Key facts
- For battery gain B, solar input is B/ηs and overnight input is B/ηn at a consistent AC measurement boundary.
- With unchanged other charges, compare (B/ηs) × export credit against (B/ηn) × overnight import price.
- The equal-cost export credit is overnight import price × ηs/ηn.
- A forgone export credit is an opportunity cost, not an additional cash payment.
- All prices and efficiency values in the worked examples are hypothetical.
Quick answer: Use solar surplus to charge your EV when the export value you give up is lower than the cost of buying enough off-peak electricity to deliver the same battery energy. Compare both options at matching measurement boundaries. A low overnight import price can make exporting solar and charging later cheaper, even though daytime charging uses more of your own generation.
This guide is for choosing a charging schedule when solar equipment already exists. Buying an array is a different decision, covered in our solar-panel sizing guide. All prices and efficiencies below are hypothetical; they are not current tariffs or equipment performance claims.
Start with the value of the electricity you could export
The U.S. Department of Energy’s solar guide identifies electricity prices and export compensation as factors in solar savings. Its discussion of utility bill credits is specific to U.S. arrangements.
If an otherwise exportable kWh goes into your car, you lose the credit that exporting it would have earned. That forgone credit is an opportunity cost. It belongs in the comparison even though no separate charging payment appears on the bill.
Use the marginal value of the next exported kWh under your contract. For a regional example, Ofgem explains that Great Britain’s Smart Export Guarantee uses export meter readings, with rates and contract terms set by the participating supplier. Other billing arrangements need their own calculation; do not assume a headline export rate always represents the credit you would actually lose.
Choose one energy boundary before multiplying prices
For a practical comparison, use AC energy at the home’s charging-supply boundary, after the solar inverter, and a common target for energy added to the vehicle battery. Assume each AC kWh diverted to the charger would otherwise have been exportable at the billing meter, with any intervening differences already accounted for.
Let B be the battery-energy target, ηs the effective solar-charging efficiency and ηn the overnight efficiency. Use decimals such as 0.90 for 90%. These efficiencies must cover the same path from the chosen AC boundary to battery gain.
- Solar AC input required: Es = B ÷ ηs.
- Overnight AC input required: En = B ÷ ηn.
- Solar opportunity cost: Es × export credit per kWh.
- Overnight purchase cost: En × off-peak import price per kWh.
The DOE/EPA charging-energy explanation notes that charging losses vary with the vehicle, charging system, battery state and weather. Equal AC inputs therefore should not automatically be treated as equal battery gains.
Our charging-loss and session-record guide explains measurement limits. A dashboard percentage change multiplied by advertised capacity is an estimate, not an independent battery-energy measurement. If efficiency is unknown, compare several stated assumptions rather than presenting one as measured.
Worked comparison: add 18 kWh to the battery
Assume adequate daytime surplus, an available car, unchanged household demand, no home battery, and charging that can finish before departure. Prices are in arbitrary currency units (CU), with matching tax treatment. Fixed charges are unchanged. These are original illustrative cases.
| Assumed case | Solar / night efficiency | Export / night price, CU/kWh | Solar opportunity cost | Night purchase cost | Lower marginal cost |
|---|---|---|---|---|---|
| A: modest export credit | 90% / 90% | 0.08 / 0.12 | 20 × 0.08 = 1.60 CU | 20 × 0.12 = 2.40 CU | Solar by 0.80 CU |
| B: higher export credit | 90% / 90% | 0.16 / 0.12 | 20 × 0.16 = 3.20 CU | 20 × 0.12 = 2.40 CU | Export and charge overnight by 0.80 CU |
| C: different charging efficiencies | 80% / 90% | 0.11 / 0.12 | 22.5 × 0.11 = 2.475 CU | 20 × 0.12 = 2.40 CU | Export and charge overnight by 0.075 CU before billing rounding |
Case C shows why comparing only unit prices can fail: solar’s export credit is lower than the overnight purchase price, but the assumed solar session needs more input energy. The 80% and 90% figures illustrate sensitivity; they do not establish that solar charging is less efficient.
For positive efficiencies, the equal-cost export credit is night import price × ηs ÷ ηn. In Case C that is 0.12 × 0.80 ÷ 0.90 = 0.1067 CU/kWh, rounded. An export credit above that threshold favors the overnight option under these assumptions.
Use your own figures
Use solar surplus or export and charge off peak?
Compare two ways to add the same battery energy: use available solar first and finish off peak, or export that solar and charge entirely off peak. Enter nonnegative single rates in one currency and on the same tax basis.
Formula and worked example
Let B be battery energy required, S available AC surplus, es solar efficiency and en off-peak efficiency as decimal fractions. Solar input used = min(S, B ÷ es). Remaining off-peak input = (B − solar input used × es) ÷ en.
Solar-first comparison cost = solar input used × export credit + remaining off-peak input × off-peak price. All-off-peak comparison cost = B ÷ en × off-peak price. The export credit is opportunity cost, not a payment to your supplier; unused solar has the same treatment in both options and cancels from the difference.
Hypothetical example: add 18 battery kWh, with 90% efficiency for each option, export credit 0.08 and off-peak price 0.12. With 20 surplus AC kWh, solar-first comparison cost is 1.60 versus 2.40, a difference of 0.80. With only 8 surplus AC kWh, solar contributes 7.2 battery kWh and 12 off-peak AC kWh supply the rest: 0.64 + 1.44 = 2.08, a difference of 0.32.
For each battery kWh that solar can supply, compare export credit ÷ es with off-peak price ÷ en. These are marginal energy comparisons, not a complete electricity bill, system-payback forecast or recommendation for every tariff. Excludes equipment cost, fixed fees, demand charges, storage cycling, changing or negative rates, export caps, net-metering credit rules and minimum charging power. Check those separately.
A cash-flow check prevents double counting
In Case B, suppose exactly 20 kWh of surplus is available. Charging now produces no export receipt and requires no overnight purchase. Exporting instead earns 3.20 CU; buying 20 kWh later costs 2.40 CU, leaving a net 0.80 CU credit across those two flows. This is the same advantage shown in the table.
Do not subtract the export receipt again from the overnight cost when also comparing against solar’s forgone export value. Those are two different accounting presentations of the same choice.
What if solar covers only part of the charge?
Using Case A’s rates and efficiencies, suppose only 8 kWh of AC surplus is available. It adds an assumed 7.2 kWh to the battery. The remaining 10.8 kWh requires 12 kWh of overnight AC input.
The combined marginal cost is 8 × 0.08 + 12 × 0.12 = 2.08 CU. All-night charging costs 2.40 CU, so this mixed schedule saves 0.32 CU in the example. Compare surplus during the hours the car is plugged in; an annual solar total cannot answer that timing question.
Set controls around departure needs and the actual contract
As one product-specific example, Tesla’s Powerwall and Wall Connector guidance distinguishes cost-focused charging from solar-focused charging. Its solar mode can consider other energy sources to meet a target when forecast surplus is insufficient. Check your own equipment’s compatibility and behavior.
Before choosing a schedule, record:
- The import and export prices for the relevant hours, including applicable taxes.
- Whether the next exported kWh earns the assumed credit, and whether changing plans changes other charges.
- Expected surplus after household loads, the vehicle’s availability and the required departure charge.
- Any grid top-up allowed by the charger when surplus falls.
- A separate storage calculation if energy passes through a home battery.
Keep existing installation costs separate from this daily scheduling comparison. Include new equipment costs when deciding whether to buy it. For consistent driving-energy inputs, use our EV efficiency unit converter and measurement guide.
Frequently asked questions
Is charging from my own solar free?
It may create no immediate purchase charge, but using exportable electricity gives up its available export value. Installation economics are a separate calculation.
What if exportable solar has no value?
If the marginal export credit is genuinely zero, using that surplus has zero forgone export income. Other constraints still matter, including charging readiness and whether the alternative import price is positive, zero or negative.
Can I use one overnight price when rates change hourly?
Only if that price accurately represents the energy you will buy. Otherwise multiply each interval’s input energy by its applicable rate and add the results; do the same for export credits forgone.
Editorial method: official sources checked 12 November 2026. Calculations are hypothetical scheduling examples, not tariff quotations, installation advice or payback forecasts.