Charging your EV from solar — when to plug in the car

An electric car is the largest energy consumer in a household — a single charging session (20–60 kWh) can exceed the daily consumption of everything else combined. That also makes it the biggest opportunity: every kilowatt-hour charged from your roof instead of the grid saves the full gap between a few cents of export value and full retail price. The catch: an EV doesn’t sip power, it gulps — and that’s where planning starts.

The threshold problem

Single-phase charging starts around 1.4 kW (6 A), typical settings are 3.7 kW, three-phase 11 kW. A 5 kWp system on an April midday delivers maybe 4 kW — of which the house takes 0.5–1 kW of base load. Real surplus is often 2.5–3.5 kW: enough to charge, but only if you hit the window. Plug in blindly at 5 pm and you’re charging grid power.

Three strategies

“Sunny Saturday”. The simplest: the car sits at home on a weekend, the forecast shows 25+ kWh — plug in during the forecast peak with current matched to the expected surplus (say 3.7 kW for 4–5 hours). Needs nothing beyond a charger with adjustable power.

The mixed strategy. On weekdays the car charges overnight in the cheap tariff window only “to commuting range” — topping up waits for the next strong solar day. An evening glance at tomorrow’s forecast decides how much battery headroom to leave for the sun.

Automatic surplus charging. The endgame: a charger in PV mode (native, or driven by Home Assistant / EVCC) continuously modulates charge current to the live surplus. This is where forecast resolution pays off: Volcast’s API (Premium) delivers 288 five-minute points per day, so the automation knows whether the cloud in 20 minutes is a passing shadow or the end of the window — and doesn’t abort the session unnecessarily.

The maths

10 kWh charged on sunshine instead of grid ≈ 2–3 €/£/$ saved, depending on your rates (retail price minus lost export value). At 1,000 km a month (~170 kWh) with half the charging from surplus, that’s a meaningful monthly sum — plus the unquantifiable part: commuting on power you harvested yourself.

FAQ

Can I surplus-charge from a regular socket? Yes — the “granny charger” (2.3 kW) actually fits moderate surpluses well. Slow, but for a car parked at home all day it’s often the most effective way to absorb the midday hump.

Charge the car or the home battery first? If the car needs charging anyway: the car. It avoids double conversion and battery cycles. The home battery wins when the car isn’t home during production hours.

How much power is left for charging? Surplus = current production − house consumption. Check it on a bidirectional meter or the Energy Dashboard; the hourly forecast tells you what it will look like tomorrow.