Solar Panel Sizing for EV Charging

How We Calculate Your Solar Panel Count

The calculation starts with your annual electricity need. We take your EV's EPA-rated efficiency (kWh per 100 miles) and multiply it by your daily mileage to get daily kWh, then scale to a full year. If you want solar to cover a portion of your home electricity, we estimate that from your monthly bill and your state's electricity rate.

Next, we look up your state's solar production factor: how many kWh a 1 kW system produces in a year. Arizona produces around 1,700 kWh/kW/yr while Washington produces closer to 1,000 kWh/kW/yr. Dividing total kWh needed by this production factor gives the system size in kW. We then divide by your chosen panel wattage and round up to get a whole panel count.

The formula: panels needed = ceil((annual kWh needed / state production factor) * 1000 / panel wattage). Because we round up to whole panels, your actual system will typically produce slightly more than your target.

Panel Wattage Matters

A higher-wattage panel produces more power from the same physical space. If you have limited roof area, choosing 450W or 500W panels instead of 350W panels can reduce your panel count by 20 to 30 percent while achieving the same annual output. The trade-off is cost: premium panels cost more per unit, though the difference is shrinking as manufacturing improves. For most homeowners, 400W panels strike the best balance of cost and efficiency. If roof space is a constraint, go higher. If budget is the priority, 350W panels from a reputable manufacturer work just as well per dollar of system cost.

Factors That Affect Your Real-World Results

Several factors can cause your real-world panel count to differ from this estimate. Shading from trees, chimneys, or neighboring buildings can reduce output by 10 to 40 percent depending on severity. Roof orientation matters too: south-facing roofs in the US capture the most sun, while east and west-facing roofs lose 15 to 20 percent. Panel degradation is also a consideration: modern panels lose about 0.5 percent of output per year, so after 25 years they produce roughly 87 percent of their original capacity. For a conservative estimate, add 10 percent to the panel count this calculator suggests. Finally, inverter efficiency (typically 96 to 98 percent) and wiring losses reduce the total system output slightly. Installers typically account for these losses when designing your system.