Home & Construction⏱ 6 min read
How to Calculate Solar Panel Savings for Your Home
Solar panels have a payback period of 6–11 years for most UK homes — but the range is wide depending on your usage, roof orientation, and export tariff. Here's how to calculate your specific case.
Solar panel calculations require more than a single "average" figure — they depend on your actual electricity consumption, roof orientation, local irradiation, and the current Smart Export Guarantee rate. Here's how to run the numbers properly.
Step 1: Estimate Annual Generation
Annual generation (kWh) = System size (kWp) x Peak sun hours x 365 x efficiency
For UK, peak sun hours by orientation (annual average hours/day):
South-facing: ~3.5 hours/day
South-east/south-west: ~3.2 hours/day
East/west: ~2.7 hours/day
System efficiency factor: ~0.8 (accounting for inverter losses, temperature, dirt)
Example: 4 kWp south-facing system in southern England
= 4 x 3.5 x 365 x 0.8 = 4,088 kWh/year
Rule of thumb for UK: 850-900 kWh per kWp per year (south-facing)
Step 2: Calculate Self-Consumption
Not all generated electricity is used by you directly.
What you don't use immediately is either exported or wasted.
Typical self-consumption rate:
Without battery storage: 30-50% of generation
With battery storage: 65-85% of generation
Example: 4,088 kWh generated, 40% self-consumption
Self-consumed: 4,088 x 0.40 = 1,635 kWh/year
Exported: 4,088 x 0.60 = 2,453 kWh/year
Step 3: Calculate Annual Savings and Income
Unit rate saved (2025 price cap): ~24.5p/kWh
Saving from self-consumption:
1,635 kWh x £0.245 = £400.58/year
Smart Export Guarantee (SEG) income from export:
SEG rates vary by supplier: 4-15p/kWh (check current offers)
At 7p/kWh: 2,453 x £0.07 = £171.71/year
Total annual benefit: £400.58 + £171.71 = £572.29/year
Step 4: Payback Period
Typical 4 kWp system cost (2025): £6,000-£8,500 installed
At £7,500 system cost, £572/year savings:
Simple payback = £7,500 / £572 = 13.1 years
With energy price rises (assume 3%/year average increase):
Year 1: £572
Year 5: ~£663
Year 10: ~£769
Year 15: ~£891
Net Present Value (NPV) over 25 years (3% discount rate):
= approximately £6,800-£9,200 net benefit
(system pays back and generates profit over 25-year lifetime)
Battery Storage: Does It Add Up?
Battery addition cost: £2,500-£4,000 (10 kWh typical)
Extra self-consumption: raises from 40% to 70%
Additional kWh self-consumed: (70%-40%) x 4,088 = 1,226 kWh extra
Additional saving: 1,226 x £0.245 = £300/year
Less export income lost: 1,226 x £0.07 = -£86/year
Net additional saving: ~£214/year
Battery payback: £3,000 / £214 = 14 years
(Battery warranty typically 10 years — marginal at current prices)
Verdict: battery storage makes financial sense mainly if you're
on a time-of-use tariff with very high peak rates.