel cost adjustments, and items equivalent to surcharges related to renewable energy.

What is important here is correctly choosing the unit price that will be subject to reduction. When you self-consume the electricity you generate, the amount of electricity you would have bought during that time is normally reduced. Therefore, the unit price used in the calculation should be based primarily on the electricity charge for the time period in which the generation was self-consumed. If you have a time-of-use contract, you need to take into account differences such as daytime, nighttime, and holiday rates. Because solar power generation mainly produces during the daytime, using the nighttime rate as-is can lead to an unrealistic assessment of the savings.

On the other hand, not all items listed on the invoice will decrease in proportion to generation. Items that change according to the amount of electricity used tend to be related to increases or decreases in self-consumption, but the basic charge based on contracted capacity or contracted demand does not necessarily fall automatically simply because generation has increased. If you anticipate a reduction in the basic charge, you need to separately confirm whether a review of the contracted demand or an actual decrease in maximum demand will occur.

To improve calculation accuracy, rather than looking only at the invoice amount, separate and organize electricity consumption, contracted power, energy charges, basic charges, and adjustment items. However, because this article does not deal with prices themselves, in practice you should use the unit price conditions stated in each company's contracts, billing statements, and internal management documents and reflect them in internal calculation sheets, etc. Rather than determining reduction amounts based only on public materials or general benchmarks, it is important to use the actual contract conditions.

Also, it is important to understand the difference between using an average unit price and using time-of-use unit prices. The average unit price is calculated by dividing the electricity charges for a given period by the amount of electricity consumed. While convenient for simple estimates, if the bill includes a basic charge or other fixed charges, the average can appear higher than the per-kWh price actually offset by generation, and as a result it may lead to an overestimation of the savings from self-consumption.

Using time-of-use unit prices makes it easier to perform calculations that more closely reflect reality. Use the purchase unit price corresponding to the time period when generation occurred and multiply it by the self-consumption amount for that period to accumulate the amount saved. When contract conditions are complex, as in business sites, calculating by time period lets you understand which time periods’ charges are being reduced by generation. For simple calculations use the average unit price, and for full-scale implementation decisions use time-of-use unit prices—choose according to your purpose.

Furthermore, it is necessary to clarify how future unit price fluctuations will be handled. Electricity rates can change due to contract changes, regulatory changes, fuel prices, usage patterns, and other factors. When considering implementation, it is common to check sensitivity under multiple assumptions, not only the current unit price. However, it is safer to avoid asserting future prices and to treat them strictly as assumptions for estimates. When using this in internal briefings or approval documents, explicitly stating that calculations were performed under these unit price assumptions will make later verification easier.

Step 4 Calculate the Reduction Amount to Avoid Overestimation

Once the conditions for generated electricity, self-consumption, and the purchase price are organized, you actually calculate the electricity cost savings. The basic idea is to multiply the amount of electricity that was self-consumed by the corresponding purchase price. When calculating by time of day, multiply the self-consumption for each time period by the price for that period, then sum on a monthly or annual basis. When calculating by month, use the monthly generation, electricity consumption, and price conditions to reflect seasonal variations.

A simple calculation method is to multiply the annual self-consumption by the average unit price of electricity. Because this method can produce a rough estimate quickly, it is convenient for initial assessments and for comparing multiple options. However, if the average unit price includes basic charges or other fixed components, the estimated savings may be larger than the actual savings. To calculate more reliably, you need to separate items that decrease in proportion to generation from items that will not decrease solely due to generation.

To avoid overestimation, it is important to use self-consumption rather than total generation. Multiplying total generation by the unit price of purchased electricity can result in treating surplus generation as a reduction in purchased electricity. This difference tends to be especially large for facilities with low daytime loads or on holidays. If there are many periods when generation exceeds consumption, the reduction amount must be calculated after excluding the surplus.

Next, consider shutdowns and output reductions. Solar photovoltaic (PV) systems do not always generate under ideal conditions. Weather, shading, dirt, snow accumulation, temperature rises, equipment shutdowns, inspection work, output curtailment, and data loss due to communication failures all affect generation performance. In pre-installation calculations, you need to anticipate these losses to some extent. In post-installation calculations, check actual performance data, clean up abnormal values and missing data, and then evaluate the amount of reduction.

Also, the calculated power generation and the reduction amount on invoices may not match exactly. If the billing period is not from the first to the last day of the month, the aggregation period for generated power may not align with the billing period. Changes in meter reading dates, holidays, number of operating days, seasonal factors, air-conditioning load, and equipment utilization rates can make the effect difficult to see with a simple comparison. Therefore, rather than judging the reduction effect solely by comparing electricity bills before and after installation, it is necessary to adopt an approach that adjusts conditions to be as similar as possible.

If a reduction in the basic charge is to be included, treat it even more cautiously. Even if solar power reduces purchased electricity during the daytime, the contracted capacity may not decrease if maximum demand occurs at a different time. For example, in facilities where maximum demand occurs during morning startups, evening overtime periods, air-conditioning loads on cloudy days, or equipment startup after holidays, solar output may not be sufficient to reduce the peak. If you expect a reduction in the basic charge, you need to overlay demand data with generation time periods and confirm.

When explaining calculation results internally, it's easier to communicate if you organize and present the assumptions rather than showing only a single number. For example, separately record the annual power generation, the assumed self-consumption rate, the self-consumption amount, the pricing conditions used, how surplus is handled, whether basic charges were included, and whether battery storage was included. This allows you to compare with actual results later and identify which assumptions were off.

The calculation of reduction amounts is not intended to produce figures that merely look precise. What matters in practice is enough validity to support decision-making and transparency that can be verified after implementation. Rather than showing large reduction figures based on overly optimistic assumptions, it is safer—both for public materials and internal documents—to conservatively organize generation, self-consumption, and price assumptions and present them in a form whose rationale can be explained.

Step 5 Review calculation conditions using actual performance data

Calculating electricity cost savings from solar power generation is not a one-time task completed before installation. By checking actual performance data after installation and revisiting the initial calculation assumptions, you can grasp the savings more accurately. This is especially true for commercial facilities, where operating hours, equipment utilization rates, HVAC loads, production volumes, and holiday operations can change, causing discrepancies between pre-installation assumptions and actual results. Verifying actual performance reveals not only the effect of the generation equipment but also the characteristics of the building’s power consumption.

After commissioning, the first thing to check is the monthly power generation. If actual output falls significantly below the simulation in any month, distinguish whether this is due to weather, equipment downtime, soiling, or shading. If it is caused by a temporary spell of bad weather, it may even out over the long term; however, if generation is only reduced on a particular string or during specific time periods, an inspection of the equipment may be necessary.

Next, check the actual amount of self-consumption. Even if generation is as expected, if the electricity cost savings are smaller than anticipated, the self-consumption rate may be low. There may be operational reasons why generated power is not being fully used, such as large surpluses on holidays, load drops during lunch breaks, changes in power usage between busy and slow seasons, or production equipment shutting down during daytime. In such cases, consider whether you can adjust the timing of power use or review the timing of air conditioning, heat storage, charging, and equipment operation.

Reconciliation with billing data is also important. Even if the generation monitoring data appears to show sufficient output, the amount of purchased electricity on the invoice may not decrease as much as expected. In that case, check for mismatches between the billing period and the generation aggregation period, increases in consumption, differences in the number of operating days, increases in air-conditioning load, changes in the contracted unit price, data loss, and so on. Be careful, because a simple before-and-after comparison alone can conflate the effects of solar power generation with changes in business activity.

Also, when verifying actual results, it is important not to annualize anomalous values as they are. Months that experience typhoons, prolonged rain, snowfall, inspection shutdowns, equipment renovations, power outages, or temporary closures will produce results different from normal months. When estimating annual savings using short-term actuals, verify whether the target period is representative. If possible, use data from multiple months or a full year and evaluate taking seasonal differences into account.

Operational improvements can further increase the amount saved. For example, shifting equipment that can operate during the daytime to power generation hours, identifying standby power on holidays, reviewing air-conditioning startup times, and aligning electricity use such as charging or heating with times of high power generation. However, operational changes that affect work quality, safety, or production schedules must be made cautiously. If you make unreasonable operational changes solely to reduce electricity costs, it may increase the burden on site or lead to equipment problems.

In reviews based on actual performance data, a practical point is not to overcomplicate the calculation formulas. To enable on-site personnel, the administrative department, and management to make judgments based on the same numbers, it is clearer to manage separately: generated power, self-consumption, reduction in purchased electricity, amount saved, surplus, and downtime. In particular, when explaining the relationship between generated power and the amount saved, make sure to share that the savings are directly linked to the reduction in purchased electricity—not the amount generated—to prevent misunderstandings.

Practical Points to Note When Calculating Solar Power Generation

When calculating electricity cost savings from solar power generation, you need to pay attention not only to the formula but also to how the data are handled. The first thing to check is which point the generation data you are using come from. Generation measured at the solar panel side, generation at the power conditioner output, the electricity measured at the point of receipt, and the electricity consumed inside the building each have different meanings. If you perform calculations without clarifying which data are being used, the basis for the claimed savings becomes unclear.

Next, it is important to align the measurement units and aggregation intervals for power generation. If generation monitoring data are on an hourly basis and energy consumption data are on a 30-minute basis, comparing them as-is will introduce discrepancies. Even when comparing only monthly aggregates, the billing period does not necessarily coincide with the calendar month. In practice, you should align the aggregation periods for generation, consumption, and billing data, and if alignment is not possible you must document that assumption.

The effects of shading should not be overlooked. Shadows from buildings, trees, adjacent equipment, signs, fences, rooftop machinery, and the like can reduce power generation during certain times of day or seasons. Even if the difference appears small when looking only at annual generation, shading during periods when the electricity purchase price is high or demand is large can have a significant impact on the amount of savings. Before installation, it is important to check how shadows fall at the site, and after installation to monitor whether there are time-of-day decreases in output.

Soiling and snowfall also affect calculations of power generation. Bird droppings, dust, pollen, fallen leaves, yellow sand, and salt-containing soiling in coastal areas can all cause reductions in power output. In snowy regions, the period during which snow remains and how easily it slides off can cause winter power generation to differ from expectations. However, cleaning and snow removal require safety considerations, and unsafe work should be avoided. When calculating reduction amounts, it is important not to entirely ignore such operational stoppages or declines.

Reduction in output due to temperature is another factor to consider. Solar power generation tends to produce more output when solar irradiance is strong, but if panel temperatures rise, output may not increase as much. Therefore, it won't necessarily reach maximum efficiency on a clear summer day. When estimating annual energy production, it's more realistic to account not only for solar irradiance but also for the effects of temperature and the installation environment.

Equipment shutdowns and communication failures must also be treated separately when calculating the amount of reduction. The judgment differs depending on whether the equipment was actually shut down and not generating, or whether generation occurred but monitoring data are missing. Before concluding that generation is low based solely on generation data, check on-site displays, meters, billing data, and anomaly logs to verify the reliability of the data itself. In particular, when using remote monitoring data, communication outages or time offsets can cause displays that differ from the actual situation.

Also, when comparing the results of calculated reductions, you should be careful about the choice of base year. If you use a year in which electricity consumption before implementation was higher than usual as the base, the post-implementation reduction may appear larger. Conversely, if the period before implementation was a year of temporary closure or reduced operations, the reduction may appear smaller. Use as representative a period as possible as the base, and if there are special factors, note them to increase the credibility of internal explanations.

Calculations of solar power generation are used in various situations such as adoption decisions, budget assessments, equipment sizing, operational improvements, and impact reporting. The required level of accuracy varies depending on which situation the calculation is for. In initial studies, grasp the overall picture with rough estimates; in the design stage, increase accuracy on a monthly and by-time-of-day basis; and after installation, verify using actual performance data—following this flow makes it easier to incorporate into practical work.

Summary

To calculate electricity bill savings from solar power generation, simply multiplying the generation by the unit price is insufficient. First, you need to understand the generation by month and by time of day and separate the portion that can actually be self-consumed. Then check the electricity purchase price and the tariff structure, and calculate by separating items that can be reduced depending on the amount of electricity from those that are unlikely to change based on generation alone. Finally, it is important to review the relationship between generation, self-consumption, and savings using actual post-installation performance data.

In practice, a key point to watch is not to confuse total generation with self-consumption. Even if generation is high, it won’t directly reduce purchased electricity unless it overlaps with power usage. Also, simple calculations using an average unit price can make it appear as if even fixed tariff items can be reduced. To improve accuracy, reconcile generation and consumption by time period and base your calculations on the actual amount by which purchased electricity was reduced.

Calculating solar power generation is useful not only for pre-installation estimates but also for verifying effectiveness after installation. By reviewing a combination of monthly generation records, time-of-day self-consumption, billing data, and outage history, it becomes easier to identify gaps between expected and actual performance. In addition, it makes it easier to find opportunities for operational improvements, such as shifting electricity use to periods when generation is high.

To get a realistic estimate of electricity cost savings, it is essential to continuously visualize generation equipment data and power usage data and compare them under consistent conditions. If you want to efficiently calculate solar generation and verify self-consumption, setting up a system that organizes generation status, energy consumption, billing data, and downtime history according to the same standards will make it easier to connect the flow from installation decision to operational improvement. By separating the amount generated from the reduction in purchased electricity and reviewing while recording the underlying assumptions, you can organize the effects of solar generation into actionable decision-making material on site.