Compare Solar Quotes: 12 Assumptions to Check

A fast payback figure is not proof of a better solar design. Two quotes can use different assumptions for roof yield, household use, export income, electricity prices, battery behaviour and replacement costs. Put the inputs side by side before comparing the result.

The 12 assumptions to record

1. Installed system size: total panel capacity in kWp, not only panel count.
2. Annual generation: predicted kWh and the source or model used.
3. Roof losses: orientation, pitch, shade, mismatch, temperature and system losses.
4. Panel degradation: how future output changes in the model.
5. Direct self-use: the percentage used in the home as it is generated.
6. Battery behaviour: usable capacity, efficiency, cycling and any tariff charging.
7. Import price: the unit rate avoided when solar is used at home.
8. Export rate: the tariff applied only to exported kWh.
9. Price inflation: whether future energy-price rises are assumed.
10. Installed cost: VAT, scaffolding, electrical work and all known extras.
11. Maintenance and replacements: inverter, battery or monitoring costs included in the model.
12. Evaluation period: years modelled and whether the battery life differs from the panel life.

Compare annual generation first

Ask each installer for the kWp, estimated annual kWh and quantified shading loss. If two systems of similar size have materially different forecasts, ask what explains the gap. A scaled roof layout and a location-based yield make the estimate easier to challenge.

Separate self-use from export

Solar used in the home normally avoids the import rate. Surplus exported to the grid earns the applicable export rate. Do not allow all generation to be valued at the higher import price. Ask how the quote estimated daytime use and whether the household pattern was actually discussed.

Run battery and panel-only cases

A battery can increase self-consumption but introduces cost, storage losses and a different warranty life. Compare the same panels with and without storage. Then test the battery using a lower export rate, a realistic cycle pattern and its usable—not headline—capacity.

Stress-test the best-looking quote

Use the payback calculator to lower self-use, lower the export rate, remove aggressive energy-price inflation and add a possible inverter or battery replacement. A proposal that remains sensible under a conservative case is more robust than one dependent on every optimistic input.

Side-by-side comparison template

Create one column per quote and one row for each assumption above. Add rows for panel and inverter models, warranties, installer accreditation, deposit protection, DNO work and exclusions. Mark any missing figure as “not stated” rather than filling it with an estimate from a different quote.

What to do next

• Use the installer questions to fill gaps.
• Use the pre-sign checklist for documents and protections.
• Get a property-specific solar quote before treating a broad online estimate as a decision.