EV Home-Charging Solar Offset Calculator
Calculate how many kilowatts of extra solar capacity you need to cover your EV's home charging. Useful when sizing a rooftop solar addition for a new or existing electric vehicle.
Last updated: June 2026
Extra solar capacity for EV
2.77 kW
2-4 kW (about 5-10 panels) covers a typical commuter. The 2.77 kW example fully offsets 12,000 mi/yr; dropping to 50% offset would halve this to under 1.4 kW.
This is the added solar array size (kW DC) to offset your chosen share of EV charging; a typical residential panel is about 0.4 kW, so divide by ~0.4 to estimate panel count.
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About this calculator
This calculator determines the additional solar panel capacity (in kW) required to offset a chosen share of your EV's home-charging energy demand. The core formula is: Required kW = ((annualMiles / 100) × efficiency / (1 − chargingLosses / 100) / 365 × (solarOffsetPct / 100)) / (sunHours × (1 − systemLosses / 100)). First, it converts annual miles into daily grid energy demand using the vehicle's efficiency (kWh/100 mi) and inflates that figure by home-charging losses (typically 10–15% for Level 2 AC charging). It then scales the result by your desired offset percentage. Finally, it divides by the effective daily solar yield — peak sun hours reduced by panel and inverter system losses — to arrive at the raw panel capacity you need to add.
How to use
Suppose you drive 12,000 mi/yr in a car rated at 30 kWh/100 mi, with 10% charging losses, 4.5 peak sun hours/day, 14% system losses, and you want to offset 80% of charging with solar. Step 1 — daily charging demand: (12,000 / 100) × 30 / (1 − 0.10) / 365 = 10.96 kWh/day. Step 2 — apply offset: 10.96 × 0.80 = 8.77 kWh/day. Step 3 — effective solar yield: 4.5 × (1 − 0.14) = 3.87 kWh/kW/day. Step 4 — required capacity: 8.77 / 3.87 ≈ 2.27 kW of additional solar panels.
Frequently asked questions
How many solar panels do I need to fully charge my electric vehicle at home?
The number of panels depends on your annual mileage, vehicle efficiency, local peak sun hours, and panel wattage. Use this calculator to find the total kilowatts required, then divide by the wattage of your chosen panel (e.g., 400 W = 0.4 kW) to get panel count. For example, 2.4 kW ÷ 0.4 kW/panel = 6 panels. Real-world results vary with shading, roof orientation, and seasonal cloud cover.
What are typical home EV charging losses and why do they matter?
Level 2 (240 V AC) chargers typically waste 10–15% of grid energy as heat in the charger and cable before it reaches the battery. DC fast chargers have different loss profiles but are rarely used for home charging. Including charging losses in your calculation ensures your solar system is sized to deliver enough net energy to the battery, not just to the charger input. Underestimating losses leads to an undersized solar addition that leaves you drawing more grid power than expected.
Why does the solar offset percentage affect the required panel capacity?
The offset percentage lets you target only a portion of your charging from solar, which is practical if you already have an existing system or if full offset is cost-prohibitive. A 50% offset requires roughly half the panel capacity of a 100% offset for the same driving habits. Partial offsets can be financially optimal when grid electricity is cheap overnight (e.g., time-of-use rates) and solar generation peaks at midday. Matching the offset to your electricity tariff and driving schedule maximises your return on investment.