How to Estimate Monthly Balances Using Solar Power Generation Simulations

When checking the benefits of an installation with a solar power generation simulation, relying only on annual generation or annual balances can lead to oversights in actual operation. Solar power generation varies by season, and facility electricity consumption also fluctuates month to month. For facilities with large cooling demand in summer, large heating or production equipment demand in winter, or large differences between holidays and busy seasons, you need to look at monthly balances to correctly assess the benefits of installation. This article explains, from a practical perspective for practitioners who search for "solar power generation simulation," how to estimate monthly balances.

Table of contents

• The importance of viewing monthly balances in solar power generation simulations

• Check monthly generation to understand seasonal variation

• Organize monthly electricity consumption and daytime demand

• Separate self-consumption and surplus electricity by month

• Read electricity cost savings by month

• Reflect generation losses and weather variability in monthly balances

• Evaluate batteries and load shifting in monthly balances

• Check risks not visible from annual balances alone

• How to compare monthly balances in vendor proposals

• Site information accuracy increases the reliability of monthly balances

• Summary

The importance of viewing monthly balances in solar power generation simulations

Solar power generation simulations sometimes emphasize annual generation and annual savings. Annual totals are an easy-to-understand indicator as an entry point for installation decisions. However, when implementing solar power generation in practice, annual totals alone are insufficient. Both generation and electricity consumption vary month to month, and there are months when the benefits are easy to realize and months when they are not.

Solar power generation changes month by month due to irradiance, sunshine hours, solar altitude, temperature, weather, snow, and the lengthening of shadows. While generation can increase from spring to summer, months can also see declines due to the rainy season, typhoons, short winter sunshine hours, snowfall, or winter shadows. Looking only at annual generation averages out these month-by-month variations and makes them hard to see.

Facility electricity consumption also changes month to month. Facilities with large cooling demand in summer, large heating or production equipment demand in winter, facilities with busy and slow seasons, or facilities with months that have many holidays versus many operating days all show large monthly variation in electricity consumption. If months with high generation coincide with months of high demand, self-consumption tends to increase; if months with high generation coincide with low demand, surplus electricity may increase.

The purpose of viewing monthly balances is to judge the effects of solar power generation more realistically. Even if annual balances look good, issues may be hidden such as large surplus in certain months, low generation in months with high demand, or smaller-than-expected winter savings. Checking monthly balances makes it easier to grasp undersized or oversized equipment capacity, the need for batteries, scope for load shifting, and operational issues after installation.

Monthly balances are also useful for internal explanations. Annual totals alone make it difficult to explain when benefits are likely to appear and which months require attention after installation. Organizing by month makes it easier to communicate to stakeholders the effectiveness in summer, winter risks, holiday surplus, and the impact of generation losses.

If you use solar power generation simulations for installation decisions, do not stop at checking annual generation—it's important to read monthly generation, consumption, self-consumption, surplus electricity, and savings separately.

Check monthly generation to understand seasonal variation

The first step in estimating monthly balances is to check monthly generation. Solar power generation varies greatly by season. Annual generation alone does not tell you how much can be generated in each month. By checking monthly generation you can identify peaks and troughs in generation and evaluate balance stability.

When looking at monthly generation, first identify months with high and low generation. Generally, generation tends to be higher in periods with high irradiance and long sunshine hours. However, while irradiance may be high in summer, panel temperature can rise and cause temperature-related losses. Spring and autumn sometimes offer a better balance between irradiance and temperature, resulting in more stable generation.

Winter generation is also important. Shorter sunshine hours and lower solar altitude in winter tend to reduce generation. Shadows from surrounding buildings, rooftop equipment, trees, handrails, and rooftop structures also lengthen in winter. In snowy areas, snow on panels or residual snow can create periods when no generation is possible. For facilities with high winter demand, the reduction in winter generation strongly affects monthly balances.

When checking monthly generation, also confirm whether regional conditions are reflected. Generation patterns differ in mountainous areas, coastal areas, basins, snowy regions, and cloudy regions. If the irradiance data used in the simulation does not match the candidate site, monthly balance estimates will be unstable. It is desirable to ensure that conditions close to the installation site, not just broad-area averages, are reflected.

Monthly generation also varies by orientation and tilt. South-facing, east-facing, west-facing, rooftop, and ground-mounted installations produce different generation profiles. Tilt angle can produce arrangements that are stronger in summer or relatively better in winter. If there are multiple installation surfaces, it is easier to judge when you check not only the overall monthly generation but also generation by surface.

By understanding monthly generation, you can find months when balances are likely stable, months when generation is likely insufficient, and months when surplus is likely to increase. In monthly balance estimation, use this seasonal variation in generation as the foundation and then overlay facility electricity consumption.

Organize monthly electricity consumption and daytime demand

To estimate monthly balances, you must organize facility electricity consumption by month as well as generation. The effect of solar power depends greatly on how much of the generated electricity can be used within the facility. Even if monthly generation is high, if daytime demand is low in that month, surplus increases and the reduction in purchased electricity may be limited.

First confirm the monthly electricity consumption. Facilities with large cooling demand in summer tend to have higher consumption in summer. Facilities with large heating, ventilation, production equipment, or hot water loads tend to have higher consumption in winter. Factories and warehouses may see consumption change with busy and slow seasons. Stores and public facilities may experience demand changes due to seasonal events, visitor numbers, or opening hours.

Next, daytime demand is important. Since solar power generates mainly during the day, total monthly consumption alone does not allow correct judgment of self-consumption. Facilities with high usage at night may have high annual or monthly consumption but limited overlap with solar generation. Conversely, facilities with a stable daytime base load can more easily use generated electricity.

If you have time-of-day usage data, overlaying monthly generation with time-of-day demand allows a more realistic estimate of self-consumption. Even without time-of-day data, you can form a hypothesis about daytime demand by organizing operating hours, holidays, main equipment operating times, air-conditioning usage times, and midday shutdowns.

Do not overlook differences between weekdays and holidays. Some facilities can self-consume during weekdays when daytime demand exists, but see demand drop and surplus increase on holidays. If the number of holidays differs by month, monthly balances are affected. Annual average self-consumption rates alone cannot capture these differences.

Also consider future changes in electricity consumption. Expansion of production equipment, air-conditioning updates, energy-saving measures, changes in operating hours, introduction of electric equipment, and adding batteries can change monthly consumption. If you have high-confidence plans, reflect them in monthly balance estimates to reduce post-installation gaps.

To correctly estimate monthly balances, you must look at both generation and consumption by month. Confirming not only how much is generated but how much can be used in the same month and time period enables a judgment closer to practical operation.

Separate self-consumption and surplus electricity by month

The core of monthly balance estimation is separating self-consumption and surplus electricity. Electricity generated by solar power splits into the portion used simultaneously within the facility and the portion that remains unused. Judging only by annual generation without separating these two can lead to misjudging actual installation benefits.

Self-consumption is the amount of generated electricity used within the facility. It is directly linked to reductions in purchased electricity and is one of the most important indicators when reading monthly balances. Even in months with high generation, if daytime demand is low, self-consumption will not increase. Conversely, in months with moderate generation but good overlap with daytime demand, self-consumption can be stable.

Surplus electricity is the generated electricity that cannot be used within the facility at that time. How surplus is treated—exported, stored in batteries, or curtailed—changes its impact on the balance. Surplus power has a different nature from self-consumed power, so it is important not to treat them as having the same value.

Viewed by month, the pattern of surplus changes significantly. In spring and autumn, when generation is high but facility demand may not be large, surplus can increase. In summer, even with high generation, self-consumption can be easier if it overlaps with cooling demand. In winter, even with high demand, self-consumption may be limited if generation is low.

Do not judge by self-consumption rate alone. With small system capacity, self-consumption rate tends to be high, but the self-consumption amount itself may be small. With large system capacity, self-consumption rate may fall while self-consumption amount increases. In monthly balances, you need to check self-consumption rate, self-consumption amount, and surplus electricity together.

Also check how monthly patterns change if you change system capacity. Increasing capacity raises generation, but whether that increase becomes self-consumption or surplus depends on the month. Summer may see more self-consumption, while spring or holiday-heavy months may see more surplus—monthly decomposition helps optimize capacity.

When combining batteries, compare month by month with and without batteries. Confirm how much surplus the battery can absorb, how much can be shifted to evening or night demand, and how much self-consumption increases after accounting for charge/discharge losses. Since surplus amounts change by month, battery effects also vary by month.

By separating self-consumption and surplus electricity by month, you can judge the benefits of solar power installation more concretely. The basis of monthly balance estimation is separating usable electricity from surplus, not just total generation.

Read electricity cost savings by month

When estimating monthly balances, you need to read electricity cost savings by month based on self-consumption. However, simply converting generation or self-consumption into monetary amounts is insufficient for practical judgment. Electricity contracts, usage time zones, maximum demand, seasonal loads, and treatment of surplus electricity change how savings appear.

First, the core of electricity cost savings is self-consumption. Using generated electricity within the facility reduces the electricity purchased from outside. Checking self-consumption by month shows in which months reductions in purchased electricity are likely. Facilities with large cooling demand in summer and high generation may see large summer savings.

On the other hand, months with high generation but also high surplus do not directly translate into reductions in purchased electricity. How surplus is handled affects the balance, so evaluate it separately from self-consumption. In monthly balances, do not confuse generation, self-consumption, and surplus electricity.

When looking at electricity cost savings, separate the part that changes with usage from the part related to contract conditions and maximum demand. Even if daytime purchased electricity decreases due to solar generation, if maximum demand occurs outside generation hours, the impact on contract-related charges may be limited. When reading savings in monthly balances, check which time-of-day purchased electricity is being reduced.

In particular, summer often sees facilities with maximum demand due to air-conditioning. If solar generation hours overlap with maximum demand, solar can contribute to peak shaving. However, if maximum demand occurs on cloudy days or in the evening, solar alone may not be expected to reduce peaks. It is important to overlay time-of-day demand and generation, not just monthly figures.

Be careful in winter as well. For facilities with large winter demand, savings may be smaller than expected since generation is low in that period. Snow and winter shadows further reduce generation. Monthly balances need to check whether generation is sufficient in high-demand months.

Reading electricity cost savings by month reveals the composition of annual balances. Even if annual figures suggest installation benefits, actual effects can be concentrated in certain months or small during winter and holidays. Decomposing by month helps identify operational issues early.

Reflect generation losses and weather variability in monthly balances

When estimating monthly balances, it is important to reflect generation losses and weather variability. Solar power generation simulations are forecasts based on certain assumptions, and actual generation varies with the year’s weather and local environment. To bring monthly balances closer to reality, identify generation-lowering factors for each month.

Generation losses include temperature losses, shading, wiring losses, power conversion losses, soiling, snow, equipment downtime, and aging. These losses do not necessarily occur uniformly throughout the year. High temperatures in summer can cause output reduction, while shadows and snow impact winter more. In spring and autumn, pollen, dust, and fallen leaves can affect generation.

In monthly balances, check which months are likely to see larger generation losses. For example, if summer generation looks optimistic relative to irradiance, confirm whether temperature losses are sufficiently reflected. If winter generation looks high, check whether winter shadows or snow are reflected. If many trees are nearby, consider seasonal variation in shading and leaf fall.

Weather variability is also important. Simulations often assume standard meteorological conditions, but in reality some years have many sunny months while others have many cloudy or rainy months. If a month’s balance is worse than the simulation, you need to distinguish whether it’s an equipment issue or weather. Therefore, keep a basis that allows comparison between predicted and actual monthly values.

Generation losses also affect self-consumption. In months where generation falls, if those months typically had large surplus originally, the impact on self-consumption may be limited. Conversely, in months where demand is large and generated electricity is almost fully used, generation losses directly affect reductions in purchased electricity. In monthly balances, check whether generation losses affect the self-consumption portion or the surplus portion.

Equipment downtime and maintenance should also be reflected in monthly balances. Periods when generation cannot occur due to regular inspections, equipment replacement, fault response, or surrounding construction can arise. It is difficult to predict all such events precisely, but when reading long-term balances, do not assume ideal continuous operation.

Reflecting generation losses and weather variability month by month makes monthly balance estimates more conservative. Rather than overestimating generation, realistically accounting for downside factors is important to reduce post-installation gaps.

Evaluate batteries and load shifting in monthly balances

When estimating monthly balances, it is important to evaluate the effects of batteries and load shifting month by month. If surplus electricity occurs from solar generation, how that surplus is utilized changes monthly balances. Batteries can allow surplus to be used in other time periods, but they do not produce the same effect in every month.

Batteries are not equipment that increases generation. They store daytime surplus for use in the evening, night, or other time periods. Therefore, a battery's effect depends on how much daytime surplus occurs and how much demand exists to discharge into. Facilities with large surplus in spring or autumn may offer significant battery charging opportunities, whereas facilities with limited generation in winter may have limited chargeable energy.

In monthly balances, compare cases with and without batteries. Without batteries, how much can be self-consumed and how much surplus occurs? With batteries, of that surplus how much can be charged and in which time periods can it be discharged? Considering charge/discharge losses, confirm how much self-consumption increases.

If you prioritize emergency use, separate that from normal self-consumption. Operating a battery to fully use it daily increases self-consumption but may leave insufficient reserve for blackouts. If you reserve a certain amount for emergencies, usable capacity for normal operation is limited. When reading monthly balances, clearly define battery operation policy.

Load shifting also affects monthly balances. If you have equipment whose operating times can be adjusted, shifting operation into solar generation hours can increase self-consumption. For instance, air-conditioning, ventilation, pumps, charging equipment, and cooling equipment that can operate during the day can be scheduled to align with generation. However, operational and production constraints may limit load flexibility.

Month by month, there are months where load shifting is more or less effective. In months with high generation and surplus, shifting loads into generation hours is easier. In months with low generation or already large demand, there may be little scope for load shifting.

Evaluating batteries and load shifting in monthly balances helps concretize post-installation operation. You can identify monthly issues not visible in annual totals: monthly surplus, insufficient charging, lack of discharge targets, and potential demand shifts.

Check risks not visible from annual balances alone

The greatest advantage of estimating monthly balances is that you can identify risks not visible from annual balances alone. Annual balances are easy to understand as yearly totals, but they average out monthly mismatches between generation and demand, surplus occurrences, and seasonal generation losses. In practice, it is important to see what lies behind those averaged numbers.

For example, annual generation may seem sufficient, but if surplus is high in spring and autumn and generation is insufficient in winter, annual figures may look favorable while facilities with high winter demand see smaller-than-expected benefits. Monthly balances show which months perform well and which months are weak.

Also, if a facility has high generation in summer that matches cooling demand, monthly balances may show large summer improvements. Conversely, in months with many holidays or reduced operation, even high generation can result in increased surplus and little contribution to the balance. Monthly decomposition reveals holiday and seasonal variation effects not visible from annual totals.

Seasonal differences in generation losses are also risks. Temperature losses in summer, shadows and snow in winter, and soiling or leaf fall in spring and autumn are month-specific factors. Looking only at an annual loss rate makes it hard to see such seasonal downside. Monthly balances show which months are more likely to have losses that affect the balance.

Over-sizing capacity is also easier to spot month by month. Increasing capacity to boost annual generation may only increase surplus in months with low demand, yielding limited contribution to the balance. By checking self-consumption and surplus by month, you can determine at which capacity surplus tends to increase.

Monthly balances are also useful for post-installation performance management. Comparing monthly actuals with forecasts after operation starts makes it easier to identify weather differences, soiling, shadows, equipment condition, and demand changes. Problems that would be detected late by looking only at annual data can be noticed earlier by viewing monthly data.

Annual balances provide a high-level indicator for installation decisions, but monthly balances show operational risks and improvement opportunities. To utilize solar power generation simulations in practice, use both appropriately.

How to compare monthly balances in vendor proposals

When you receive solar power generation simulations from multiple vendors, comparing monthly balances is extremely important. Even for the same facility or site, differences in system capacity, installation area, irradiance, shading, loss rates, and electricity usage assumptions change how monthly balances appear.

First check whether monthly generation is shown. Annual generation alone does not reveal seasonal differences in generation. If monthly generation is provided, you can check summer and winter generation trends, rainy season or snowfall impacts, and seasonal shading changes. Proposals without monthly generation make it difficult to judge balance stability.

Next, see whether monthly self-consumption and surplus electricity are shown. Annual self-consumption rates alone do not show which months have large surplus or which months have alignment between demand and generation. When comparing vendor proposals, confirm whether they separate monthly self-consumption and surplus electricity.

The granularity of electricity usage data is another comparison point. Proposals that estimate monthly balances from annual usage alone differ in accuracy from those that reflect monthly and time-of-day usage. Check whether weekends vs. weekdays, seasonal variation, and operating hour differences are reflected.

Compare how generation losses are treated. Monthly generation changes depending on how much temperature, shading, soiling, snow, wiring, conversion, and aging are reflected. If summer generation looks too high, check temperature losses; if winter generation looks too high, check shading or snow assumptions. If loss rates are presented only as an aggregate value, confirm how monthly effects are treated.

For proposals that include batteries, compare monthly balances with and without batteries. Showing only results with batteries can obscure issues of surplus or self-consumption with solar alone. Check monthly charging, discharging, surplus reduction, and charge/discharge losses to realistically judge battery effectiveness.

When comparing vendor proposals, do not simply choose the proposal with the best-looking annual balance; prioritize proposals whose monthly breakdown matches site conditions and facility operation. A proposal with explainable monthly balances is also useful for post-installation performance management.

Site information accuracy increases the reliability of monthly balances

Accurate site information is essential to increase the reliability of monthly balances. Solar power generation simulations calculate generation based on installation site conditions. If the candidate installation area, orientation, tilt, shading, obstacles, surrounding environment, connection equipment, or maintenance access routes are inaccurate, monthly generation, self-consumption, and surplus electricity estimates can deviate from reality.

For rooftop projects, accurately grasp roof dimensions, orientation, slope, rooftop equipment, handrails, rooftop structures, piping, drains, inspection openings, and positional relationships with surrounding buildings. Winter shadows, summer temperature environment, and installation constraints due to rooftop equipment affect monthly generation. If equipment or piping added after the drawings exist, update the simulation after a site survey.

For ground-mounted projects, confirm site boundaries, trees, utility poles, surrounding structures, slopes, embankments, drainage channels, maintenance paths, and candidate connection points. Trees and terrain relate to shading, soiling, wind, and snow management. Even if it appears you can use the entire site, you may need to reserve maintenance paths, drainage, or snow storage space, which can change installable capacity.

Site information also affects self-consumption. If panel layout or orientation changes, generation hours change. If generation hours change, overlap with facility daytime demand changes. In other words, site information accuracy affects not only monthly generation but also monthly self-consumption and surplus electricity.

Accurately recording site information also helps post-installation performance management. If predictions and actuals diverge, it is easier to determine whether shading, soiling, snow, equipment condition, or demand changes are the cause. If you have matching monthly balance baselines and site information, you can more quickly identify reasons for generation decline or balance deterioration.

Accurate site information is also important when comparing vendor proposals. Sharing the same site conditions with each vendor makes it easier to judge whether differences in monthly balances are due to design policy or differing assumptions. If site conditions remain ambiguous, comparing monthly balances is unstable.

To make monthly balance estimates reliable, prepare both electricity data and accurate site information. By accurately overlaying generation-side conditions and consumption-side conditions, you can make solar power generation simulations practical documents.

Summary

To estimate monthly balances using solar power generation simulations, you need to check not only annual generation but also monthly generation, monthly electricity consumption, self-consumption, surplus electricity, generation losses, weather variability, and the effects of batteries and load shifting separately. Annual balances are useful to grasp the big picture, but operational risks and seasonal effects cannot be understood without monthly analysis.

First, check monthly generation to understand seasonal variation. Confirm summer generation, winter generation, rainy season or snowfall impacts, temperature losses, and the lengthening of shadows to map generation peaks and troughs. Next, organize monthly electricity consumption and daytime demand. Even in months with high generation, surplus increases if daytime demand is low. It is important to see the overlap between generation and demand.

Estimate self-consumption and surplus electricity separately by month. Self-consumption is closely tied to reductions in purchased electricity, while surplus electricity’s impact on the balance depends on how it is handled. Do not judge by self-consumption rate alone—check actual self-consumption amount together with surplus electricity.

When reading electricity cost savings by month, focus on self-consumption. However, separate parts that vary with usage from parts related to contract conditions and maximum demand. Overlaying monthly and time-of-day generation and demand makes it easier to judge the realism of savings.

Reflect generation losses and weather variability in monthly balances. Temperature losses in summer, shadows and snow in winter, and soiling or leaf fall in spring and autumn are month-specific factors that reduce generation. Since weather variability also exists, do not treat standard simulation results as absolute—identify downside factors.

When considering batteries and load shifting, evaluate them month by month. Confirm which months generate surplus, how much can be charged to batteries, and which time periods can be discharged to. For load shifting, check whether demand can realistically be shifted to generation hours. Annual totals alone make it difficult to judge the effectiveness of batteries and load shifting.

Viewing monthly balances uncovers risks that annual balances hide: months when demand is large but generation is low, months with large surplus, weaker winter effects, and months with many holidays that increase surplus. When comparing vendor proposals, prefer proposals that can explain monthly breakdowns, not just the best-looking annual balance.

Finally, accurate site information is the foundation for reliable monthly balances. If you can accurately record installation candidate areas, rooftop equipment, obstacles, trees, site boundaries, orientation, tilt, maintenance access routes, and candidate connection points on site, you can bring predictions of monthly generation, self-consumption, and surplus electricity closer to reality.

If you want to improve the accuracy of monthly balance estimates by precisely recording site installation candidate areas, rooftop equipment, obstacles, trees, site boundaries, orientation, tilt, maintenance access routes, and candidate connection points on site, using LRTK, an iPhone-mounted high-precision GNSS positioning device, is effective. High-precision location information from the site makes it easier to organize shading and obstacles, feasible installation areas, wiring routes, and maintenance access routes, enabling consistent comparison of vendor proposals, pre-construction checks, and post-installation performance management. To correctly estimate monthly balances with a solar power generation simulation, it is important to prepare both accurate electricity data and accurate site information.