Energy
Estimate how much of your EV charging can be supplied by solar power.
Enter your numbers to see the answer.
Solar EV charging calculator guide
If you have rooftop solar, some of your driving can be powered from your own roof; this calculator estimates how much. Enter daily miles, efficiency in miles per kWh, the usable solar output your system produces on an average day, and the share of that output you can direct to the car. It works out the daily energy the EV needs, the solar kWh actually available to it, and reports coverage as a percentage capped at 100%, with the grid remainder alongside. A 5 kW array in a sunny US location yields roughly 20–25 kWh on an average day and 30 or more in midsummer, so 25 kWh is a fair default; a 4 kW UK system averages closer to 10 kWh across the year. The share matters because solar peaks at midday while most cars are away—if the car is home in the daytime you might allocate 60–80%, but a commuter that charges at night gets solar only indirectly, through a home battery or export credits. Use NREL’s PVWatts for a location-specific output figure.
Daily EV energy (kWh) = daily miles ÷ efficiency (mi/kWh). Solar allocated to EV (kWh) = usable solar output (kWh/day) × share % ÷ 100. Solar coverage (%) = min(100, solar allocated ÷ daily EV energy × 100). Grid remaining (kWh) = max(0, daily EV energy − solar allocated).
Worked example: 40 miles a day at 3.5 mi/kWh, 25 kWh of usable solar a day, and 60% of it allocated to the car. Daily EV energy = 40 ÷ 3.5 = 11.4 kWh. Solar for the EV = 25 × 60 ÷ 100 = 15 kWh. Coverage = 15 ÷ 11.43 × 100 = 131%, which the calculator caps at 100% covered, with 0 kWh remaining from the grid and about 3.6 kWh of allocated solar left over for the house or export. In December, when the same array might make 12 kWh, the sum becomes 12 × 0.6 = 7.2 kWh, coverage 7.2 ÷ 11.43 = 63%, and 4.2 kWh comes from the grid.
Daily miles has a kilometre option (1 mile = 1.609344 km). Efficiency is miles per kWh; convert kWh/100 km with 62.137 ÷ (kWh/100 km) and Wh/mi with 1,000 ÷ Wh/mi. Solar output is kilowatt-hours per day of usable generation, not the array’s kW rating; a rough annual-average conversion is array kW × 4 to 5 in sunny climates and × 2.5 in northern Europe. The share is a percentage of that daily output. Coverage is shown as a whole-number percentage.
This is a modelled estimate, not a meter reading. Real consumption moves with weather, occupancy, appliance age, standing charges, and tariff structure, and equipment rarely runs at its nameplate rating. Use the figure to compare options against each other, then confirm against your own bill and the manufacturer's specification before committing to a purchase. On this page the figure rests entirely on miles driven per day, ev efficiency, usable solar output, solar allocated to ev, market / jurisdiction, effective date, data source, update owner and review date, so start there if the solar ev charging calculator returns something you did not expect.
Good to know: coverage above 100% does not mean free charging every day—the tool caps the figure and the surplus is only useful if the car is actually plugged in while the sun is out. Annual average solar output hides a 3–4× swing between June and December in temperate climates, so run the numbers for your worst month too. Charging losses of 10–15% are not included, so the kWh the charger must pull is higher than the daily need shown. And a percentage of solar going to the car is only achievable with a solar-aware charger or careful scheduling; without one, a daytime charger pulls from the grid the moment a cloud passes.
Energy results depend on tariffs, equipment behavior, region, weather, and installation assumptions. Verify rates and electrical decisions with the utility, manufacturer, or qualified professional.
Sources
How it works
Estimate how much of your EV charging can be supplied by solar power. This tool explains the calculation so you can adjust the assumptions to match your situation.
Worked example
With Miles driven per day = 40 mi · EV efficiency = 3.5 mi / kWh · Usable solar output = 25 kWh / day · Solar allocated to EV = 60 % → 100 % covered (of daily driving energy supplied by solar). Change an input above and this example updates with your numbers.
Common questions
Driving 40 miles a day at 3.5 mi/kWh needs 11.4 kWh, or about 12.8 kWh after charging losses. In a sunny US location an array makes roughly 4–5 kWh per kW of capacity per day, so about 3 kW—eight to ten 350 W panels—dedicated to the car covers it on an average day. Northern Europe needs roughly double.
Not directly—the panels make nothing after dark. You can charge at night from a home battery that stored daytime solar, or export solar in the day and offset the cost with net-metering or export credits. This calculator treats the share you allocate to the car as usable regardless of timing, so lower it if the car is only home at night.
At 3.5 mi/kWh, 40 miles is 11.4 kWh. With 25 kWh of daily solar and 60% allocated, 15 kWh is available and coverage is 100%, leaving 3.6 kWh spare. With 12 kWh of winter output the same 60% share gives 7.2 kWh, 63% coverage, and 4.2 kWh from the grid.
Often, because an EV adds 3,000–4,500 kWh a year to household demand—40 miles a day is about 4,170 kWh—which raises the amount of self-generated electricity you can use instead of exporting at a low rate. The saving equals the kWh you self-consume times your retail rate; at $0.16 per kWh, 15 kWh a day is about $2.40, or $876 a year.
A wallbox with a current clamp on the incoming supply that varies charging power to match your export surplus, so the car absorbs solar that would otherwise go to the grid. Because charging cannot run below about 1.4 kW (6 A at 230 V), it needs a decent surplus to start, which is why the default share is 60% rather than 100%.