6 considerations for calculating solar power generation and battery capacity

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Table of Contents

• Reasons to consider power generation calculations and battery capacity together

• Approach 1: Organize generation benchmarks by annual, monthly, and daily amounts

• Approach 2: Understand the time windows available for self-consumption and the amount of surplus electricity

• Approach 3: Consider batteries in terms of kW as well as kWh

• Approach 4: Reverse-calculate the required capacity from the amount of electricity you want to use at night

• Approach 5: Consider usable (effective) capacity, including charge/discharge losses and state-of-charge control

• Approach 6: Assess surpluses and shortages by examining seasonal variations

• Order of checks that practitioners should confirm when proposing equipment

• Summary

Why consider power generation calculations and battery capacity together

When evaluating solar power systems, the calculation of generation output and the assessment of battery capacity are often discussed separately. However, in practice it's easier to make decisions if these two are not considered independently. This is because a battery isn't simply better if it's made larger when generation is higher; the required capacity changes depending on when surplus occurs and when you want to use electricity.

For example, even a system with a large annual power generation can have little surplus, and the amount of electricity that can be diverted to battery storage is limited if the facility uses a lot of electricity on-site during the daytime. Conversely, residences and facilities with low daytime consumption tend to produce more surplus, increasing the potential for battery utilization. In other words, battery capacity cannot be determined by calculating solar power generation alone; it is necessary to consider surplus electricity together with nighttime demand.

What you should grasp first here is the difference between kW and kWh. kW refers to the output scale of equipment: for a solar PV system it means the system capacity, and for a battery it relates to the ability to charge or discharge at any one time. By contrast, kWh is the amount of electricity generated or stored over a certain period. When considering solar generation you ultimately organize values in kWh, and battery capacity is also basically thought of in kWh. However, in practice batteries require attention to both kWh and kW, so it is important to clarify this difference early on.

You should also clarify the purpose of combining battery storage. The way you think about the required capacity differs depending on whether you want to shift daytime surplus to the night to raise self-consumption, prioritize backup during power outages, or reduce peak power. If your sole objective is to improve self-consumption, the main focus should be on the balance between the amount of energy surplus during the day and the amount used at night. On the other hand, if you place strong emphasis on emergency preparedness, you also need to consider how many hours you want to power the necessary equipment.

The greatest advantage of considering solar generation and battery capacity together is that it makes it easier to see the overall system’s surpluses and shortfalls. If the solar system is oversized and only increases daytime surplus, efficiency will suffer, and if you make only the battery larger without sufficient daytime surplus, it cannot be fully charged. In other words, generation capacity and battery capacity are not independent optimal solutions but should be optimized as a combination.

In this article, we organize these ideas from six perspectives. Starting with how to set annual generation, and then sequentially examining surplus energy, nighttime consumption, charge/discharge losses, and seasonal variations, the concept of battery capacity becomes much clearer. This is compiled as useful material for those who want to take solar power generation calculations beyond mere generation figures and connect them to practical system planning.

Approach 1: Organize power generation criteria by annual, monthly, and daily output

The initial idea is to organize the standards for power generation into three categories: annual, monthly, and daily. When linking solar power generation to battery capacity, looking only at annual kWh makes it difficult to understand how much surplus is generated on a daily basis. Conversely, looking only at daily amounts makes seasonal differences hard to see. Therefore, it is important to keep annual, monthly, and daily figures as numbers that serve different roles.

First, annual power generation is useful for getting a rough sense of system scale. For example, with a system capacity of 10 kW, if you assume a regional reference generation of about 1,050 kWh per kW per year, the annual generation will be around 10,500 kWh. Having this figure makes it easier to outline the system and compare alternatives. However, this figure alone does not show how much surplus there is per day, which is necessary for determining battery capacity.

Next, we look at monthly power generation. Monthly power generation is useful for understanding seasonal differences. In spring and autumn it is relatively easy to generate power stably; in summer solar irradiation is strong but there are high-temperature losses; and in winter, daylight hours are short and shadows tend to lengthen, so generation tends to fall. If you do not check these differences, you can easily overlook phenomena such as a storage battery being able to fully charge in summer but hardly being charged in winter. In other words, monthly power generation is the figure used to read seasonal variations in how the storage battery is used.

Finally, looking at daily amounts makes the connection to battery capacity suddenly much clearer. For example, assume a 10 kW system generates about 28 kWh per day on average. If 15 kWh of that is used during the day by a facility or a household, the surplus is about 13 kWh. If you are installing a storage battery, the starting point for thinking about capacity is how much of that 13 kWh you want to shift to the night. For considering a storage battery, the daily surplus of 13 kWh is far more useful than the annual 10,500 kWh.

Thus, annual, monthly, and daily figures are not competing numbers but serve different purposes. Use the annual figure to grasp the scale of the installation, the monthly figure to check seasonal variations, and the daily figure to link to battery capacity. Thinking in that order makes the relationship between solar generation and battery capacity much easier to organize. Rather than immediately deciding how many kWh the battery should be, first having generation figures on these three scales is very important when planning based on self-consumption.

Approach 2: Understand the time periods available for self-consumption and the amount of surplus electricity

The second approach is to identify the hours when self-consumption is possible and the amount of surplus electricity. The most important thing when considering battery capacity is to know how much of the electricity generated during the daytime is consumed on site and how much is left over. This is because, fundamentally, only this surplus can be stored in the battery. Even if total generation is large, in facilities that use most of it during the day, the amount of electricity that can be allocated to the battery is not very great.

For example, in facilities such as factories or offices that are operating fully during the day, much of the solar power generated is consumed on-site. In such cases, while the reduction in electricity costs can be significant, the required battery capacity may not end up being as large as expected. Conversely, in homes where occupants are often away during the day or in facilities with low daytime demand, surplus electricity tends to arise. In that case, the benefit of using batteries to shift that surplus to nighttime becomes greater. In other words, when planning battery capacity it is more practical to consider the size of the surplus rather than the total generation volume itself.

The important point here is not to look at surplus electricity only in terms of an annual average. For example, even if the yearly total appears to show a large surplus, in reality it may be concentrated in spring and autumn, while in summer the surplus can be reduced due to air-conditioning demand. Alternatively, in winter generation may be lower, which could increase the number of days when batteries cannot be sufficiently charged. In other words, surplus electricity is a matter of time periods that include seasonal variation, and it can be hard to see by simple subtraction alone.

Also, the orientation of the installation changes how surplus appears. South-facing systems tend to concentrate generation around noon, and if daytime demand is low the surplus tends to be large. With east-west distributed systems, the generation period widens, which can increase the hours during which self-consumption is easier. In other words, the amount of surplus electricity is also influenced by the system configuration. When considering batteries, it is essential not just to look at total generation but to check how much surplus occurs in each time period.

In practice, it's easiest to start by estimating the surplus from the average daily generation minus daytime usage, and then adjust that estimate month by month. Once you can interpret this surplus, discussions about battery capacity become much more concrete. In other words, the true entry point to battery capacity is not the generation amount itself but grasping the surplus energy.

Approach 3: Consider battery storage not only in kWh but also in kW

The third way of thinking is to consider batteries not only in kWh but also in kW. When it comes to battery capacity, the discussion tends to concentrate on how many kWh of battery to install. Of course, how much energy can be stored is important. However, in practice that alone is not enough. That's because batteries are important not only for "how much can be stored" but also for "the output (power) at which they can be charged and discharged."

For example, even if you have a 10 kWh battery, if it doesn't have the output to handle a large load at once, it may not be able to adequately cover nighttime use. Conversely, even if the output is high, if the total amount that can be stored is small, it will only be able to provide support for a short time. In other words, you can't tell whether a battery is really suitable for a facility or a home by looking only at kWh; you must consider kW as well.

When considered together with calculations of solar power generation, this difference becomes even more important. Even if there is a large daytime surplus, you cannot capture all of it if the battery's charging power cannot keep up. Conversely, at facilities where a large load suddenly ramps up at night, operations may not meet expectations even if electricity is stored, if the battery's discharging power is insufficient. In short, when connecting solar power generation to batteries, you need to check not only the amount but also the power flow.

Also, in practice it’s advisable to check not only the average load but also how peak loads appear. Facilities that have a small, steady load during the night and facilities where the load suddenly surges during specific time periods require batteries with different characteristics. For the former, it’s relatively easy to prioritize energy capacity, while for the latter it makes sense to place more emphasis on power output as well. In other words, when considering battery capacity you need to look not just at surplus kWh and nighttime usage kWh, but also at the flow of output over time.

With this way of thinking, you stop viewing a battery's "size" as a single number. In practice, only by looking at both the amount in kWh and the output in kW can you readily assess whether the battery capacity is appropriate. If you want to directly link calculations of solar power generation to a battery, this perspective is indispensable.

Approach 4: Reverse-calculate the required capacity from the amount of electricity you plan to use at night

The fourth approach is to work backwards from the amount of energy you want to use at night to determine the required capacity. A common tendency when considering battery capacity is to try to decide the capacity based on the surplus daytime energy. Of course, how much surplus there is during the day is important. However, for batteries intended for self-consumption, it is even more important to determine "what and how much you want to cover at night." This is because the value of a battery is determined by how you shift the surplus daytime electricity to meet nighttime demand.

For example, if a facility uses an average of 8 kWh at night, it needs a usable capacity that can at least cover roughly that amount. On the other hand, for a facility with 20 kWh of nighttime usage, a battery of around 8 kWh would not be enough and would only provide partial supplementation. In other words, battery capacity is easier to determine by working backward from the size of nighttime demand rather than just from how much surplus there is.

What’s important here is that nighttime demand also fluctuates. It isn’t constant from day to day; it can change by day of the week and by season. In households, loads tend to be higher from the evening into the night, while in businesses standby power after closing and the operation of specific equipment may be the only remaining loads. In other words, by understanding not only the total nighttime consumption but also how much is used at which times, the required capacity becomes much clearer.

Also, even if there is sufficient daytime surplus, making the battery too large becomes less meaningful if nighttime demand is low. Conversely, even if nighttime demand is high, installing a large-capacity battery may not reach full charge if daytime surplus is small. In other words, for practical purposes it is best to determine the required capacity by considering both daytime surplus and nighttime demand together. Deciding based on only one of them tends to result in an oversized or undersized system.

Using this approach makes discussions about battery storage capacity much more realistic. It shows which nighttime loads, and to what extent, can be covered by a battery of a given kWh capacity. It is an important step in connecting analyses that start from solar generation amounts to actual nighttime usage.

Approach 5: Consider effective capacity including charge/discharge losses and state-of-charge control

The fifth consideration is to think in terms of effective capacity (usable capacity), including charge/discharge losses and state-of-charge control. When evaluating a battery, people sometimes assume that the rated capacity is the amount that can be used as-is. However, in reality batteries incur certain losses during charging and discharging, and they often include state-of-charge controls for longevity and safety. In other words, nominal capacity and the actually usable capacity are not the same.

For example, even if you have a 10 kWh battery, you cannot necessarily use that 10 kWh at 100 % every day. There are losses during charging and discharging, and operational controls may also keep some portion in reserve. As a result, the amount of energy that can actually be allocated to nighttime demand is less than the nominal capacity. In other words, when considering battery capacity, you need to think in terms of usable (effective) capacity rather than the displayed size.

If you overlook this point, you are likely to underestimate the required capacity. If you think that because you want to use 8 kWh at night an 8 kWh battery will be sufficient, it may actually be insufficient due to losses and state-of-charge control. Conversely, even if you assume that a daytime surplus of 10 kWh can be fully absorbed by a 10 kWh battery, charging losses and operating conditions may prevent it from fitting. In other words, battery capacity should be converted not only from theoretical charge/discharge quantities but into effective values.

Also, by incorporating the concept of effective capacity, the connection with solar power generation becomes more realistic. For example, even if there is a daytime surplus of 15 kWh, that does not directly translate into 15 kWh of nighttime use. Some of it is lost through the battery, and there are also operating practices that do not use the entire amount. If you calculate without knowing this, you may overestimate the self-consumption rate and the effect of reducing electricity bills.

In practice, if you base your considerations on the amount of energy actually usable in everyday operation rather than the battery’s nominal capacity, subsequent explanations become much more reliable. When linking calculations of solar PV generation to battery capacity, incorporating the perspective of these losses and state-of-charge control makes the numbers much more representative of real-world conditions.

Approach 6: Judge surpluses and shortages by observing seasonal differences

The sixth approach is to judge surpluses and shortfalls by looking at seasonal variations. When considering the relationship between solar power generation and battery capacity, deciding capacity based only on the annual average tends to result in overcapacity or undercapacity depending on the season. This is because solar power generation differs considerably between spring, summer, autumn, and winter, and as a result the amount of surplus electricity also changes.

For example, in spring and autumn power generation is relatively stable and surpluses are more likely to occur. On the other hand, in summer power generation is high, but if cooling demand is high self-consumption also increases. In winter power generation falls, so there may be more days when the battery cannot be sufficiently charged. In other words, a capacity that is just right in one season may be too large or too small in another.

Therefore, when determining battery storage capacity, it is advisable to examine the relationship between generation and demand for at least the four seasons: spring, summer, autumn, and winter. In spring there is a large surplus and charging is easy, but nighttime demand may not be very large. In winter, nighttime demand is high, but daytime generation may be low and the battery may not reach full charge. In summer, generation is high, but self-consumption also increases, so the surplus may not be as large as expected. In this way, by looking at seasonal surpluses and shortages, the suitability of the battery capacity becomes much clearer.

Also, this approach can be used to compare equipment sizes. It helps decide whether it's better to make the solar installation slightly larger, to make the battery storage slightly smaller, or, conversely, to secure capacity with winter prioritized. What was ambiguous when considered as an annual total becomes much clearer by looking at seasonal variations.

In system planning for self-consumption, it is important to look not only at averages but also at surpluses and shortages. This is because battery capacity is not used in the same way all the time. Precisely for that reason, in practice it is important to consider battery capacity with an awareness of seasonal variations.

Sequence of Checks Operations Staff Should Follow When Proposing Equipment

Given the six ideas covered so far, the sequence of checks that a practitioner should follow when proposing equipment becomes clear. The first thing to do is grasp the outline of annual power generation from the equipment capacity and local conditions. After that, break it down into monthly generation to see which seasons are likely to produce surpluses. Next, look at the overlap with daytime demand to estimate the rough magnitude of self-consumption. By this point, the relationship among generation, self-consumption, and surplus will be largely clarified.

Next, overlay the relationship between daytime surplus and nighttime demand and consider how much of that you want to shift with the battery. At this stage, thinking in terms of the battery’s usable capacity rather than its nominal capacity will help keep later explanations consistent. Also, by looking at differences between summer and winter, it becomes easier to judge whether the system is oversized or will be too short in winter. In other words, when connecting solar PV generation calculations to battery capacity, it’s clearer to organize them in the following order: generation, self-consumption, surplus, nighttime demand, usable capacity, and seasonal variation.

Also, it is important not only to keep the numbers but also to document the assumptions. What is the equipment capacity in kW, which coefficient is used for annual generation, how were monthly generation figures assessed, how was daytime demand set, how was nighttime demand set, and how was effective capacity considered? If these assumptions are organized, you will not be confused when changing the equipment capacity or battery capacity later. Conversely, if only numbers remain, it becomes difficult to explain why that capacity was chosen.

Furthermore, the accuracy of collecting site conditions is also crucial. If the roof orientation, obstacle positions, elevation differences, or surrounding environment are unclear, projections of surplus electricity and of time-of-day characteristics will be rough. In particular, which times of day receive shading directly affects both the self-consumption rate and the scope for battery charging. For that reason, equipment proposals often place greater emphasis on the quality of site-condition data than on the calculation formulas.

Summary

When considering solar power generation and battery capacity, it's important not to look only at the total generation but to organize generation into three timeframes—annual, monthly, and daily—identify the daytime periods when you can self‑consume and the amount of surplus power, think about the battery in terms of kW as well as kWh, work backwards from the amount of energy you want to use at night, view the battery as effective capacity including charge/discharge losses and state‑of‑charge control, and finally judge seasonal surpluses and deficits. Simply following this sequence makes solar and battery planning far more practical.

What is especially important is not to consider battery storage capacity on its own. Only by looking at how much surplus there is in solar power generation, when that surplus occurs, and how much you want to use at night will the appropriate capacity become apparent. In other words, it is more reasonable to consider the calculation of solar power generation and the assessment of battery storage capacity as a single, integrated process.

Also, if you really want to increase this level of accuracy, it is essential to accurately grasp the site conditions. If the roof orientation, positions of surrounding obstructions, elevation differences, and how shadows fall are ambiguous, predictions of generation time windows and forecasts of surplus electricity will be prone to variation. In particular, for projects that consider self-consumption and battery storage, the timing of shading matters greatly.

In that respect, LRTK, an iPhone-mounted GNSS high-precision positioning device, is extremely effective as a means of accurately grasping on-site positional relationships. Because it makes it easier to accurately record candidate equipment locations and the positions of surrounding obstacles on site, it helps facilitate consideration of solar power generation and battery storage capacity that takes into account surplus energy and shading conditions. If you want to organize calculations of solar power generation and battery capacity into truly usable figures, properly capturing site conditions with a method like LRTK is a major advantage.