Features → What surplus means

What "surplus" means

There are two numbers a solar charging service could chase, and they are not refinements of each other. One is honest arithmetic; the other buys electricity on purpose, and exists because for some households the alternative is not charging on solar at all.

Excess: what is actually spare

Production minus what the house is drawing. If the panels make 5 kW and the house is using 1.5, then 3.5 kW is genuinely spare, and every watt of it sent to a car is a watt that would otherwise have been exported for a few cents.

This is the only setting that knows whether the energy going into your car would have been sold cheaply or bought back expensively — which is the whole economic case for solar charging. It needs something that can see the grid: a meter, a hybrid inverter, or a Shelly clamped at the supply point.

Gross: production alone, house ignored

Some equipment reports what the panels make and nothing else. Most microinverter setups are like this — APsystems, Hoymiles — because there is no meter in an array and the gateway only ever hears from the panels.

On gross, the cars charge against total production and the house is not subtracted. So while a car is charging, the house runs on grid power. This buys electricity, deliberately. It exists so that a household without a meter has a worse experience rather than no experience.

The allowance is doing a different job here

Beside the setting there is a figure in kilowatts held back for the house. Under excess it is a safety margin — a cushion so the car does not claim the last watt and start pulling from the grid on every passing cloud.

Under gross it is something else entirely: with no meter, it is the only estimate of house load the engine has. It stops being a margin and becomes the house allowance itself. Set it to roughly what your house actually draws in the afternoon and the arithmetic comes out close; leave it at zero and you will buy every watt your house uses while a car is charging.

Two smaller settings in the same place

A production floor, below which solar charging simply stops. Dawn and dusk produce a few hundred watts of noise that is not worth waking a car for, and this is what stops it trying.

Your inverter's AC rating, if you set it. The house and the cars together have to fit inside what the inverter can actually pass, which is a different limit from what your supply allows and is occasionally the smaller of the two.

A negative allowance is possible and does something specific: it overdraws on purpose and imports the difference. It also disables filling the home battery first, because the two instructions contradict each other — you cannot both hold energy back for the battery and deliberately pull more than you have.

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