Are home batteries worth it on a small solar array?

When the payback maths works for storage on a sub-5 kWp solar setup, and when it doesn't.

This piece works through whether a typical UK home with a small solar array (under 5 kWp installed capacity) gets its money back on a 5 kWh battery. The numbers depend on tariff structure more than on the panels, which is the headline conclusion.

The setup we're modelling

A representative configuration based on publicly available statistics: a 4.1 kWp array on a south-east-facing roof, a two-adult household, an electric vehicle charged overnight, and a smart EV tariff with a wide peak-to-trough delta. The battery being considered is 5 kWh of LFP, installed at roughly £4,000.

Best answer for most homes

On a smart EV tariff with a wide peak-to-trough spread, a 5 kWh battery on a sub-5 kWp solar setup gets to a 7-10 year payback. On a flat tariff the case collapses to 15-17 years.

Self-consumption uplift

Without storage, a typical UK 4 kWp installation self-consumes around 28% of its generation. The rest is exported. Adding a 5 kWh battery lifts that to around 54% on a tariff with low export rates, which is worth roughly £215 a year at the typical 23p delta between import and export rates.

Tariff arbitrage

The bigger contribution, often overlooked in installer quotes, is cycling the battery from cheap overnight rates into the more expensive daytime window. On a tariff with a 25p peak-to-trough delta and disciplined scheduling, this adds roughly £350-400 a year. The exact figure depends on how disciplined the household is about checking forecasts and scheduling.

Where it doesn't work

On a flat-rate tariff with no peak window, the case collapses to the self-consumption uplift alone, and the payback period becomes uncomfortable (15-17 years). On a tariff with a narrow peak window (less than 15p delta), the payback period sits between 10-13 years, which is at the edge of warranty cover and barely beating an inflation-linked savings account.

What we'd recommend

The case for a battery on a small solar array makes sense if the household is on a wide-delta smart tariff (Octopus Intelligent Go, Cosy or Flux equivalents) and willing to engage with scheduling. On a flat tariff, the case is much weaker. Better in those cases to increase the array first, then revisit storage if and when a smart tariff becomes available.