Should you add a home battery to a small solar array?

The honest answer, before the analysis, is that we kept arriving at maybe. A small array generates the wrong shape of energy for a small house. The peaks are short, the dips are long, and the bits that match your evening load are exactly the bits you do not have. A battery promises to flatten this out. The brochure says it pays for itself in seven years.

So we measured. The roof was 4.1 kWp, south-facing, with a small chimney shading the bottom row from about 4pm in winter. The house was a 2010-build three-bed semi on the Surrey-Hampshire border with two adults and an EV. The tariff for the test year was Octopus Intelligent Go. We pulled the half-hourly export and import data, sized a 5 kWh battery against it offline, and asked four versions of the same question.

The narrow question

If you assume a battery is purely a self-consumption device, the maths is straightforward. Take every kWh you exported, ask whether a 5 kWh battery, charged from the panels and discharged in the evening, would have caught it. Over a year, the answer was a touch under a thousand kWh of export turned into self-consumption. At a delta of about 23p per kWh between export rate and import rate, that is around £230 a year.

The wider question

Now widen it. A modern smart-tariff household does not just self-consume from solar. It cycles the battery from cheap overnight electricity, runs the house off it through the expensive daytime peak, and re-imports cheap electricity later if needed. On Intelligent Go that delta is closer to 18p per kWh on the right days. Over a year of disciplined cycling, that is several hundred pounds more.

The complication is that this strategy stops being purely about your solar. The array becomes the smaller of two energy sources for the battery. The thing earning you money is the tariff structure, not the panels.

What we changed our mind about

Twice. The first time, from yes to no, when we noticed that the modelled savings assumed a level of behavioural alignment with the tariff that real households rarely sustain. The second time, from no to maybe, when we ran the same data against a household with no EV and a smaller daytime load.

What we landed on is this. If your tariff has a meaningful peak-to-trough delta and you are disciplined about scheduling, a battery probably earns its keep within ten to twelve years. If you are not, and you are looking at the battery as a "fit and forget" addition to your solar, the case is much weaker. You would be better off increasing the array first.

This piece will be updated when we have a second year of data. Reader data welcome: hello@energystack.uk.