"How many solar panels do I need for my EV?" has a precise answer, and it starts with your odometer, not your roof. Here is the sizing process a good installer follows - so you can check their work.
Step 1: Get the EV's annual energy need
Annual miles / efficiency = kWh. At 3.5 miles/kWh (a typical modern crossover; efficient sedans hit 4+):
| Annual driving | EV energy need |
|---|---|
| 8,000 miles | ~2,300 kWh |
| 12,000 miles (US average) | ~3,400 kWh |
| 15,000 miles | ~4,300 kWh |
| 20,000 miles (high commuter) | ~5,700 kWh |
Step 2: Add the house
Pull 12 months of bills and total the kWh (see reading your bill). US average is ~10,500 kWh; a gas-heated UK home uses ~2,700-4,000 kWh; Australian homes ~5,000-7,000. Then decide what else electrifies in the next 5 years - a heat pump adds 2,000-5,000 kWh - because resizing later costs more than sizing now.
Step 3: Divide by your sun
| Location | Annual yield per kWp | System for house (8,000 kWh) + EV (3,400 kWh) = 11,400 kWh |
|---|---|---|
| Southwest US | ~1,700 kWh | 6.7 kWp (~16 panels) |
| Average US | ~1,400 kWh | 8.1 kWp (~19 panels) |
| Northern US / UK | ~950-1,100 kWh | 10.4-12 kWp (24-28 panels) |
| Australia (Sydney/Brisbane) | ~1,450-1,600 kWh | 7.1-7.9 kWp (~17-19 panels) |
Yields from NREL PVWatts and CEC/MCS equivalents with modern ~430-450W panels. Shading, roof angle and orientation move these by 10-25% - the installer's shade report is the document to scrutinize.
Step 4: The reality checks
- Roof space: a modern panel needs ~18-20 sq ft (1.8 m²). A 19-panel system wants a clear 350+ sq ft face, ideally south-ish (US/UK) or north-ish (AU), unshaded 9 AM-3 PM.
- Charging pattern honesty: solar only directly charges the car when the car is home in daylight. If it is not, you are sizing for the house plus exporting for the car's benefit - which only pays where export rates are decent or a home battery time-shifts it. Be honest here or the payback math lies to you.
- Electrical capacity: larger systems can trigger panel or interconnection upgrades (US 120% rule). Your installer's load calc covers this - ask to see it.
- Budget and incentives: US 30% federal credit, UK 0% VAT, AU STCs and state schemes all cut the real price; get 3 quotes because installed $/W varies by 30%+ between installers for identical hardware.
Mistakes installers will not correct for you
- Sizing to last year's bills. You bought the EV to drive it - size for house + EV, not the pre-EV history.
- Optimistic yield assumptions. Every quote should state the assumed kWh/kWp and shade losses; compare those numbers across quotes, not just the price.
- Ignoring the export rate. In low feed-in markets (most of Australia, NEM 3.0 California), an oversized system sells surplus for pennies. Pair sizing with a plan: EV midday charging, a battery, or a smaller system.
- No consumption monitoring. Spec a monitor (Emporia, Sense, or the inverter's) from day one; you cannot tune what you cannot see.
What to read next
- Solar + EV payback: when the numbers work
- Heat pump + EV: planning total capacity
- Solar-aware chargers that divert surplus into the car
Sources: NREL PVWatts, MCS (UK), Clean Energy Council (AU), EIA household consumption data. Checked November 2026.