Engineer's Second Opinion — Solar PV & Battery Quote
At a glance
Executive summary
Verdict: don't sign as quoted. The system is fundamentally sound but oversized in the wrong place, and the headline payback figure won't survive contact with a real export tariff. The biggest problem is the "6-year payback" claim: it only holds if the installer values your exported electricity at the price you pay for imports, which isn't how any export tariff works — on a realistic tariff you're looking at 9–11 years. The biggest cost finding is the battery: 13.5 kWh against a realistic overnight requirement of 5–6 kWh, which is roughly £4,000 of storage this household will rarely fill. The good news is the panel (tier-one, 25-year performance warranty) and the scaffolding line (fixed, no provisional sums) — both sound as quoted. Renegotiate the battery size and the workmanship warranty and this becomes a fair deal, roughly £2,500 less than quoted.
Diagram 1 — Your system as quoted
Flagged in red: the battery is the one component in this chain that doesn't match the load it's serving.
Findings in full
Panel selection
- What the quote says
- Tier-one panel, 25-year performance warranty.
- What the data says
- The manufacturer's datasheet backs up the warranty claim, and the panel sits in the tier-one bracket independent trackers recognise.
- Why it matters
- A 25-year performance warranty from a tier-one manufacturer is a genuine long-term guarantee, not marketing.
- What to do
- Nothing — accept this line as quoted.
System sizing
- What the quote says
- 4.3 kWp array against a stated annual usage of 2,900 kWh.
- What the data says
- That ratio is generous rather than wrong — defensible if generation is modelled on your actual usage pattern, not a national average.
- Why it matters
- An array sized against the wrong baseline inflates every downstream saving figure, including the payback claim.
- What to do
- Ask for a generation forecast built on your own smart-meter data before accepting the sizing.
Battery sizing
- What the quote says
- 13.5 kWh battery, presented as standard for a system of this size.
- What the data says
- A realistic overnight requirement for this household is 5–6 kWh — the battery is sized at roughly 2.5× what will routinely be used.
- Why it matters
- That's roughly £4,000 of storage capacity this household cannot routinely fill, sitting idle in the quote as if it were essential.
- What to do
- Ask for a requote with a battery sized to your actual overnight usage, not a standard package size.
Payback claim
- What the quote says
- Payback in 6 years.
- What the data says
- The calculation silently assumes exported electricity is paid at import prices. On a real export tariff, realistic payback is 9–11 years.
- Why it matters
- This is the quote's biggest problem — it's the number most likely to be quoted back to you as the reason to sign today.
- What to do
- Ask the installer to rebuild the payback calculation using your smart-meter data and the export tariff you can actually get, and to show the working.
Workmanship warranty
- What the quote says
- Two years on workmanship.
- What the data says
- Reputable installers in this market commonly offer five years or more on workmanship.
- Why it matters
- Two years is thin cover for the period when installation faults are most likely to surface.
- What to do
- Ask what extending the workmanship warranty to five years would cost, and get it in writing.
Access & scaffolding
- What the quote says
- Scaffolding included as a fixed line item.
- What the data says
- No provisional sums attached — the price doesn't move once work starts.
- Why it matters
- Provisional sums on access and scaffolding are a common place for quotes to balloon after signing. This one doesn't have that risk.
- What to do
- Confirm in writing that this line stays fixed whatever the installer finds on the day.
Diagram 2 — The battery-sizing gap
The hatched portion of the quoted battery sits above the household's realistic overnight requirement.
Diagram 3 — Payback: claimed vs realistic
Illustrative only. The installer's line assumes export at import prices; the realistic line uses a real export tariff.
The exact questions to put back to your installer
- “Rebuild the payback calculation using my smart-meter data and the export tariff I can actually get — and show your working.”← Payback claim
- “Requote with a battery sized to my overnight usage.”← Battery sizing
- “What does extending the workmanship warranty to five years cost?”← Workmanship warranty
- “Confirm the inverter is matched to the array as installed — not oversized for an expansion I haven't ordered.”← System sizing
- “Confirm the scaffolding line stays fixed whatever you find on the day.”← Access & scaffolding
Verdict
Do not sign as quoted. The system is fundamentally sound but oversized in the wrong place. Renegotiate the battery and the warranty and this becomes a fair deal at roughly £2,500 less.
What a fair deal looks like: same array, right-sized battery, five-year warranty — roughly £2,500 less.
Your report is delivered within 48 hours (Mon–Fri).
— Darren Emery, B.Sc (Hons), M.Sc.
Methodology & independence
- Inputs used: your installer's quote, checked line by line; your smart-meter export where available; real export tariffs (not list price); manufacturer datasheets for the equipment specified.
- What we check: specification and sizing against your actual usage, the arithmetic behind claimed savings and payback, warranty terms against market norms, and where provisional sums could let the price move after signing.
- Independence: no products sold, no commission from installers or manufacturers. You pay us, so we answer only to you.
- Turnaround: within 48 hours, Monday to Friday, from the point your quote and usage data are received.
This sample is illustrative and generalised, with details redacted and altered. Every real report is built line-by-line from your actual quote and usage.