EV Home-Charging Solar Offset Calculator
Find how much extra solar you need to cover your EV's home charging, from annual miles, vehicle efficiency, charging losses, sun hours, system losses, and your offset target.
Last updated: May 2026
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About this calculator
An electric car is just a large new electrical load, so sizing solar to cover it follows the same logic as whole-home sizing but starts from miles instead of a utility bill. First convert driving to energy at the battery: annual miles divided by 100, times the vehicle's consumption in kWh per 100 miles, gives the energy the car stores per year. But home charging is not perfectly efficient — AC-to-DC conversion, cabling, and battery thermal management lose roughly 8–15% — so you divide by (1 − charging losses) to find the energy that must actually come from the wall. Dividing by 365 turns that into an average daily draw, and the offset target lets you size for all of it or just a share. The remaining step is identical to any solar sizing: divide the daily energy you want to cover by the productive sun, which is peak sun hours reduced by the array's loss factor (1 − system losses), to get the additional DC kilowatts of panels needed. Every input pulls the result: more miles or a less efficient vehicle raise it, charging losses inflate it, and sunnier sites or fewer system losses shrink it. The output is the extra array size to bolt onto your existing system so your driving runs on sunshine rather than the grid.
How to use
Worked example. You drive 12,000 miles a year in an EV that uses 28 kWh per 100 miles, your home charging is 90% efficient (10% losses), your roof sees 4.5 peak sun hours, your array has 18% system losses, and you want to cover 100% of charging with solar. Step 1 — energy at the battery: 12,000 ÷ 100 = 120, times 28 = 3,360 kWh/year. Step 2 — energy from the wall after charging losses: 3,360 ÷ 0.90 = 3,733 kWh/year. Step 3 — daily draw: 3,733 ÷ 365 = 10.23 kWh/day, and at a 100% offset target that stays 10.23 kWh/day. Step 4 — convert to panels' worth of capacity: productive sun is 4.5 × (1 − 0.18) = 3.69 effective hours, so 10.23 ÷ 3.69 = 2.77 kW of extra solar — about seven 400 W panels. Drive 18,000 miles a year and that grows to roughly 4.2 kW; choose a thriftier 24 kWh/100 mi vehicle and it shrinks back below 2.4 kW.
Frequently asked questions
How many solar panels do I need to charge my electric car?
Size it from your driving, not a guess. Convert annual miles to energy (miles ÷ 100 × your car's kWh per 100 miles), divide by your charging efficiency, then by 365 for a daily figure, then by your peak sun hours after system losses to get the extra kilowatts of solar. A typical 12,000-mile-a-year EV needs roughly 2.5–3 kW of additional solar — about 6–8 standard 400 W panels. Drive more, drive a thirstier vehicle, or live somewhere cloudy and you'll need more; an efficient car in a sunny climate needs fewer.
Why do home EV charging losses matter when sizing solar?
Because the energy your panels must supply is the energy drawn from the wall, not the energy that ends up in the battery. Level 1 and Level 2 AC charging loses roughly 8–15% to the onboard charger's AC-to-DC conversion, wiring resistance, and battery thermal management. If your car needs 3,360 kWh at the battery but charging is 90% efficient, the wall actually pulls about 3,733 kWh — and your solar has to cover that larger number. Ignoring charging losses undersizes the array by 10% or more, which is why the calculator divides by (1 − losses).
Is it cheaper to charge an EV with solar than from the grid?
Almost always, once the panels are paid off. Grid charging costs your electricity rate per kWh; solar charging costs essentially nothing per kWh after the system's payback period, because the marginal sunshine is free. Adding a few panels to cover driving is often the highest-value expansion of a solar system because an EV is a large, predictable, year-round load. Sizing the extra capacity precisely — covering 100% of charging or whatever share you choose — lets you compare the upfront panel cost against years of avoided charging bills.