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Where the sun in your car came from

The dashboard says a car charged twelve kilowatt-hours and that most of it was free. That is a claim about the past, and it is worth knowing exactly how it is worked out — including the two places where it deliberately understates.

Three sources, and they add up

Every kilowatt-hour that went into a car came from the roof, the house battery, or the grid. Those three are the whole of it and they sum to the total — if they did not, the accounting would be hiding something.

At each moment Sunplug knows what the site was doing: production, battery flow, and what crossed the meter. It knows what the cars were drawing. So it can say what fraction of that instant's charging was covered by sun, and it records that as it happens rather than reconstructing it later from daily totals — which would average away every cloud.

Two cars share the credit in proportion

When two cars charge at once, the sun is divided between them in proportion to what each was actually drawing. A car taking 7 kW alongside one taking 3 kW gets 70% of that moment's solar.

This is the only division that survives contact with reality. Splitting it evenly would credit a car that was barely trickling with as much sunshine as one running at full current, and the household's numbers would stop adding up the moment the two cars charged at different rates — which is most of the time, for the reasons on the two-cars page.

Charging away from home is not a fourth source

A motorway rapid, a workplace charger, a friend's driveway. That energy went into your car and it is worth showing, but it must not enter the solar split, and the reason is a rounding error that turns into a rout.

Solar is divided in proportion to draw. A car on a 150 kW rapid is drawing twenty times what a car on a home wallbox draws — so if that were folded in, it would take about 95% of the household's solar credit for an hour it spent fifty miles away, and the car genuinely sitting on your drive in the sun would be shown a twentieth of the sunshine it actually used.

So away energy is counted, shown, and kept out of the split.

Why the savings figure is smaller than the truth

The saving is what those kilowatt-hours would have cost you had you bought them. That needs a price, and prices vary — which means an assumption, and the assumption is made against us.

Where the tariff has cheap and expensive days, the calculation assumes the cheap one. Free solar on an expensive day saved you far more than the figure says; the figure is what you would certainly have saved rather than what you probably did. A number that flatters is worse than useless, because the first time a reader checks it against a bill it loses all its credibility and takes the rest of the dashboard with it.

How far back it goes

Readings arrive every few seconds, which is the right resolution for deciding and the wrong one for remembering. So history is rolled down a ladder: the raw samples are kept a week, five-minute buckets for over a year, and one row per day forever.

The per-car split survives at every level, so "how much of this car's charging was free" is answerable for this afternoon and for two summers ago. Years of daily rows cost kilobytes.

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