Online solar price figures are snapshots, not quotations for your roof. Access, scaffolding, mounting system, array size, inverter architecture, battery capacity, consumer-unit work, monitoring, bird protection and warranty scope can all change the installed total. Use current written quotations for financial decisions.
What should a solar quotation make clear?
- Panel count, individual module rating and total array kWp.
- Inverter type and AC rating, including whether storage uses a hybrid or separate AC-coupled inverter.
- Battery nominal and usable kWh, charge/discharge power and warranty terms.
- Scaffolding, mounting, electrical protection, monitoring, commissioning and certification.
- Any roof, distribution-board, meter or export-limitation work that is excluded.
- The generation, self-consumption, import, export and tariff assumptions behind projected savings.
Compare like with like. A lower total may omit work or use different system capacity; a higher total is not automatically better.
Build annual benefit from separate flows
Solar electricity used in the home usually has a different value from electricity exported. Battery charging adds conversion losses, and tariff optimisation can add off-peak imports. Keep each flow visible.
Avoided import is not simply annual generation multiplied by the import tariff. Only solar or battery energy that replaces an import earns that avoided-import value. Exported energy uses the entered export rate. Standing charges normally remain and should not be counted as a solar saving.
Calculate simple payback consistently
Simple payback is understandable but incomplete. It does not automatically model tariff inflation, maintenance, component replacement, finance costs, degradation, opportunity cost or the time value of money. Use it as one comparison, and make sure competing scenarios use the same assumptions.
Battery payback is incremental
To test whether storage improves economics, compare solar only with solar plus battery. Divide the additional battery cost by the additional annual benefit created by the battery. Do not credit the battery with savings already produced by solar. If losses and added cost outweigh extra self-consumption or tariff shifting, the correct conclusion may be that the battery worsens financial payback.
Use tariffs you can actually access
Import, off-peak and export rates can change. Export eligibility, metering requirements and treatment of grid-charged battery exports depend on supplier terms. Ofgem’s Smart Export Guarantee guidance explains the scheme framework, but it does not make every battery export automatically eligible.
Run sensitivity cases rather than one optimistic forecast: lower and higher self-consumption, current tariffs, a reduced export rate, and no tariff optimisation. This shows which assumptions drive the result.
Check VAT and eligibility against current guidance
VAT treatment depends on the measure, installation, location and eligibility. GOV.UK states that qualifying installer-supplied energy-saving products can receive reduced treatment, with different rules in Great Britain and Northern Ireland. Electrical storage batteries are covered in specified circumstances. Your installer is responsible for applying the correct rate, so verify the current official guidance rather than deducting VAT yourself.
A transparent example without a made-up system price
Assume an estimate shows 2,400 kWh/year of avoided standard-rate imports and 1,000 kWh/year of export. Multiply those flows by the household’s entered import and export tariffs. If a tariff-optimised battery imports another 800 kWh off-peak, include that charging cost and ensure battery-delivered energy plus losses reconcile with the charge.
Then enter the actual solar-only quotation and actual battery add-on quotation separately. The calculator can show annual benefit and payback without presenting an illustrative national price as though it applies to the property.
A practical comparison checklist
- Use the same demand, roof and tariff assumptions for every quote.
- Compare solar-only economics before adding storage.
- Check that annual generation is plausible for location and orientation.
- Ask how self-consumption was estimated and whether interval data was used.
- Separate export income from avoided import.
- Stress-test tariffs and include likely component replacement in your wider decision.
Use your own quotation and tariffs.
Compare grid, solar, battery and tariff scenarios side by side.