5 Points to Note When Calculating Solar Power Generation Before Installing a Storage Battery

The purpose of calculating solar power generation before installing a storage battery is not simply to know "how much will be generated annually." In practice, it is necessary to confirm when the generated electricity can be used, during which time periods it will be surplus, how much can be charged into the battery, and whether the conditions are in place for the expected benefits to be realized after installation. If the calculation of solar power generation remains rough, judgments tend to be off when deciding the storage battery’s capacity and operating methods. This article organizes five points to check when calculating solar power generation before installing a storage battery, aimed at practitioners.

Table of Contents

• Establish the assumptions for power generation calculations before determining the battery capacity.

• View not only annual power generation but also daily and time-of-day power generation.

• Consider self-consumption and surplus electricity separately.

• Conservatively estimate the chargeable capacity, allowing for losses and degradation.

• Verify the calculation results, including post-deployment operational scenarios.

• Summary

Establish the assumptions for power generation calculations before determining battery capacity

When considering the introduction of a storage battery, attention often first goes to the battery’s capacity and how it will be used during power outages. However, in practice it is important to organize the calculation conditions for solar power generation before deciding the battery capacity. This is because a storage battery is not a device that generates electricity, but a device that temporarily stores and uses electricity produced by solar power or supplied from the grid. If you consider only the battery capacity while the expected generation is still unclear, you are likely to make mistaken judgments such as failing to secure enough power to charge the battery or, conversely, the capacity being too small relative to the available generation.

First, the basic conditions I want to confirm are the size of the solar power system, the installation orientation, the tilt angle, the installation location, surrounding shading, and whether it is an existing or a newly installed system. Even with the same system capacity, energy production varies depending on local solar irradiation conditions, the direction of the roof or mounting structure, and the times when the panel surface is shaded. If you judge “approximately this much” based only on system capacity, you may overestimate the surplus power available for battery storage. Particularly for building roofs, factories, retail stores, warehouses, and vacant land—where the installation environment differs from project to project—calculations that reflect site conditions, not just standard generation estimates, are necessary.

In generation calculations, you decide how far to examine annual generation, monthly generation, daily generation, and generation by time of day. When considering battery installation, annual generation alone is often insufficient. Even if the annual total appears adequate, in reality surplus can be concentrated during the daytime while there is little or no generation in the mornings, evenings, or at night. Because batteries are equipment designed to smooth out this time difference, using coarse calculation granularity makes it difficult to assess operational effectiveness. At minimum, you should check monthly trends, and if possible organize representative days and generation patterns by time of day.

Care should also be taken in how the solar irradiation values and generation coefficients used in calculations are handled. In practice, estimated generation is based on past weather trends and region-specific solar irradiation conditions, but actual generation varies with weather, temperature, snowfall, soiling, and equipment condition. Calculated values are only projections under fixed conditions and do not guarantee the same annual generation every year. When explaining the benefits of introducing battery storage, considering a range—such as a standard year, a low-generation year, and a high-generation year—helps avoid excessive expectations.

When adding a storage battery to an existing solar power installation, prioritize actual performance data over simulation values. If you have past generation records, amounts of electricity sold, amounts of electricity purchased, electricity consumption, peak power, and records of generation stoppages, you can make judgments closer to reality than with desk calculations. However, there are caveats with past data. Data gaps due to communication errors, differences in measurement units, equipment downtime periods, seasonal factors, and changes in business operations mean the past cannot be used as-is as the basis for future projections. When calculating generation before installing a battery, you need not only to gather data but also to confirm whether the period is appropriate to use in the calculations.

When considering a newly installed photovoltaic power generation system and battery storage at the same time, you need to anticipate not only the layout on the drawings but also changes in the surrounding environment. If adjacent buildings, trees, equipment, signs, fences, or mountain shadows cast shade on the generation surface, output may drop during certain seasons or times of day. Because battery storage systems store surplus electricity, if daytime generation is less than expected, the amount of charge will also be reduced. When firming up the assumptions for generation output calculations, checking the timing of shadow occurrence, the area affected by shadows, and seasonal differences will make it easier to realistically assess the required capacity of the battery.

Thus, when calculating solar power generation prior to installing a storage battery, the starting point is to organize not only the system capacity but also the generation conditions, installation environment, existing data, calculation units, and factors causing variability. By clarifying how much power will be generated and when, before comparing capacities or functions, it becomes easier to evaluate options that match the intended purpose of installing the storage battery.

View not only annual power generation but also daily and time-of-day power generation

When calculating solar power generation prior to installing a storage battery, be especially careful not to judge based solely on annual generation. Annual generation is useful for understanding the overall scale of the system’s output, but it does not provide enough information to plan a storage battery’s charge and discharge schedule. Because storage batteries typically store power generated during the daytime and use it in the evening or during periods of high electricity demand, you need to check when the generation occurs.

For example, a system that appears to produce enough electricity over the year may still have limited available charging capacity in winter in regions with low winter solar radiation. Conversely, in spring and autumn there may be higher generation and more periods when daytime generation exceeds consumption. The effectiveness of a battery depends on how much the times when surplus occurs differ from the times when power is needed. Therefore, it is important to look not only at the annual total generation but also at generation by month, by day, and by hour.

When you look at daily power generation, you can see the differences in output on sunny, cloudy, and rainy days. If you base assumptions only on sunny days to make the benefits of installing a storage battery look larger, in actual operation the number of days with insufficient charging will increase and you may not be able to shift as much power as expected. Conversely, if you use only bad-weather days as your baseline, the calculations become unnecessarily conservative and you may underestimate the potential for using the storage battery. In practice, checking operation not only for an average day but also for days with low generation makes it easier to explain the results after installation.

When looking at generation by time of day, pay particular attention to the generation curves in the morning, around midday, and in the afternoon. Solar power generation increases during the day and decreases toward the evening, but peak times can shift depending on installation orientation and shading. East-facing roofs tend to generate more in the morning, while west-facing roofs tend to generate more in the afternoon. Even when facing south, shadows from surrounding buildings in the morning or afternoon can shave the generation curve. The spare capacity to charge a battery is determined by the overlap of this generation curve and power consumption, so checking by time of day is essential.

In facilities that use a lot of power during the daytime, much of the generated electricity may be consumed on-site, leaving little surplus to be routed to batteries. Conversely, in facilities where daytime usage is low and consumption increases in the evening and thereafter, it becomes easier to store daytime surplus in batteries. Because households, offices, shops, factories, and warehouses use electricity differently, the role batteries can be expected to play varies even with the same amount of solar generation. It is important to look not only at generation calculations but also to compare them with consumption by time of day.

When assessing battery capacity, you should check not only the peak generation but also how long it can be charged continuously. Even if a large surplus occurs for a short time, depending on the battery's charging power and operational settings, you may not be able to store all of it. Also, on days when generation is unstable, the timing of charging and discharging may not proceed as expected. By looking at daily and hourly generation, you can identify days when the battery is likely to become fully charged, days when it cannot be sufficiently charged, and days when surpluses are unlikely to occur.

When performing calculations by time period, it is also important to align the time granularity of generation and consumption. If generation is in 30-minute intervals, consumption is given as monthly totals, and sold electricity as daily totals, you cannot correctly determine how much electricity can be directed to batteries. In practice there may be limits to the granularity of data you can obtain, but you need to convert them to at least the same time unit for comparison. Calculating without aligning units can lead to large misestimates of surplus power.

When considering the introduction of battery storage, annual generation serves as an indicator to grasp the overall scale, monthly generation as an indicator to observe seasonal differences, and daily and hourly generation as indicators for considering actual charging and discharging, so it is important to use them appropriately. The more closely you examine the calculations for photovoltaic generation, the more concrete the image of battery operation becomes. To avoid ending up after installation thinking "it doesn't charge as much as I expected" or "there is little power available at night," carefully check for timing mismatches during the calculation stage.

Consider self-consumption and surplus electricity separately

When calculating solar power generation prior to installing a storage battery, it is important not to assume that all generated electricity can be stored in the battery. Under the typical self-consumption model, electricity produced by solar generation is first used to meet loads occurring at the same time; any portion that cannot be consumed on-site becomes surplus power and is then sold back to the grid or used to charge the storage battery. In other words, the amount that can potentially be charged into the battery is not the total generation but the surplus remaining after subtracting consumption at the same time from generation.

If this point is mistaken, it can lead to overestimating battery storage capacity. For example, even if daytime generation is large, if consumption such as air conditioning, mechanical equipment, lighting, water heating, and charging devices is high during the same period, the surplus will be small. A high-generating system does not necessarily mean the battery can be fully charged. Conversely, in buildings with low daytime consumption, a larger proportion of generation remains as surplus, making it easier to utilize battery storage.

To separate self-consumption from surplus electricity, you need to overlay generation and consumption by time period. Periods when generation exceeds consumption create a surplus, and periods when consumption exceeds generation require making up the shortfall by purchasing electricity or similar means. Battery storage is effective when charged during periods of surplus and discharged during periods of shortfall. Therefore, it is important not to rely solely on calculations of solar power generation but to also check demand-side data.

The concept of the self-consumption rate is also useful. The self-consumption rate is an indicator that shows the proportion of generated electricity that is used on-site. However, when considering the introduction of battery storage, you should not simply judge a high self-consumption rate as good and a low one as bad; it is necessary to check how much surplus remains. If the self-consumption rate is very high, most of the generated electricity is being used during the daytime, so there may be little surplus available to divert to batteries. Conversely, if the self-consumption rate is low, there may be a large daytime surplus and room to shift that to nighttime or peak periods using batteries.

However, a large surplus does not necessarily mean that installing battery storage is appropriate. What matters is when and how much of the electricity stored in the battery can be used. If there is steady consumption at night or in the early morning, it is easier to expect an effect from shifting daytime surplus. Conversely, in facilities with low nighttime consumption, even if the battery is charged it may not be fully used and operations may carry the energy over to the next day. A battery storage system is a receptacle for surplus power, and it can only be utilized when there is demand to discharge to.

For existing installations that sell electricity, it may be possible to estimate the scale of surplus from the actual exported electricity. However, the exported amount does not necessarily equal the amount that can be charged into a battery. Measurement units and measurement points for exported electricity, reverse-flow restrictions, output control, equipment outages, contract conditions, and other factors can cause discrepancies between the exported amount and the amount actually available to charge the battery. Also, when a battery is installed, part of the electricity that was previously sold will be diverted to on-site consumption, so the flow of electricity changes before and after installation. In calculations, exported electricity, surplus energy, and the amount charged into the battery must be treated separately.

When checking self-consumption, you should also pay attention to equipment whose daytime load can temporarily vary greatly. In factories and workshops, consumption changes with the operating status of machines, and in stores the air-conditioning load varies with opening hours and seasons. In homes, daytime consumption also fluctuates with the time spent at home, hot water use, heating and cooling, and the way electrical appliances are used. Calculating based only on averages evens out days when a surplus does and does not occur, making it difficult to grasp the battery’s actual state of charge.

In calculations before installing a storage battery, it is important to organize step by step the generation amount, simultaneous consumption, surplus energy, chargeable capacity, and dischargeable capacity. Rather than making the simplistic judgment that a large generation amount makes a battery effective, or that having a surplus means you should simply increase capacity, checking the flow of electricity by time of day allows an assessment that closely matches post-installation operation. A major practical point is that calculations of solar power generation should not be done only from the generation side but viewed in combination with the demand side.

Make conservative estimates of chargeable capacity to account for losses and degradation

When calculating solar power generation to consider installing a battery storage system, it is important not to treat the calculated generation as the usable amount of electricity. Generated power incurs various losses as it flows from the panels through electrical equipment to the loads and the battery. Also, both the solar power system and the battery experience performance changes over long-term use. If these factors are not accounted for in pre-installation calculations, you may overestimate the actual usable amount of electricity.

The output of solar power generation is affected by factors such as output reduction due to temperature rise, wiring losses, conversion losses, dirt, snow accumulation, shading, equipment standby or shutdown, and degradation over time. A panel’s rated output is a value given under certain test conditions and does not always correspond to actual on-site generation. In summer, even when sunlight is abundant the panel temperature can rise and reduce generation efficiency. In winter, although temperatures may be advantageous, generation can be affected by sunlight hours, snow accumulation, and weather. Calculating without accounting for these variations can lead to overestimating the surplus available to send to a battery.

There are also losses within the battery. A battery cannot deliver the full amount of energy that was charged into it; losses occur during charging, storage, discharging, and conversion. In addition, for safety and lifespan considerations, the actually usable capacity may be set smaller than the rated capacity. When considering battery capacity, it is necessary to take into account not only the rated capacity but also the actual usable capacity, charge/discharge efficiency, the operational minimum state of charge, and the reserve capacity to keep available during power outages.

When considering the usable capacity of a battery from solar power generation, it's easier to organize if you separate generation, surplus energy, chargeable capacity, and the usable energy after discharge. Even if generation is 100, part of it will be consumed on-site during the day, and the remaining surplus is stored in the battery. Moreover, because of losses during charging and discharging, the amount available at night will be less than the surplus energy. If you treat the calculated generation amount as the usable energy from the battery as-is, it will often fail to match the actual experience after installation.

Anticipating degradation over time is also important. Solar power generation systems are equipment used for long periods, and their power output changes little by little as the years go by. Batteries can also see their usable capacity gradually decline as they undergo repeated charge and discharge cycles. If you judge based only on calculations for the first year after installation, it may become difficult to operate as expected after a few years. When considering battery installation, it is reassuring to assume not only the first-year power generation and storage capacity but also those after a certain period has passed.

When considering losses and degradation, there is no need to be overly pessimistic, but you should avoid calculations that are so optimistic they cannot be justified. In practice, explicitly state the calculation assumptions and compare a standard case, a low-generation case, and cases with changed consumption to provide a range for decision-making. Especially for internal approvals and customer explanations, being able to explain how much impact changes in conditions would have is more likely to be trusted than showing only the best-case scenario.

Handling shadows and soiling is also important. The effects of shading can be difficult to see from annual generation figures alone. Even short periods of shading, if they fall on part of the generating surface, can reduce output. Also, soiling and deposits such as bird droppings, dust, fallen leaves, and snow tend to occur with varying likelihood depending on the installation environment. When calculating expected generation before introducing battery storage, it is necessary to check for factors that can reduce output, including post-installation maintenance.

Also, pay attention to the conversion paths of the equipment. Configurations such as direct charging from solar PV to the battery, AC-coupled configurations, or retrofitting a battery to existing equipment can change the number of conversions and the way losses are considered depending on the system layout. What’s important here is not memorizing specific device names or method names, but understanding for your project where power is converted, where it is measured, and where it enters the battery. If you perform calculations without knowing the measurement points and the flow of power, you are likely to misunderstand the relationship between generated energy and charged energy.

When calculating solar power generation before introducing battery storage, it is important to take a conservative approach when estimating the usable energy. Being conservative does not mean understating the expected benefits of the installation. Rather, it is a practical measure to reflect real-world losses and degradation so that the gap between projected and actual performance after installation is not large. By building a margin into the calculated generation figures and accounting for the battery’s charge/discharge losses and its usable capacity, you can make an installation decision that is closer to reality.

Verify calculation results, including operational scenarios after deployment

Calculating solar power generation provides a basis for deciding whether to install a storage battery, but the calculation results alone are not sufficient. What’s important is clearly defining how that generated power will be used. Storage batteries have several purposes, such as using daytime surplus power at night, reducing electricity purchases during peak hours, securing power during outages, and increasing self-consumption of renewable energy. If the purpose differs, both the way you assess the required generation and the way you determine battery capacity will change.

First, clarify the purpose of introducing a battery storage system. Whether you prioritize smoothing electricity bills, outage resilience, utilizing daytime surpluses, or overall facility energy management will change what needs to be calculated. For example, if you prioritize outage resilience, you need to check not only the normal annual power generation but also the loads you want to run during an outage, the required duration, generation on bad-weather days, and the reserve capacity to keep in the battery. If you prioritize self-consumption, the relationship between surplus electricity and nighttime consumption becomes important.

Next, assume how the storage battery will be charged and discharged on a daily basis. The usable capacity will vary depending on whether you charge immediately when surplus occurs during the day and discharge in the evening and later, discharge during specific time periods, or leave a certain amount reserved as a backup for power outages. When calculating solar power generation, it is important to consider not only days with high output but also operations on cloudy or rainy days when the battery may not be sufficiently charged. Calculating on the assumption that the battery becomes fully charged every day tends to produce discrepancies with actual operation.

The facility's operating schedule is also a major factor. If consumption differs significantly between weekdays and holidays, the way generation and demand overlap will change. In factories, whether it is a working day or a closed day; in stores, business hours; in offices, workdays and days off; and in residences, occupants' at-home status—all of these affect how generated electricity is used. Facilities with low consumption on holidays will have increased surplus power and make it easier to charge batteries, but it is necessary to check whether that power can be used up during the night or the following day. If you base planning only on working days, you may overlook holiday surpluses or operational constraints.

When considering peak management, compare the timing of power generation with that of maximum demand. At facilities where the peak occurs around noon, when solar power generation is high, generation can often help reduce the peak. Conversely, at facilities where the peak occurs in the evening or early morning, solar power generation alone is difficult to rely on, and the timing of battery discharge becomes important. In this case, you need to check not only how much can be charged during the day but also whether the battery’s remaining charge can be maintained until the peak time and whether it will not be consumed earlier by other loads.

When planning operations for a power outage, more realistic condition-setting is required. Rather than assuming that all equipment can be used as usual during an outage, narrow down the loads to be prioritized and calculate the required amount of energy. Lighting, communications equipment, refrigeration, management equipment, water supply systems, etc.—priority loads differ depending on the facility. Because solar power generation is affected by weather and time of day, there is no guarantee of sufficient generation during an outage. It is important to organize in advance how much charge to maintain in the battery storage and how to assess the likelihood of recharging during daytime.

In post-installation operation scenarios, control settings and operating rules are also reviewed. A battery storage system will not automatically operate optimally just by being installed; settings and management tailored to the intended purpose are required. Consider which time periods to charge, how low to discharge (what remaining level to leave), how much to keep in reserve for power outages, and whether to change settings seasonally. Even if the calculated solar power generation is sufficient, if the operational settings do not align with the objectives, the expected effects will be difficult to achieve.

Also, it is a good idea to decide in advance how you will verify performance after installation. By determining which data you will use after installation to check the power generation, surplus electricity, charging amount, discharging amount, and self-consumption assumed in your calculations, you will make it easier to improve operations. If you can compare the pre-installation calculations with the post-installation actual performance, you can identify the effects of weather, changes in consumption patterns, the need to review settings, and the condition of the equipment. Calculations can be used not only for the installation decision but also as management indicators after installation.

When calculating solar generation before introducing battery storage, it is important not to stop at producing generation figures but to verify, including the operational objectives, charge/discharge patterns, facility operating schedule, use during outages, and post-installation verification methods. By mapping the calculation results into operational scenarios, it becomes easier to determine whether the battery’s capacity and settings match the site’s actual usage.

Summary

Before installing a battery storage system, when calculating solar power generation it is important to check not only the magnitude of generation but also, in order, when that power is produced, how much is self-consumed, how much remains as surplus, and how much can be charged into the battery. Looking only at annual generation gives an understanding of the overall scale of the system, but is insufficient to judge the effectiveness of a battery. By checking generation by month, by day, and by time of day, and overlaying it with consumption, the actual amount of power that can be directed to the battery becomes clearer.

What you should be especially careful about is not to confuse generated energy with the amount that can be charged. Of the electricity generated, the portion used at the same time is self-consumption, and the remainder becomes surplus power. What can be charged into the battery is mainly this surplus portion, and you also need to take into account losses during charging and discharging and limits on usable capacity. If you treat the calculated generation amount as the amount of energy you can use from the battery, you are likely to see a gap between expectations and reality after installation.

In addition, solar power generation fluctuates with weather, season, installation orientation, shading, soiling, temperature, equipment condition, and ageing. Storage batteries also require consideration of charge/discharge efficiency, usable capacity, and operational reserve settings. When evaluating the benefits of installation, consider not only favorable conditions but also days with low generation and days when consumption patterns change, so you can make a judgment that is closer to reality.

It is also necessary to clarify the purpose of introducing a battery storage system. Whether you want to increase self-consumption, implement peak shaving, or prioritize preparedness for power outages will change how you view the required generation and how you use the battery. By mapping the calculated solar PV generation to charging/discharging operation scenarios, facility operating schedules, priority loads, and post-installation verification methods, you can more easily connect desk calculations with on-site operations.

When practitioners carry out "solar power generation calculations," they need a perspective that not only determines the total generated power but also identifies the amount of electricity that can be utilized by batteries. By organizing the calculation conditions, checking generation and consumption by time of day, anticipating losses and degradation, and considering post-installation operation, they can improve the accuracy of battery capacity selection and installation decisions.

If you want to efficiently proceed from calculating solar power generation to evaluating the introduction of battery storage, monitoring generation status, and organizing site-specific conditions, it is important to establish a management system that allows integrated confirmation of generation output, installation environment, consumption patterns, and operational objectives. By reviewing calculation results while cross-checking them against site conditions and actual performance data, you can more easily not only make pre-installation decisions but also improve post-installation operations.