A cheaper quote may cover less work. Compare scope, assumptions and responsibility before comparing totals.
Make the scope visible
Create a table with panel capacity, inverter model, installation work, scaffolding, connection, monitoring and aftercare. Mark anything excluded. If one quote includes a battery and the other does not, ask for separate prices before making a comparison.
Challenge the savings assumptions
Ask both companies to use the same household consumption, electricity price and export price. A savings estimate built around an optimistic self-use rate is not comparable with a cautious one. Keep financing costs and any assumed support payments visible rather than burying them in one payback figure.
Ask who solves a problem
Distinguish a product warranty from installation workmanship and performance estimates. Ask who handles a fault, labour, access equipment and replacement delivery. Request a written handover list and a clear payment schedule. Keep verbal promises in the written specification.
Normalise the figures before choosing
Convert every proposal into the same comparison: total installed price, array capacity, forecast annual generation and expected direct use. Keep the battery as a separate line. Cost per kWp can highlight a difference, but it cannot explain awkward access, electrical upgrades or stronger aftercare. Note whether tax and all compulsory work are included. A lower equipment price is not necessarily a lower finished-project price.
Test the calculation with one shared example
Use an example household generating 4,000 kWh and directly using 1,600 kWh. At €0.30 per purchased kWh and €0.08 per exported kWh, that gives €480 of avoided purchases and €192 of export income before running costs. Ask each installer to reproduce the calculation with your own data. If their answers differ, identify the changed assumption instead of comparing the final payback figures in isolation.
Make exclusions and substitutions easy to spot
Mark every item as included, excluded or awaiting survey. Pay particular attention to scaffolding, meter work, consumer-unit changes, removal of old equipment and the monitoring subscription, if any. Ask what happens if the named panel or inverter is unavailable. A substitute should come with a revised specification and an explanation of the effect on compatibility, warranty and the agreed price.
Your planning checklist
- Same system boundaries and consumption data for every offer.
- Separate prices for storage, finance and optional services.
- Named equipment, exclusions and substitution arrangements.
- Payment stages linked to clearly described project milestones.
Common questions
Is the shortest payback estimate the best quote?
Only if the inputs and scope are comparable. An optimistic electricity price, higher assumed self-use or omitted cost can shorten the number without improving the installation. Compare those inputs first.
What should a useful handover include?
A record of the installed equipment, relevant commissioning documents, monitoring access, operating instructions, warranty details and the fault-reporting contact. Agree the list before choosing the installer so it forms part of the expected delivery.