Summary
The VoltSolar calculator estimates how many solar panels you need to cover your EV's charging, what those panels cost, and how your costs compare with buying gasoline or grid electricity. This page explains every step of the calculation, the default numbers we use, where those numbers come from, and what the calculator does not include, so you can check our work and judge how far to rely on it.
1. What you enter
The calculator needs only a few inputs. You can change any of them.
| Input | What it means | Default |
|---|---|---|
| EV model | Sets the car's efficiency (miles per kWh), from a list of 76 US models | Tesla Model Y Long Range |
| Location | Sets average peak sun hours, electricity price and gas price for your region (19 regions) | California |
| Daily miles | How far you drive on a typical day (5 to 150 miles) | 35 miles |
| Panel size | Watts per solar panel (375 W, 400 W or 440 W) | 400 W |
| Home charger | Charging power (3.6 to 11.5 kW) | 9.6 kW (40 amps) |
| When you charge | Daytime, night or mixed; used for the battery suggestion | Mixed |
| Electricity and gas price | Optional: replace the regional average with your own price | Regional average |
2. How the calculation works, step by step
Step 1: Energy your EV needs. Daily energy (kWh) = daily miles ÷ the car's miles per kWh. Yearly energy = daily energy × 365. We use EPA combined ratings, which include the energy lost while charging. For example, the 2024 Tesla Model Y Long Range is rated at about 29 kWh per 100 miles, or about 3.45 miles per kWh.
Step 2: What solar produces where you live. Daily output per 1 kW of panels = peak sun hours × 0.82. "Peak sun hours" is the number of hours per day of full-strength sunlight, averaged over the year. The 0.82 allows for 18% system losses (inverter, wiring, heat, dust and other real-world losses). That is a little more cautious than the 14% default in the PVWatts calculator from the National Laboratory of the Rockies (formerly NREL).
Step 3: Number of panels. System size (kW) = daily EV energy ÷ daily output per kW. Panels = system size ÷ panel size, rounded up to a whole panel.
Step 4: Roof space and charging time. Roof space = panels × 20.5 square feet (a typical 400 W panel). Daily charging time = daily EV energy ÷ charger power.
Step 5: Cost of the panels. System cost = system size in watts × $3.00 per watt, with no tax credit. This is a slightly cautious national average for installed home solar: EnergySage reports about $2.60 per watt in 2026 and SolarReviews about $3 per watt, and small systems usually cost more per watt.
Step 6: What you would pay without solar. Gas vehicle: yearly miles ÷ the MPG of a similar gas vehicle × local gas price. We use 32 MPG for sedans and hatchbacks, 27 MPG for SUVs, 21 MPG for pickup trucks and 18 MPG for cargo vans. Grid charging: yearly EV energy × local electricity price.
Step 7: Payback and long-term savings. Payback vs gas = system cost ÷ yearly gas cost. Payback vs grid = system cost ÷ yearly grid charging cost. 20-year savings = gas savings over 20 years, assuming gas prices rise 3% a year, minus the system cost. We treat the electricity from panels you already own as free, so the calculator counts only the upfront cost of the panels (no loan interest or repairs).
Step 8: Battery suggestion and CO₂. If you charge at night, we suggest a home battery that holds your daily EV energy plus 15%. If you charge partly during the day ("mixed"), we suggest 60% of your daily EV energy. Both are rounded up to a whole kWh. CO₂ avoided = gallons of gasoline you no longer burn × 19.6 pounds of CO₂ per gallon (EPA: 8,887 grams per gallon).
3. Default numbers and where they come from
Every default below has a source and a date, so you can check it or replace it with your own number.
| Assumption | Value we use | Source and date |
|---|---|---|
| EV efficiency | EPA combined rating for each model | EPA / FuelEconomy.gov |
| Peak sun hours | 3.95 to 6.54 hours a day, depending on region (US average 4.98) | PVWatts v8 / NSRDB data, one representative city per state (compiled by The Green Watt, April 2026) |
| System losses | 18% | Our cautious choice (PVWatts default: 14%) |
| Solar cost | $3.00 per watt, installed, before incentives | EnergySage and SolarReviews, 2026 |
| Federal tax credit | 0% | IRS: Section 25D ended for systems installed after Dec. 31, 2025 |
| Electricity price | Average home price for each state or region, $0.13 to $0.48 per kWh (US average $0.18) | EIA, Electric Power Monthly, July 2026 |
| Gas price | Average regular gas price, updated weekly: EIA's own figure for the US and 8 large states; other states use their AAA average (October 4, 2026) adjusted by the national weekly change | EIA weekly retail gasoline prices; AAA |
| Comparison gas vehicle | 32 MPG (sedan), 27 MPG (SUV), 21 MPG (pickup), 18 MPG (cargo van) | Typical EPA combined ratings for similar gas vehicles |
| Gas price increase | 3% a year (20-year savings only) | Our assumption |
| CO₂ per gallon of gas | 19.6 pounds | EPA |
| Roof space per panel | 20.5 square feet | Typical 400 W panel size |
4. A worked example
Here is what the calculator shows for a Tesla Model Y Long Range driven 35 miles a day, using the US national average settings (4.98 peak sun hours, $0.18 per kWh, $4.37 per gallon), 400 W panels and a 9.6 kW charger:
| Result | Value | How we got it |
|---|---|---|
| Daily energy needed | 10.1 kWh | 35 miles ÷ 3.45 miles per kWh |
| Yearly energy needed | About 3,700 kWh | 10.1 × 365 |
| Output per kW of panels | 4.08 kWh a day | 4.98 hours × 0.82 |
| Panels needed | 7 panels (2.8 kW) | 10.1 ÷ 4.08 = 2.48 kW = 6.2 panels, rounded up to 7 × 400 W |
| Roof space | About 144 square feet | 7 × 20.5 |
| Charging time | About 1 hour a day | 10.1 kWh ÷ 9.6 kW |
| System cost | $8,400 | 2,800 W × $3.00 |
| Yearly gas cost avoided | About $2,068 | 12,775 miles ÷ 27 MPG (similar gas SUV) × $4.37 |
| Yearly grid charging cost avoided | About $667 | 3,700 kWh × $0.18 |
| Payback vs gas | About 4.1 years | $8,400 ÷ $2,068 |
| Payback vs grid charging | About 12.6 years | $8,400 ÷ $667 |
| Suggested battery (mixed charging) | 7 kWh | 60% of 10.1 kWh = 6.1, rounded up |
| CO₂ avoided | About 9,300 pounds a year | 473 gallons × 19.6 pounds |
Which payback should you look at?
If you are deciding whether to buy an EV instead of a gas car, the payback vs gas is useful. If you already own an EV and are deciding whether to add solar, the payback vs grid charging is the fairer comparison, and it is usually much longer. The calculator shows both, and its checklist uses the payback vs grid charging (10 years or less counts as a pass). The checklist's score counts only real tests; notes such as the home battery suggestion and the tax credit reminder do not lower it.
5. What the calculator does not include
We want you to know the limits of this tool:
Shade and roof direction: we assume an unshaded, south-facing roof. East- or west-facing roofs and shade reduce output.
Seasons: peak sun hours are a yearly average. In winter, your panels produce less and your EV uses more energy, so you may rely on the grid more in cold months.
Your utility's solar rules: we assume every kWh your panels produce is worth the full retail price. That is only true with 1:1 net metering, or if you charge directly from the panels while the sun shines. Many utilities pay less for exported solar.
Exact local data: each region uses one average for sunshine and prices. Your town, rate plan and installer's price can be different.
Other costs: loan interest, repairs, an inverter replacement, panel wear (about 0.5% less output per year) and changes in electricity prices are not included.
Incentives: state and utility rebates are not included, so your real cost may be lower.
For a firm number, ask a local installer for a quote. You can also check your roof's real solar potential with the free PVWatts calculator.
6. How we keep the data up to date
Gas prices update automatically each week from the EIA, and the calculator shows which week it is using. We check electricity prices every few months and update them when the EIA publishes new figures. The dates of the current data are shown in the table above. If you think a number is wrong or out of date, please tell us through our contact page, and we will check it and correct it.