SolarToolsHow This Tool Works
Most solar sizing tools treat your home and your EV as separate problems. They're not. Once you have an EV, your home's electricity load effectively doubles — and sizing solar for one without the other leaves money on the table. The Solar + EV Combined System Sizing Calculator sizes a single integrated system that powers both your home appliances and your daily driving.
Typical result: a 900 kWh/month home needs about 6-8 panels; add a daily 40 km EV commute and you're at 10-12 panels total. The math is additive — the calculator shows the breakdown so you can see exactly how much of your system is "home" vs "EV." This matters for installer quotes: many installers size only for current home use and undersize the system, leaving you grid-charging your EV at $0.16/kWh when you could be solar-charging for $0.06/kWh.
The calculator includes a future-proofing factor (default 2% annual home usage growth) so your system isn't undersized the day after installation. New heat pump, new AC, second EV — these all add load. The growth factor gives you headroom without oversizing today.
- Monthly home kWh — from your utility bill. Use the last 12-month average, not the latest month (which may be seasonal).
- Annual home growth — leave at 2% unless you have specific plans (heat pump install, EV #2, home addition). 0% if you're confident your usage is stable.
- Daily EV driving — your typical daily kilometres. Be honest — underestimating means undersized solar.
- EV efficiency — real-world km/kWh. Check your car's trip computer over a few weeks of normal driving.
- Charging days — daily commuters: 5-7. Weekend drivers: 1-2.
- Solar conditions — peak sun hours from NREL/PVGIS, panel wattage from your installer quote, leave losses at 14%.
Get quotes for the combined panel count, not just home panels. Many installers will offer a smaller system at a lower price — but you'll pay the difference in grid electricity for your EV within 2-3 years.
When to Use This Calculator
Why combined sizing matters
Sizing solar for home + EV together is almost always cheaper than sizing them separately. Reasons: (1) one installation labor cost vs two, (2) one inverter can handle the combined load, (3) net metering credits from home surplus can offset EV charging, (4) incremental panel cost ($150-250 each) is much less than separate systems. The combined system approach typically saves $3,000-5,000 vs separate home-solar and EV-charger-with-battery installs.
The EV doubles your electricity load
A typical US home uses 900 kWh/month (30 kWh/day). A daily 40 km EV commute adds 6-7 kWh/day — about 20% more load. But the EV load is "smart" — you can schedule it to charge during solar production peaks (midday) or off-peak grid hours (night). This load-shifting flexibility is valuable: a combined system lets you absorb your own solar production rather than exporting it at low net-metering rates.
Should you oversize or right-size?
Right-size for today, oversize by 10-20% for tomorrow. Solar panels degrade 0.4-0.55% per year, so a 10 kW system is 9.0 kW by year 20. If you add a second EV in year 3, a heat pump in year 5, or a pool in year 7, your load grows while your production shrinks. Installers often recommend "maxing out" your roof (filling available space with panels) because incremental panel cost is small compared to installation labor — adding 4 more panels during initial install costs $800-1,200 vs $4,000+ as a separate retrofit.
The net metering trap
If your utility has good net metering (1:1 credit), you can oversize aggressively — surplus production credits your evening/night usage. If your utility has bad net metering (NEM 3.0 in California, low export rates in Australia), oversizing wastes money because surplus exports are worth 30-50% of retail. In NEM 3.0 territory, combine solar with a home battery instead of oversizing — the battery banks daytime surplus for evening EV charging.
Real-world example: 900 kWh home + 40 km EV/day
Home: 30 kWh/day. EV: 6.7 kWh/day. Total: 36.7 kWh/day. At 5 peak sun hours and 450W panels with 14% losses, per-panel output is 1.93 kWh/day. Required panels: 19 panels (8.55 kW system). This is a typical "full electrification" system — covers home + EV for a US household. Annual production: ~13,400 kWh. Annual savings at $0.16/kWh: ~$2,150. At an installed cost of ~$25,000 (after any state rebates), payback is ~11 years. Add a $10k battery and payback extends to 13-14 years — but you also gain outage protection.
Frequently Asked Questions
Typically 30-50% bigger. A 6 kW home solar system becomes 8-9 kW when you add an EV driven 40 km/day. The exact number depends on your daily driving distance, EV efficiency, and local sun hours. Use the calculator above for your specific case.
Slightly oversize for future use. EVs tend to be driven more than petrol cars (cheap per-km fuel). Plan for 1.5-2x your expected daily driving within 2 years. Adding panels later costs 3-4x more than oversizing during initial installation.
Yes, that's exactly what this calculator sizes for. A combined 8-12 kW system (20-30 panels) typically covers a 900 kWh/month home plus a 40 km/day EV commute in a sunny climate. The EV charges from surplus solar during the day or from a home battery at night.
Add 4-6 more panels per additional EV (assuming similar daily driving). If you anticipate a second EV within 3-5 years, oversize the initial system by 25-30%. Retrofitting panels later costs 3-4x more per panel than including them in the initial install.
Yes, significantly. With good net metering (1:1 credit), you can oversize aggressively — surplus exports credit your night EV charging. With poor net metering (NEM 3.0, low export rates), oversized solar wastes money — pair a right-sized system with a home battery instead.
It depends. If you charge your EV during the day (work-from-home, smart charger), you don't need a battery. If you charge at night, a battery banks daytime solar for evening EV charging — especially valuable in poor-net-metering regions. A 10-13 kWh battery covers one full EV charge plus overnight home use.
Further Reading
Deep-dive articles and guides related to this calculator.
EV Charging at Home: Costs, Speeds, and Savings Explained
Level 1 vs Level 2 charging, real-world costs, public vs home charging economics, and how much an EV...
Solar Fundamentals: From Sunlight to Socket
How solar panels turn sunlight into electricity, the components of a residential system, and the phy...
Energy & Solar Glossary
Plain-English definitions of every term used in this calculator and across the site.