Features → Do you need a battery

Do you need a home battery?

Short answer: no. Solar charging works without one, and in daylight a car is arguably the better battery of the two — it is large, it is already paid for, and it is the thing you actually want filled.

What a battery actually changes

Not "whether you can charge on solar" — you can either way. What it changes is what happens to the sunshine your car is not taking at that instant.

Without a battery, surplus your car cannot absorb is exported for whatever your feed-in rate is, which is usually a few cents. With a battery, it is stored and displaces electricity you would have bought this evening at a much higher rate. The battery is not helping your car; it is capturing the surplus your car leaves behind.

Where a battery genuinely helps the charging itself

It smooths cloud. A passing cloud drops production for ninety seconds. Without a battery that shortfall either comes off the car's current or off the grid. With one, it comes off the battery and the car carries on at a steady rate — which, given what stop-start costs, is worth more than the raw kilowatt-hours suggest.

It extends the afternoon at the edges. Morning and evening production is often below the level a car will accept at all. A battery can accumulate those otherwise-wasted watts and give them back as a usable rate.

Where it changes the picture completely

If your inverter is held to an export limit — nothing at all, or a couple of kilowatts — the presence of a battery changes what recovering your curtailed solar costs. With one, the extra demand Sunplug creates is met from the battery and nothing is bought. Without one, it comes from a small grid import instead: a real cost, small, and usually far smaller than the sunshine it unlocks. The whole argument →

The honest arithmetic

A home battery is a large purchase justified mostly by your evening consumption and your tariff spread, not by your car. If you are weighing one specifically to charge an EV better, the money usually does more as panels — more array means more hours per year above the rate a car will accept, which is the actual constraint on solar charging.

If you already have a battery, the order things are filled in is the setting worth understanding.

Stuck? Email me.