The honest answer is: it depends — but not as much as the internet makes out. Across 200 Decarbr-matched installs, the median three-bed semi saves around £720 in year one and pays back in seven to nine years.
Three factors move the number more than anything else: the tariff you export on, your daytime usage pattern, and whether you pair with a battery. Everything else — panel brand, inverter size, a few degrees of orientation — matters less than any of those three.
The numbers at a glance
A typical 4kWp south-facing array in the South of England generates roughly 3,800 kWh a year. Self-consumed electricity is worth about 28p per kWh; exported electricity pays 15p on the best SEG tariffs. The split between those two is where the numbers diverge between households.
What changes the answer
- Working-from-home households self-consume 60–70% of generation. Payback ~6 years.
- Empty-weekday households self-consume 25–35%. Payback 9–11 years.
- EV owners who charge during the day flip this entirely — self-consumption can exceed 80% if the car is sunshine-scheduled.
With a battery vs without
A 5kWh battery typically shifts another 1,200–1,500 kWh/year from export to self-consumption. That adds around £160/year in savings — but a 5kWh battery costs £3,000–£4,500. On tariff arbitrage alone, payback on the battery is 8+ years. Paired with a flexible import tariff, that drops to 4–6 years.