Summary

Rooftop solar and home batteries are widely proposed as a way to decarbonise and decentralise the power system, reducing bills for households while easing pressure on the grid. The value is arguably greatest for vulnerable households, who face the highest energy burdens but are least able to afford the upfront cost.

Yet despite the rapid rise of both technologies, there is limited causal evidence on what happens when a household actually installs them. Moreover, the little evidence that does exist comes almost entirely from early adopters - who tend to be wealthier and more engaged than the population as a whole. Evidence on home batteries is thinner still, and is largely modelled rather than measured empirically.

To close this gap, Centre for Net Zero worked with Octopus Energy to embed a randomised controlled trial into a programme providing free solar panels and batteries to 708 households under Great Britain's Warm Home Discount Scheme. This trial provides one of the only experimental estimates of the impact of residential solar, and the first for solar combined with battery storage, on energy demand and the grid. Crucially, it gives us real-world insight into how these technologies reduce bills in vulnerable or fuel-poor households specifically.

Findings:

① Solar and battery installation cut electricity bills by around 62%, or £681 per year. Households reduced direct grid electricity draw by 38% and, accounting for power sent back to the grid, cut net imports by 77%. This could avoid 4.4 tonnes of CO₂e per household over an assumed fifteen-year life. Savings are largest in summer, when estimated bills fall close to zero, but persist through winter.

② Batteries shifted demand away from the evening peak, including in winter. Grid imports between 16:00 and 20:00 fell by 66%, and this load-shifting held even in December, when there was minimal solar generation to store.

③ The battery contributes materially to the bill saving. It is worth roughly £148 per year – 13% of the baseline bill – and delivers far more peak reduction, and therefore system value, than solar alone.

④ Tariffs determine how much value the installation creates for consumers and the grid. Around one in four households moved to a solar-specific tariff, but about half stayed on a flat tariff, removing the battery's ability to respond to time-varying prices. Survey evidence points to low awareness as a key barrier.

⑤ The case for subsidy depends on which benefits are counted, and how. Current UK VAT relief on solar and batteries represents good value once learning-by-doing benefits are included. Weighting benefits by recipient income, a fully subsidised programme for low-income households is estimated to enhance social welfare.