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Solar Surplus Charging

Your car charges when your roof delivers, not the grid. Franzl continuously adjusts the charging power to match the real surplus.

What Franzl does

As soon as your roof produces more than the house currently needs, your car starts charging, with exactly the power that's left over. If there's more, Franzl ramps up. If a cloud rolls through, it ramps down instead of quietly switching to grid power. And when there's nothing left over, it pauses before the car can tap into your battery or the grid.

All of this happens automatically. In the app, you see what's going on, in Franzl's own words: "I'm charging your car at 12 A on solar surplus."

Why this isn't trivial

Grid export is the wrong measurement. Many systems treat "what flows into the grid" as surplus. But by that point, the home battery and water heater have often already helped themselves, leaving seemingly nothing for the car, even though the roof is producing at full power. Franzl calculates surplus from production and actual household consumption, and distributes it in your order: the car can come before the home battery, if that's what you want. (The technology behind it →)

No wallbox can go below 6 amps. The charging standard doesn't allow wallboxes to charge below 6 A; on three phases, that's a good 4 kW minimum. Smaller amounts of surplus would otherwise go to waste. If your car itself supports current control (Tesla, for example), Franzl passes the real target straight through to the car and keeps charging with 2–3 kW of pure solar power, no grid share at all.

A short cloud is no reason to switch anything off. Franzl rides out dips of a few minutes with hysteresis, instead of switching the wallbox's contactor on and off every minute. Only once there's truly no partial surplus left does it pause, with a grace period and a restart lockout that spare the hardware.

What's in it for you: an honest example

Let's say you drive 11,000 km a year on electricity (~1,900 kWh), and from now on you charge half of that from surplus instead of from the grid. Each of those kilowatt-hours then costs you the feed-in tariff you're forgoing (say, 8 ct) instead of the grid price (say, 30 ct): around €200 a year, more if you drive further or your grid prices are higher.

How much this actually saves you depends on your roof, your driving habits, and your tariff; the assumptions above are deliberately conservative. After the first few weeks, Franzl shows you the real number, calculated from measured kilowatt-hours.

Modes for when you're in a hurry

Surplus charging is the default, but not the law: "Charge" charges immediately at the current you set on the slider. "Stop" stops charging, no exceptions. A daily target ("80% by 6:00 am tomorrow") combines both: as much sun as possible, as much grid as necessary, in the cheapest hours.