5 Steps to Calculate the Amount of Solar Power Used for EV Charging

Table of Contents

• Approach to Calculating Solar Power Generation for EV Charging

• Step 1: Organize the required electrical energy from the EV's driving distance

• Step 2: Decide the time windows available for charging and the usage patterns

• Step 3 Estimate solar power generation on a daily and monthly basis

• Step 4: Add margin to account for conversion losses and charging losses

• Step 5 Determine by combining self-consumption, energy storage, and grid utilization

• Commonly overlooked checks in power generation calculations for EV charging

• How to use calculation results to drive operational improvements

• Summary

Approach to Calculating Solar Power Generation for EV Charging

When using solar power for EV charging, the first thing to keep in mind is not to judge solely by the capacity of the solar power system. Even if the system has a certain output, the actual amount of electricity you can obtain varies with season, weather, installation angle, orientation, shading, temperature, and system losses. Also, the EV does not necessarily use the same amount of energy every day; required charging varies depending on commuting, business travel, weekend use, and whether long-distance trips are made. Therefore, when calculating the amount of solar power to use for EV charging, it is important to estimate the generation side and the consumption side separately, and finally account for time-of-day and losses.

The basic approach to calculating solar power generation is to determine the amount of electricity produced over a given period by multiplying the rated capacity of the generation equipment by coefficients related to solar irradiance and equipment losses. When used for homes or businesses, you need to look not only at annual generation but also at monthly, daily, and hourly breakdowns. Because EV charging can require a substantial amount of energy, even if the annual total appears sufficient, it can be inadequate during winter, prolonged rainy periods, or for charging in the evening.

In practice, you first determine the amount of energy an EV requires and then check how much of that can be supplied by solar power. For example, if daily driving distances are short, part of the demand may be covered by charging with daytime surplus generation. On the other hand, in operations where vehicles return at night or are out during the day for work, the times when generation occurs and charging is possible do not coincide, so it is necessary to plan on the assumption of using energy storage and grid power.

Also, calculations for covering EV charging with solar power cannot be based solely on whether generation exceeds charging; that is insufficient. Whether the generated electricity is used simultaneously within the building, prioritized for the EV, stored in battery storage, or the surplus used for other purposes changes the required equipment capacity and the control strategy. Clarifying whether the purpose of the calculation is to determine the size of the generation system, to assess the utilization of existing equipment, or to forecast electricity consumption after EV introduction makes it easier to use the calculation results for practical decision-making.

This article outlines a five-step process for calculating the amount of solar power available for EV charging. Without relying on specific devices or service names, and to enable practitioners to move easily from rough estimates to detailed evaluation, it verifies, in order, the required energy, charging time windows, solar generation, losses, and operating conditions.

Step 1 Organize the required electric energy from the EV's driving distance

The first step is to clarify how much electricity an EV actually uses. If you begin by calculating solar power generation, it's easy to focus only on the capacity of the generation equipment, but when viewing this from the perspective of EV charging you should first grasp the required energy on the demand side — namely the EV. Estimating generation without knowing the required energy makes it difficult to judge whether the equipment is adequate or oversized, and how much can be covered by daytime charging.

The required energy for an EV is basically determined by driving distance and energy consumption. Energy consumption refers to how far you can travel on a given amount of electricity, or how much electricity is used to travel a certain distance. In practice, it is important to account for actual operating conditions as well as the figures in the vehicle catalog. If you frequently drive at high speeds, carry heavy loads, use heating or air conditioning a lot, encounter many hills, or repeatedly make short trips with frequent stops, energy consumption can be higher than expected.

As the first step in the calculation, set the average daily driving distance. For commercial vehicles, using daily reports, driving logs, delivery routes, and number of visits, and separating weekdays and holidays as well as peak and normal seasons, will make the estimate closer to reality. Even for personal use, organize not only commuting distance but also shopping, drop-offs and pick-ups, and weekend or holiday travel. Assuming the same distance every day can cause you to overlook days with long-distance travel and resulting charging shortages, so it is important to check not only the average but also the conditions for days when you drive somewhat more.

Next, convert driving distance into electrical energy. For example, by assuming the amount of electrical energy required to drive 1 km (0.6 mi) and multiplying it by the daily driving distance, you can estimate the energy required for that day's driving. Conversely, if you use the distance that can be driven per a given amount of energy, divide the driving distance by that value to obtain the required energy. What is being calculated here is the amount of electrical energy consumed from the battery for the vehicle to run, which does not exactly match the energy supplied by the charger. Because there are conversion losses during charging, those losses will be accounted for in a later step.

When handling multiple EVs for business use, calculate the required energy for each vehicle separately. Treating all vehicles uniformly can obscure the needs of vehicles with longer driving distances or later return times. Vehicles that return during the day, those that return in the evening or later, and those that are out all day differ in the proportion of solar generation that can be used directly for charging. Therefore, in addition to the number of vehicles, organizing each vehicle’s driving distance, return time, and next-day departure time will make subsequent charging plans more realistic.

At this stage you do not need to calculate the required energy with excessive precision. However, at a minimum you should separately identify the required energy for a typical day, for a day with heavier-than-normal driving, and for a day when multiple vehicles are being charged simultaneously. Because calculations of solar power generation are influenced by natural conditions, organizing the demand side with some allowance will help you identify conditions likely to cause power shortages or charging waits earlier.

Step 2 Decide the time periods available for charging and the usage patterns

The next thing to check is when the EV can be charged. Because solar power generation mainly occurs during the daytime, if the EV is not parked during those hours, the amount of generated electricity that can be used directly for charging is limited. If you overlook this, annual generation may appear sufficient, but in reality the amount available for EV charging is likely to be small.

In calculations for using solar power generation for EV charging, it is important to look at the overlap between generation and the hours when charging is possible. For example, in an operation where company vehicles are out from morning until evening and return in the evening to charge overnight, the peak hours of solar generation and the charging hours are misaligned. In that case, daytime generation may be used for building loads, and a higher proportion of EV charging may rely on grid power. On the other hand, facilities with many vehicles parked during the day or operations where vehicles return after short outings make it easier to use solar generation directly for charging.

When organizing charging time windows, check the vehicle’s return time, the next departure time, and the required charging completion time. Rather than roughly dividing times as “charge at night” or “charge during the day,” consider precisely from when to when vehicles can be connected; this will reveal the required charger output and the number of chargers needed. For example, if vehicles can remain connected for long periods, it may be possible to meet the required amount by charging relatively slowly. If charging must be completed in a short time, higher charging output will be necessary, which tends to concentrate power demand during that time window.

Solar PV generation generally increases gradually from the morning, becomes largest around noon, and decreases in the evening. It varies with the season, weather, and installation conditions, but basically the output is concentrated in a few hours during the daytime. Therefore, whether an EV is connected during the day greatly affects the proportion of solar generation that can be used directly. When calculating generation, it is desirable to check not only the total daily generation but also how much is generated during the periods when charging is possible.

Also, check the building's electricity usage patterns. In offices and stores, daytime electricity consumption can increase due to air conditioning, lighting, equipment, and facility operations. Electricity generated by solar PV is used not only for EV charging but also for building loads. If EV charging overlaps with periods of high building load, solar generation alone may be insufficient and purchases from the grid may increase. Conversely, if EVs can be charged during periods of low building load, it becomes easier to utilize surplus generation.

You should also decide the charging priorities. Whether you prioritize EV charging, on-site building self-consumption, or—if a storage battery is available—charging the storage battery will change the required controls and how to interpret calculation results. Over-prioritizing EV charging may increase the building’s purchased electricity, while over-prioritizing the building load may delay completion of EV charging. If the purpose of the calculations is operational decision-making, it is important to determine in advance which load to prioritize.

Step 3 Estimate solar power generation on a daily and monthly basis

Once you have organized the EV's required energy and charging time windows, the next step is to estimate the solar power generation. The important thing here is not to judge based only on annual generation. Because EV charging is tied to daily operations, you need to check month-by-month and day-by-day variations in generation; otherwise it becomes difficult to grasp when actual charging shortages or surpluses will occur.

In estimating photovoltaic power generation, one takes into account the rated capacity of the generation equipment, the solar irradiance conditions of the installation area, the panels’ orientation and tilt, temperature effects, losses associated with power conversion, wiring losses, and degradation due to soiling and aging. The general approach is to multiply the system capacity by the solar irradiance over a given period and by loss coefficients to determine the expected electricity generation. In practice, it is realistic to first check the approximate magnitude relative to demand using an estimate before conducting detailed simulations, and then improve accuracy by adding detailed conditions.

The reason for looking at it by month is that there are large differences in power generation depending on the season. In summer, sunlight hours are longer and power generation tends to increase, but high temperatures can reduce generation efficiency. In winter, temperature conditions can be advantageous in some respects, but because sunlight hours are shorter and the sun’s altitude is lower, generation can be reduced depending on installation conditions. During the rainy season or extended periods of rain, generation can drop significantly compared with clear days. When planning to cover EV charging with solar power, it is necessary to check not only the annual average but also how much of the required energy can be supplied in months with low generation.

The reason for examining things on a daily basis is that both EV usage and power generation vary day to day. Even if weekdays are mainly for short-distance use, there can be particular days with long-distance trips. On the generation side, output differs between sunny, cloudy, and rainy days. Calculating using only averages can overlook the risk of failing to meet required charging on low-generation days. In particular, when vehicles used for business need a certain amount of charge by the next morning, it is important to base calculations on the minimum required charge and to include contingencies for adverse weather.

When viewing solar generation as the amount available for EV charging, you need to consider subtracting the portion used by the building load from the total generation. For example, even if panels are producing during the daytime, if the building’s electricity use is high at that time, the surplus available for EV charging will be small. Conversely, when the building load is low—such as on holidays or during non‑operational hours—and an EV is connected, it is easier to allocate generation to charging. In this way, solar generation itself and the amount of generation available for EV charging should be treated as separate quantities.

If you have an existing solar power system, checking past generation records increases the reliability of your calculations. If you know monthly generation results, daily generation results, and hourly generation trends, you can specifically identify overlaps with the time periods available for EV charging. However, even when using past performance, future generation may not be the same as before if shading increases, the system becomes soiled, nearby buildings change, or the equipment degrades. When using historical values, it is important to confirm whether the same conditions will continue going forward.

For new installations, we make a rough estimate based on the area's solar irradiation conditions and the planned installation conditions. For roof installations we check orientation and tilt; for ground-mounted systems we check surrounding shading and array spacing; for installations above parking lots we also check shadows from structures and impacts on vehicle traffic flow. For EV charging, it is important not only to maximize generation but also to consider whether generation will be available during the times when charging is desired. If vehicles return in the afternoon, for example, confirming how much generation remains in the afternoon and adding a time-of-day perspective increases practicality.

Step 4: Account for conversion and charging losses to provide a margin

When comparing solar power generation with the electrical energy required by an EV, losses are easy to overlook. The electricity generated does not go directly into the EV’s battery. Losses occur in processes such as converting the generated DC power to AC, supplying power to the vehicle via the charger, and charging the battery on the vehicle side. Therefore, the electrical energy supplied from the charging equipment should be estimated to be higher than the energy the EV uses for driving.

In the calculations, first determine the electrical energy required for EV driving, and then account for charging losses. For example, even if the energy required for driving is a fixed amount, the energy supplied by the charger will be greater than that. Furthermore, if conversions or control processes occur between solar power generation and EV charging, those losses should also be considered. When routed through a storage battery, losses during charging and discharging are added, so the required amount of generation may increase compared with using solar power directly for the EV.

What is important here is not to underestimate losses. In the preliminary estimation phase, detailed equipment specifications are often undecided, so it is safer to use a coefficient that allows some margin rather than assigning exact values. In practice, once the equipment configuration is finalized, you refine the details to reflect the specifications of the power conversion equipment, charging equipment, wiring, and energy storage equipment. By allowing a margin at an early stage, you can more easily avoid later problems such as insufficient power generation, inadequate charging time, or higher-than-expected purchased electricity.

Also, the relationship between charging power and charging time should be examined together with losses. Even if the required energy is the same, the load on equipment and the coincident power demand vary depending on whether charging is done in a short time or over a long time. Assuming short-duration charging increases the charging power, which may affect the building’s peak demand. If high-power charging is performed during periods when solar PV output is insufficient, reliance on grid power can increase.

Calculations of solar power generation for EV charging check not only for insufficient generated energy but also for insufficient power output. Energy (electricity) is the total over a given period, while power output indicates how much electricity can be supplied at a given moment. Even if a day's generated energy exceeds the required charging energy, if the output during the desired charging period is insufficient, charging may not be possible with solar power alone. In particular, when generation output fluctuates due to passing clouds or when multiple vehicles are charging simultaneously, operations must account for output variability.

In calculations that reflect losses, the basic approach is to estimate the required electric energy slightly higher and to view the power generation somewhat conservatively. If you assume optimistic conditions on the demand side and also optimistic ones on the generation side, shortages are likely to occur in actual operation. Conversely, if you err too far on the safe side, the system size may become excessively large. Therefore, it is practical to perform calculations for multiple cases — normal conditions, bad-weather conditions, high-usage conditions, etc. — and decide up to which conditions you want to rely on solar power generation.

Step 5 Determine by combining self-consumption, battery storage, and grid use

In the final step, you determine the operation based on solar power generation, the EV's required energy, charging time windows, and losses. While the idea of covering all EV charging solely with solar power is easy to understand, in practice it is not simple to match them exactly because of weather and vehicle usage variability. In reality, a practical approach is to maximize self-consumption of solar power, make up any shortfall from the grid, and combine battery storage as needed.

If you prioritize self-consumption, it is important whether the EV can be connected during the periods when generation is occurring. If the vehicle is parked for long periods during the day, it becomes easier to use the generated electricity directly for charging. Charging the EV during periods of low building load also helps make use of surplus generation. Conversely, if the vehicle is absent during the day and returns at night, the share of solar generation used directly will decrease. In that case, consider adding energy storage or planning on using the grid at night.

Combining battery storage makes it easier to shift daytime generation to EV charging in the evening and later. However, because losses increase when energy passes through a battery, simply matching generation and charging volumes is not sufficient. The required capacity and control strategy also change depending on whether the storage is used solely for EV charging or also for building load leveling and emergency use. Even when considering storage for the purpose of EV charging, it is important to design it in conjunction with the building's overall power operation.

When combining grid use, it is important to understand the share that can be covered by solar power generation. Rather than asking whether everything can be covered by solar, check how much energy can be supplied by solar over the year, how much purchased electricity can be reduced by daytime charging, and how large the shortfall will be in winter or during adverse weather. When operational staff prepare explanatory materials, organizing figures not as a simple comparison of generation and charging but into direct solar use, storage-mediated use, and grid-supplied portions makes it easier for stakeholders to understand.

When operating multiple EVs, control that avoids simultaneous charging is also effective. If all vehicles begin charging immediately upon returning, the temporary power demand becomes large. By changing the charging order according to departure times and required charging amounts, it becomes easier to take advantage of the remaining solar generation time and periods when building load is low. The calculated solar generation amounts can be used not only to determine equipment capacity but also as input for deciding charging schedules.

What we want to verify with this procedure is whether the calculation results translate into operational decision-making. Rather than a simple conclusion that generation is sufficient or insufficient, we clarify how much can normally be covered by solar, how much the grid would need to be relied on on days with high driving distances, and what contingency measures exist if rainy weather continues. Because EV charging is directly tied to daily work and life, operations must consider not only computational optimization but also missed charging, sudden trips, vehicle swaps, and the operational burden on users.

Commonly Overlooked Points in Power Generation Calculations for EV Charging

When calculating the amount of solar power generation to be used for EV charging, a common oversight is judging solely by the annual generation. Even if the annual total exceeds the EV's required energy, in practice the vehicle may be absent during the daytime and unable to charge, generation may be insufficient in winter, and prolonged rainy weather may increase grid usage. Because EV charging is heavily constrained by time of day, checking monthly, daily, and hourly breakdowns is more important than looking only at the annual total.

Another point often overlooked is the interaction with building load. At facilities with solar power generation, the electricity produced may be consumed by the building first. If daytime air conditioning and equipment operation are substantial, the surplus available for EV charging may be less than expected. Conversely, on holidays or during low-load periods the surplus can increase. When introducing EV charging, it is important to check, together with existing power usage data, how much spare capacity is available at each time of day.

Shading effects are also important. The way shadows appear varies depending on the installation location—on roofs, over parking lots, or ground-mounted within the site. Surrounding buildings, trees, utility poles, signs, equipment, and seasonal changes in the sun’s altitude can cause actual generation to be lower than expected. For EV charging in particular, daytime generation often coincides with charging windows, so even short periods of shading can affect operations. Even at the rough estimation stage, checking the times when shading is likely can help avoid unrealistic expectations.

Output settings of charging equipment are another easy-to-overlook point. To meet the amount of energy an EV requires, both the available charging time and the charging power must be satisfied. If the vehicle can be connected for a long time, a lower power may be sufficient, but if it needs to be charged in a short time, higher power is required. Performing high-power charging while solar power generation output is fluctuating can increase the need for support from the grid. Along with calculations of generated power, it is necessary to align the charging schedule with real-world operations.

Also, you should consider the potential future increase in the number of EVs. Even if you start with just one vehicle, if there is a possibility that the number will increase to multiple vehicles in a few years, judging equipment based only on the electricity demand needed now can lead to shortages later. However, overestimating future needs too much can leave you with a large surplus in the early stages. Checking together whether you can expand in stages, whether there is space to add chargers, and whether the power contract and electrical service equipment have spare capacity will make it easier to establish a long-term plan.

How to Turn Calculation Results into Operational Improvements

Calculations of solar power generation should not be limited to pre-installation assessment; they can also be used to improve operations after installation. By comparing the power generation, EV energy consumption, and charging time periods assumed in the calculations with actual results, you can identify where the discrepancies lie. If generation is lower than expected, check for shading, soiling, equipment faults, weather conditions, or incorrect assumptions about installation conditions. If EV charging amounts are higher than expected, review driving distance, energy efficiency, use of heating and cooling, loading conditions, and driving patterns.

In operational improvements, adjusting charging times can often be effective. For example, simply charging some vehicles that had previously been charged all together in the evening during lunch breaks or immediately after return can increase the direct utilization rate of solar power generation. If there are multiple vehicles, prioritize those that will depart early the next day and schedule vehicles with more leeway during periods of higher generation; this can increase self-consumption while avoiding charging shortages. Rather than treating calculation results as fixed, it is important to adjust charging rules while monitoring actual performance.

It is also important to visualize generation and charging volumes. If you can see daily power generation, EV charging, building consumption, and electricity purchased from the grid, it becomes easier to explain how much solar generation is contributing to EV charging. Especially when operational staff give internal presentations, organizing not only the total generation but also the time periods when generation occurred, when charging occurred, and which portions experienced shortfalls makes it easier to identify the next improvement measures.

In power operations that include EV charging, handling exception days is also important. Even if normal days are fine, on days with multiple long trips, weeks of sustained bad weather, or days when facility power use increases, things may not go according to calculations. If you record such days as exceptions and check under which conditions shortages occurred, it can help improve charging rules and equipment settings. Conversely, if there are consecutive days with abundant surplus generation, you can consider moving charging times earlier or using the surplus for other purposes.

By periodically reviewing the differences between calculations and actual results, the combination of solar power generation and EV charging becomes more practical. Pre-installation estimates are only projections, and actual generation and driving conditions can vary. That is why it is important to record the assumptions used in the calculations and compare them with actual performance after installation. Keeping those assumptions makes it easier to explain why a particular capacity was chosen and why that charging time was selected, even if the person responsible for the equipment changes.

Summary

To calculate the amount of solar generation to be used for EV charging, it is important not just to look at the capacity of the generation equipment but to check, in sequence, the EV’s required energy, the time windows when charging is possible, the monthly and daily variability of solar generation, conversion losses, building loads, and the combinations of storage and grid use. In particular, judging solely by annual generation can easily overlook mismatches between actual charging periods and generation periods. For EV charging, it is important not only how much is generated but when it is generated and when charging can occur.

In practice, first calculate the EV’s required energy from the driving distance, then clarify the available charging time. Next, estimate solar photovoltaic generation by month, day, and time of day, account for losses and include a margin. Finally, decide how to combine self-consumption, storage, and grid use. Following this sequence lets you understand not only whether generation will be sufficient, but also under which conditions shortages are likely and which operating strategies make effective use of solar power.

Combining EV charging and solar power is not something that ends with just calculations. After implementation, by reviewing actual power generation, charging records, and driving records and by revising charging times and operational rules, you can tailor usage to better fit actual conditions. Especially at facilities that handle multiple EVs, it is important to set charging priorities based on each vehicle’s driving distance and return time. By using the calculated solar generation not only for equipment planning but also for daily operational improvements, it becomes easier to achieve both stable EV charging and self-consumption.

If you want to concretely understand the amount of solar generation available for EV charging, it is helpful to review site conditions, generation output, shading, charging times, and vehicle operations together. It is important to develop a realistic deployment plan by comparing expected generation with actual usage. Rather than relying solely on specific equipment or service names, organizing driving distances, charging time windows, building loads, weather variability, and losses, and continuously refining calculations and operations to match site conditions, form the basics of practically integrating solar power and EV charging.