Charge an EV from PV surplus
Works out how much solar power really reaches the car — and answers the question the wallbox purchase hangs on: is phase switching worth it? The surplus duration curve comes from your system, your location and your household profile, not from a sample curve. For 10 kWp near Berlin with 4,000 kWh household consumption, 8,606 kWh of surplus remains across 3,090 hours; single-phase captures 8,014 kWh of it, three-phase only 5,151.
The formulas behind the calculator
Every number above can be recomputed: the full calculation path, all assumptions and the data source with retrieval date — plus cross-validation against independent references. Disclosed, not claimed.
An estimate based on the stated assumptions. The final design must be checked by a qualified professional against the rules that apply where you are.
Data as of: 2026-08-06
| Step | Formula | Value | Provenance |
|---|---|---|---|
| Minimum power, single-phase | U * I_min | 1.38 kW | exact |
| Minimum power, three-phase | sqrt(3) * U_L * I_min | 4.1569 kW | exact |
| Advantage of the single-phase wallbox | E_1ph - E_3ph | 2,863.4 kWh/a | exact |
- Formula
P_min,1ph = V · I_min = 1,380 W · P_min,3ph = √3 · V_L · I_min = 4,157 W- Valid for
- Surplus duration curve from the hourly TMY grid (1,155 DACH grid points, PVGIS-SARAH3 2005–2023) with a synthetic household load profile, or a measured curve from your own monitoring — which takes precedence.
- Not covered
- Charging losses in the vehicle, the car's battery management, minimum session duration, and vehicle availability — if you commute during the day you cannot use the midday surplus. The surplus duration curve is an input, not a simulation.
- Data sources
- Bundesnetzagentur SMARD electricity price analysis, StromAuskunft price index (as of 2026-08-06) and EEG remuneration rates per § 48 EEG 2023 · Q3/2026 (EEG-Sätze H2/2026) · retrieved 2026-08-06
Frequently asked questions
Why won't my three-phase wallbox start charging on midday solar surplus?
Because of the minimum charging power at the smallest permitted current of 6 A: three-phase that is √3 · 400 V · 6 A = 4,157 W, single-phase only 230 V · 6 A = 1,380 W. Below about 4.1 kW of surplus a three-phase wallbox cannot start surplus charging at all — on a 10 kWp array that forfeits several thousand kilowatt-hours a year that a single-phase box would capture.
Is a phase-switching wallbox worth the premium?
Phase switching starts at the single-phase threshold and steps up when surplus is large — so its usable energy equals the single-phase case, and its benefit is charging speed, not energy captured. The calculator weighs the premium against the annual extra benefit, where each kilowatt-hour is worth only the difference between grid price and forgone feed-in payment, and reports the payback time.
Where does the surplus curve come from, and what is left out?
The surplus duration curve is an input from your monitoring portal, not a simulation — without site weather data a generated hourly curve would be an invention with decimal places. Not covered are charging losses in the vehicle, the car's battery management, minimum session durations, and vehicle presence: if you commute during the day, the midday surplus is unavailable to you.
Where do the 1.38 and 4.16 kW thresholds come from?
From the charging standard: the smallest current a car accepts is 6 A. Single-phase that is 230 V × 6 A = 1,380 W. Three-phase all three conductors count: √3 × 400 V × 6 A = 4,157 W. Below that the wallbox cannot regulate, only pause — which is why the threshold is hard. Neither value appears in any wallbox datasheet, although the purchase decision rests on them.
How much surplus does the three-phase wallbox actually lose?
In the example duration curve of a 10 kWp system, the single-phase wallbox charges 7,609 kWh per year from solar, the three-phase one only 4,746 kWh — 60% more for single-phase. Valued at grid price minus feed-in (35 − 7.9 ct) that is €2,062 versus €1,286 of annual benefit. The €500 premium for phase switching pays back in about half a year on this curve.
Is there a limit to single-phase charging?
Yes, the unbalanced-load limit: in Germany, VDE-AR-N 4100 allows at most 4.6 kVA of asymmetry per phase — so single-phase charging ends at about 20 A (4.6 kW). For surplus charging that is rarely the constraint, because the single-phase wallbox earns its advantage exactly in the range below. If you regularly need fast charging you need all three phases — phase switching combines both.
How much solar surplus is left for the car?
It depends on what the household takes first. For 10 kWp near Berlin and 4,000 kWh household consumption (family profile) the calculator finds 8,606 kWh of surplus spread over 3,090 hours a year. Not all of it is usable: a single-phase wallbox starts at about 1.4 kW and captures 8,014 kWh, a three-phase one needs roughly 4.2 kW and reaches only 5,151 kWh. The 2,863 kWh difference is exactly the surplus that falls between the two starting thresholds.
Is a phase-switching wallbox worth the money?
In the reference case clearly yes: the 500-euro premium pays back in 0.6 years. The reason is the starting threshold. At the 6 A minimum charging current a three-phase wallbox needs about 4.2 kW before it starts at all — on an ordinary spring day that is never reached. Single-phase needs 1.4 kW. The smaller the array and the larger the household consumption, the more clearly the switching option wins.
Where does the surplus duration curve come from?
From the same hourly chain as the self-consumption calculator: hourly yield of your system from the TMY grid of your location, minus the household load from the synthetic profile, with the remainder sorted in descending order. Previously the curve was a list and therefore not editable through the form at all — the sample curve simply stayed. If you have a measured curve from your monitoring portal, use it: it beats any simulation, and the calculator gives it precedence.
Why does the household type change the result?
Because it decides when the household takes the power itself. A working-household profile uses little at midday and leaves plenty of surplus; a home-office profile eats into exactly the midday peak the car would charge from. The calculator uses the same five profiles as the self-consumption calculator. The profile is a model assumption, not a measurement — it captures the shape of a typical day, not your particular Tuesday.
What does this calculator leave out?
Above all one thing: whether the car is there. The calculation assumes charging is possible in every surplus hour. Anyone commuting during the day does not have the midday surplus available precisely when it is largest — the real benefit is then considerably smaller. Also not modelled: charging losses in the vehicle, a home battery competing for the same surplus, and control lag of the wallbox under fluctuating cloud cover.