How our solar savings calculator works
This page documents what the calculator does with each input, so you can audit a result and understand its limits. For a practical calculation you can follow by hand, use the step-by-step solar savings guide.
Step 1: Estimate annual generation
Annual generation is the kWh your solar array is expected to produce in a year. You can enter a known annual figure or ask the calculator to estimate one from your outward postcode, system size, roof direction and pitch using its configured PVGIS evidence.
Step 2: Split generation into self-use and export
The solar-only calculator estimates how much annual generation is used directly in the home from annual generation and household electricity use. You can provide known annual household consumption or explicitly use the sourced household planning fallback. The remainder is treated as exported.
Step 3: Value both buckets
| Bucket | Formula | Why it matters |
|---|---|---|
| Self-used solar | Self-used kWh x import unit rate | Represents bill savings from buying less electricity. |
| Exported solar | Exported kWh x export tariff | Represents SEG or export tariff income. |
Step 4: Calculate payback and net value
Annual solar benefit is bill savings plus export income. Solar-only simple payback is upfront solar cost divided by annual benefit. The solar-only result also shows 10-year and 20-year constant-price illustrations so you can see beyond the break-even point.
How the battery comparison works
Once the solar-only result is ready, you can opt into a battery comparison. Enter the usable battery capacity and the incremental installed battery cost, then choose Compare battery. The comparison estimates annual solar energy sent into the battery and energy later delivered from the battery to the home. It values avoided grid imports and displaced export separately before showing the net annual battery benefit and battery-only simple payback.
What the calculator doesn't include
- Finance interest or opportunity cost.
- Future energy price inflation or export tariff changes.
- Panel or battery degradation, maintenance, replacement schedules or lifetime savings.
- Grid charging or time-of-use and dynamic-tariff battery arbitrage.
- Automatic battery sizing, product recommendations or product-specific control modes.
- Roof repairs, planning constraints, DNO limits or structural survey outcomes.
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