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

A 4 kWp roof in Surrey, a 5 kWh battery and a year of half-hourly meter data. We ran the numbers four different ways and changed our mind twice.

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.

The narrow question

If you assume a battery is purely a self-consumption device, the maths is straightforward. Take every kWh you exported, and 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 narrow case: A 5 kWh battery at £4,000 paying back £230 a year of self-consumption uplift. Seventeen-year payback. The brochure number is a different number.

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.

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 something close to 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, the case is much weaker and you would be better off increasing the array first.