Features → The battery decides first

The battery decides first

A house with a battery and a car has two things that want the same sunshine, and they are not interchangeable. Sunplug fills the battery first by default, and the reasoning is about what each store is worth rather than about which is more deserving.

Why the battery goes first

A kilowatt-hour in your house battery will be used by your house this evening, displacing electricity you would have bought at the evening rate — which is usually the most expensive rate you pay.

The same kilowatt-hour in a car displaces charging you would have done overnight, on off-peak, which is the cheapest rate you pay. It also sits there until you drive, and a car that is going to be plugged in again tomorrow afternoon is in no hurry.

So the same energy is worth more in the battery than in the car, most of the time, and "most of the time" is what a default should be built for.

What the reserve protects

Battery-first governs the order things are filled. The reserve governs how much is kept: a percentage below which the cars may not draw the battery down.

Forty percent by default. That is not a technical limit — it is the answer to "how much of tonight do you want covered before a car is allowed to eat into it". A household that comes home at six and cooks wants a substantial reserve. A household that is out all evening might sensibly set it lower.

The reserve matters most in the case people do not anticipate: an overcast afternoon where the cars would otherwise quietly drain the battery into themselves, leaving you buying peak-rate electricity at seven o'clock to cook with. Without a reserve that is not a bug, it is the system doing exactly what it was told.

When to turn it round

Car-first is the right setting less often than it feels, but it is genuinely right sometimes:

• You are leaving early tomorrow and the car needs to be full more than the house needs to be covered.
• Your battery is large relative to your array and rarely fills, so battery-first means the cars never see any sun at all.
• Your evening rate is not much above your overnight rate, which removes most of the arithmetic above.

Borrowing from a full battery

A battery at 95% can barely take any more, and at 100% the surplus leaves for the grid at export rates. In that window it is nearly free to let a car draw a little more than the meter shows as spare — round the charging current up, keep the car going through a short cloud — and let the battery cover the difference, because the same sun puts it straight back.

Nearly free is not free: a round trip through the pack loses ten to fifteen percent and every discharge is wear. And on a slow inverter feed it can look, on your own inverter app, like the car is charging from the battery for an hour at a time. So it is off unless you switch it on, under Advanced → Solar tracking, and it never draws the battery below your reserve. With it off, the cars take only what the meter shows as spare; a reading that is a few minutes old can still put a little on the battery as the sun changes, in either setting.

The interaction worth knowing

If you set a negative house allowance — deliberately overdrawing and importing the difference — battery-first is switched off, because the two instructions contradict each other. You cannot simultaneously hold energy back for the battery and instruct the system to pull more than it has. That is not a limitation so much as arithmetic refusing to be told otherwise.

Without a battery

None of this applies and the setting does nothing. Solar goes to the house and whatever is spare goes to the cars, which is the simpler world. Whether you should get one →

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