Solar EV Charging Stations: How Do You Match Solar Power with Charging Demand

To match solar with EV charging, start with three numbers: daily charging demand in kWh, charging power in kW, and how much solar energy is available while the vehicles are actually charging. If 20 EVs each need 18 kWh, total demand is 360 kWh/day. A 100 kW solar array might generate 370 kWh that day, but if only 280 kWh is produced while the EVs are charging, direct solar coverage is about 78%. The rest has to come from the grid, a battery, or charging shifted to another time.

Putting 100 kW of solar beside 100 kW of EV chargers does not mean the two systems are automatically matched. Solar output moves up and down through the day. Cars also arrive, leave, and charge at different times. You have to look at energy, power, and timing together.

Calculate Daily EV Energy Demand First

If the charging site already operates, real charging records are the best starting point. Check energy delivered per session, plug-in time, actual charging time, and the highest simultaneous load.

For a new fleet, daily mileage gives you a useful first estimate:

Daily battery energy = number of vehicles × daily distance × energy consumption

Take 30 service vehicles that each drive 50 miles per day. If you use 0.30 kWh/mile as the planning assumption:

30 × 50 × 0.30 = 450 kWh/day

The site will need to supply more than 450 kWh because charging is not perfectly efficient. If the early design assumes 90% overall charging efficiency:

450 ÷ 0.90 = 500 kWh/day from the site

That 90% is only a planning assumption. It is not a fixed number for every EV and charger. Once the equipment is known, replace it with measured or manufacturer-supported efficiency data.

For solar sizing, that 500 kWh/day figure tells you far more than simply knowing that 30 EVs use the parking lot.

Match kWh and kW Separately

kWh tells you how much energy the vehicles need. kW tells you how fast you need to deliver it.

A 10 kW charger running at full power for three hours could theoretically deliver:

10 kW × 3 hours = 30 kWh

Solar works the same way. A 100 kW PV array does not produce 100 kW all day. Output rises and falls with sunlight, temperature, shade, module direction, and weather.

The time vehicles stay connected also changes the amount of charging power you need.

If the fleet needs 500 kWh from the site, shortening the charging window quickly increases the required average power:

This is why a workplace where cars sit for most of the day can often manage with much less simultaneous charging power than a fleet with only a few hours between routes.

The U.S. Department of Energy's Alternative Fuels Data Center lists Level 2 charging equipment at roughly 2.9–19.2 kW. The vehicle itself may also accept less power than the charger's maximum rating.^[1]

Estimate Solar Production for the Actual Site

For an early feasibility check, you can estimate daily PV production with:

Daily PV energy ≈ array capacity × equivalent solar hours × system performance factor

For example, assume a 100 kW array, 4.5 equivalent solar hours, and an 82% combined performance factor:

100 × 4.5 × 0.82 ≈ 369 kWh/day

Do not treat 4.5 hours or 82% as universal project values. Location, tilt, orientation, temperature, shading, inverter design, and system losses can move the result quite a lot.

The National Renewable Energy Laboratory (NREL) provides the PVWatts calculator for location-specific grid-connected PV estimates. PVWatts can model system size, tilt, azimuth, and losses and can provide hourly results. For EV charging, those hourly results are much more useful than one annual production number.^[2]

For commercial roofs, warehouses, and parking canopies, usable area also affects which module format makes sense. Tongwei's industrial and commercial PV application scenarios show how different module formats are used in commercial projects.

Calculate How Much Solar Actually Overlaps With Charging

This is where a system that looks balanced on paper can turn out to be poorly matched in real operation.

Imagine two sites that both use 400 kWh per day for EV charging:

The daily energy demand is identical. The difference is timing. Office vehicles are parked during the strongest solar hours, while delivery vans return after most PV production is over.

A useful metric is:

Direct solar coverage = PV energy used by EVs during charging hours ÷ total EV charging energy

If the vehicles use 400 kWh and 310 kWh comes directly from PV while the system is generating:

310 ÷ 400 × 100 = 77.5%

The remaining 90 kWh has to come from the grid or from energy stored earlier.

Research by the National Renewable Energy Laboratory (NREL) has examined solar-synchronized charging, where flexible EV demand is moved toward stronger solar-production periods instead of letting every vehicle start charging as soon as it plugs in.^[3]

A Workplace Example: 138 kW of Chargers Does Not Mean You Need 138 kW Continuously

Consider a workplace with 24 EVs. Each one needs an average of 14 kWh during the workday:

24 × 14 = 336 kWh

Using the same 90% planning efficiency:

336 ÷ 0.90 ≈ 373 kWh/day

If the vehicles remain connected for eight hours:

373 ÷ 8 ≈ 46.6 kW average charging power

The parking lot could still have twelve 11.5 kW chargers:

12 × 11.5 = 138 kW installed charger capacity

But if managed charging caps the site at 70 kW, the theoretical energy available over eight hours is:

70 × 8 = 560 kWh

That is comfortably above the 373 kWh actually required.

Now add the earlier 100 kW PV example, producing about 369 kWh on the modeled day. The daily totals look almost identical:

373 kWh EV demand vs. 369 kWh PV generation

But once you look hour by hour, perhaps only 290 kWh of PV overlaps with usable EV demand:

290 ÷ 373 × 100 ≈ 77.7% direct solar coverage

The remaining gap is:

373 − 290 = 83 kWh

That 83 kWh timing gap matters more than the small difference between 373 kWh of charging demand and 369 kWh of total solar production. Adding more modules may not solve the problem if the extra power arrives when the vehicles do not need it.

Overnight Fleets Need Energy Shifting, Not Just More Solar

Now consider 40 delivery vans returning to a depot at 7:00 p.m. Each one needs 45 kWh of battery energy:

40 × 45 = 1,800 kWh

At the same illustrative 90% charging efficiency:

1,800 ÷ 0.90 = 2,000 kWh from the site

If all vans must be ready by 5:00 a.m., the ten-hour charging window requires an average of:

2,000 ÷ 10 = 200 kW

A 550 kW PV array using the same simplified 4.5-hour and 82% assumptions would generate roughly:

550 × 4.5 × 0.82 ≈ 2,030 kWh/day

So the PV system produces about the same daily energy that the fleet consumes. The problem is that the vans arrive after sunset. Direct solar charging would still be low.

The site could export or use the daytime solar elsewhere and import electricity overnight. Another option is to store some of the midday production.

If the battery must deliver 1,200 kWh to the overnight load, its nominal capacity needs to be higher than 1,200 kWh once usable state-of-charge range, round-trip losses, reserve requirements, and degradation are included.

Fast-Charging Stations Must Handle Short Power Peaks

A public site with four 150 kW DC fast chargers has a maximum connected charging capacity of:

4 × 150 = 600 kW

Suppose the station handles 25 sessions each day, averaging 35 kWh per session:

25 × 35 = 875 kWh/day

That daily total does not tell the whole story. If several EVs arrive together, demand can jump into the hundreds of kilowatts within minutes.

A PV system that generates 875 kWh over the day cannot automatically cover a 600 kW charging peak at a particular moment.

REopt analysis from the National Renewable Energy Laboratory (NREL) examined more than 10,000 combinations of utility rates and solar resources for DC fast charging. The analysis shows why PV and storage do different jobs: PV can reduce purchased energy, while batteries can help with short, high-power events and demand-charge management.^[4]

Use Managed Charging Before Automatically Adding More Battery Capacity

An EV that arrives at 8:00 a.m., leaves at 5:00 p.m., and needs 24 kWh does not have to start charging immediately.

At an average charging rate of 6 kW:

24 ÷ 6 = 4 hours of charging

The vehicle is plugged in for nine hours, so there are roughly five hours of scheduling flexibility.

Instead of doing most of the charging early in the morning, the control system can move more of it into late morning and early afternoon, when solar production is stronger.

Across 30 vehicles, shifting 10 kWh per vehicle creates:

30 × 10 = 300 kWh of flexible charging load

DOE describes managed charging as adaptive control of EV charging based on vehicle energy needs and other system goals. Potential benefits include reducing electrical-equipment upgrades and making better use of on-site generation.^[5]

Departure time still comes first. If a vehicle must leave at 1:00 p.m., you cannot delay required charging just to make the site's solar-use percentage look better.

Size Battery kW and kWh for Different Jobs

Battery kW tells you how much power the battery can deliver at once. Battery kWh tells you how long it can keep doing it.

Suppose a battery needs to cut an EV charging peak by 150 kW for one hour.

Before reserve and efficiency adjustments:

150 kW × 1 hour = 150 kWh

A 500 kWh battery limited to 50 kW has plenty of stored energy, but it cannot knock 150 kW off the peak.

A 200 kW / 100 kWh battery has enough power, but not enough usable energy to hold 150 kW for a full hour.

DOE makes the same distinction between storage power capacity and energy capacity. It also notes that storage can move solar electricity from the time it is generated to a later period when the load needs it.^[6]

Check Building and Utility Capacity Together

EV chargers usually share the site's electrical system with the building.

Suppose the workplace has 500 kW of usable site capacity and the building already reaches a 380 kW peak.

If all twelve 11.5 kW chargers run at full output:

380 + 138 = 518 kW

That is 18 kW above the assumed site limit.

Using the earlier 70 kW managed-charging ceiling:

380 + 70 = 450 kW

Now there is 50 kW of headroom.

This simple calculation does not prove that the existing electrical service is adequate. Transformer capacity, cables, switchgear, protection, formal load calculations, and local utility requirements still need engineering review. PV production also should not simply be subtracted from required service capacity unless the controls, design, and utility rules allow it.

DOE's EVI-LOCATE planning guidance considers nearby transformer capacity, existing peak load, service-panel capacity, and charger power when evaluating charging infrastructure.^[7]

Larger charging projects may also need transformer, service, feeder, metering, or interconnection upgrades. DOE charging-site guidance recommends involving the utility early because the required work can vary greatly from one site to another.^[8]

Convert the Solar Target Into a Practical Module Layout

Once you have a target PV capacity, turn it into a rough module count.

For a 120 kW DC system using 655 W modules:

120,000 W ÷ 655 W ≈ 183.2

So the early layout needs about 184 modules before final string sizing and electrical design.

Tongwei's TWMNH-66HD module page lists power output up to 655 W. But the module count is only the beginning. Roof or carport geometry, module dimensions, string voltage, current, inverter limits, structural loading, and maintenance access all affect the final layout.

Use the latest electrical values from Tongwei's product specifications. On larger commercial projects, also check physical module format against inverter and construction limits instead of choosing purely by maximum wattage.

Check Seasonal Production and Future Fleet Growth

A design that looks fine on a strong summer day can behave very differently in winter.

Suppose EV charging demand stays at 400 kWh/day and the modeled PV system generates 440 kWh on one representative day:

440 ÷ 400 × 100 = 110% of the EVs' daily energy requirement

That does not mean direct solar coverage is 110%. The EVs can use no more than their 400 kWh demand, and even that only works if charging happens when the solar energy is available. Extra generation may feed other building loads, charge a battery, go to the grid, or be curtailed.

If winter production falls to 250 kWh:

250 ÷ 400 × 100 = 62.5% of daily EV energy demand

For working fleets, test normal days as well as low-solar periods, high-mileage days, and several cloudy days in a row.

Fleet growth matters too. If 30 EVs each need 16 kWh/day:

30 × 16 = 480 kWh/day

If the fleet later grows to 45 vehicles:

45 × 16 = 720 kWh/day

That is a:

(720 − 480) ÷ 480 × 100 = 50%

increase in charging demand.

You do not necessarily need to install all future PV capacity on day one. Still, leaving room for switchgear, conduit, inverters, roof zones, and future parking-canopy structure is much easier during the first build than after the site is finished.

Include the Utility Tariff in the Design

Solar charging economics depend on when electricity is imported, not only on the number of imported kWh.

Commercial tariffs may include energy charges, time-of-use pricing, monthly demand charges, seasonal rates, fixed charges, export compensation, and dedicated EV rates.

For a simple example, suppose the utility charges $18/kW per month for demand. If managed charging and storage reduce the billing peak by 200 kW:

200 × $18 = $3,600/month

The $18/kW figure is only an example. The real calculation has to use the tariff that applies to the project.

This is why two charging stations with similar equipment can have very different battery economics. Their demand charges, time-of-use periods, or solar-export rules may be completely different.

Model the Whole Site on the Same Timeline

For early analysis, put building load, EV demand, and PV production on the same hourly timeline. If short peaks or demand charges matter, 15-minute data is even more useful.

These example values show why annual totals can hide the real problem. At noon, 95 kW of solar cuts grid imports significantly. By 6:00 p.m., only 5 kW of PV remains, so almost all of the building and EV load comes from the grid.

The same model can also track maximum grid demand, direct solar-to-EV energy, exports, battery use, and whether any vehicle misses its required departure charge.

Compare the Model With Real Charging Data After Commissioning

Real driving and charging behavior can drift quite far from the assumptions used during design.

Suppose the project expected 15 kWh per charging session but measured 22 kWh during the first six months:

(22 − 15) ÷ 15 × 100 ≈ 46.7% higher than planned

That is big enough to justify recalculating daily charging energy, solar overlap, charging schedules, and future grid capacity.

Useful operating metrics include:

• daily and monthly EV charging energy;
• average kWh per session;
• peak EV charging demand;
• whole-site peak demand;
• PV generation;
• direct solar consumption;
• grid imports;
• battery throughput where storage is installed;
• failed charging sessions;
• vehicles leaving below their required charge.

If the design expected 75% direct solar coverage but the site is achieving only 52%, check the charging schedule before buying more equipment. Vehicles may be arriving later, chargers may be starting immediately instead of following the PV curve, or the building may be using more midday solar than expected.

Keep Electrical Safety and Local Approval in the Final Design

Final cable sizing, transformer loading, switchgear, overcurrent protection, PV interconnection, battery installation, and charger installation must follow the rules that apply at the project location.

In the United States, the Alternative Fuels Data Center notes that EV charging requirements are based on national or international building and electrical standards but are adopted through state and local processes. Permitting, accessibility, and electrical requirements can therefore vary by jurisdiction.^[9]

The calculations in this article are useful for feasibility and energy planning. They do not replace site-specific electrical design, utility approval, or professional engineering for construction.

Conclusion

Match solar and EV charging with three numbers: daily charging energy, required charging power, and PV output during the same charging hours. If EVs need 400 kWh/day but only 300 kWh of solar overlaps with charging, direct solar coverage is 75%. Shift flexible charging into solar hours first, then size grid capacity or storage for the remaining 100 kWh and any short power peaks.