Ten questions to ask before you accept a solar proposal

The ten questions

1. What load data was the system sized on?Interval data from your meter is better than monthly bills, and monthly bills are better than a rule of thumb. The sizing is only as good as the data behind it.
2. What tariff does the saving assume, and how will it change?Ask for the tariff and escalation rate used in the savings calculation, and what happens to the case if tariffs rise more slowly than assumed.
3. Where does the yield estimate come from?Look for the simulation method and the weather data. A P50 figure is the output expected in a typical year. A P90 figure is a more cautious estimate that output should exceed in nine years out of ten. Lenders usually look at both.
4. What degradation and availability are assumed?Panels lose a little output each year and systems are sometimes offline. Check that both are in the model.
5. What is included, and what is not?Grid connection, approvals, structural checks, roof repairs, cabling upgrades, metering and monitoring are often excluded or priced separately.
6. What is actually guaranteed?A performance guarantee should say how output is measured, over what period and what the contractor does if it falls short. “Designed to produce” is not a guarantee.
7. Who maintains it, and what does that cost?Get the scope in writing: inspections, cleaning, response times and who pays for parts.
8. Which warranties apply, and who honours them?Separate product warranties from workmanship warranties, and ask what happens if the manufacturer or installer is no longer around.
9. Who owns the performance data?You should be able to see your own monitoring data, and another party should be able to take over monitoring if you change contractor.
10. What happens at the end, or if you want out?For leases, power purchase agreements and financed systems, check early-termination terms, the buy-out price and what happens if you sell or re-let the property.

Compare like with like

Put every bid into the same table: scope, price, yield assumption, guarantee, maintenance and exclusions. The cheapest headline price is often the one with the most exclusions.

An independent tender makes the comparison fair, because the specification is written before suppliers see it and every bid is judged on the same criteria. That is what we do in our procurement work.