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Updated March 2026 — You've just received an £8,000–£12,000 solar installation quote. How do you know if it’s any good? As electrical engineers who review dozens of solar quotes every month, we've seen patterns.
Your quote should include a detailed shading analysis showing how nearby trees, chimneys, or neighbouring buildings affect your panels throughout the year.
Why it matters: Even partial shading can reduce output by 30–40%. If they haven't analysed shading, they're guessing your system performance.
What to ask: “Can I see the shading analysis for my specific roof?”
Good sign: They provide a detailed report with 3D modelling or drone photos showing sun paths.
Bad sign: “Don't worry, there's plenty of sun” or generic “your roof faces south so it's fine.”
Does every quote from this installer seem to be 4kW or 6kW, regardless of house size or energy use?
Why it matters: Your system should be sized based on your energy consumption, roof space, and budget — not a one-size-fits-all approach.
What to ask: “How did you determine this system size is right for my usage?”
Good sign: They reviewed your electricity bills and sized the system to your actual consumption.
Bad sign: “This is our most popular size” or “Everyone gets 12 panels.”
Your quote should include (or offer to provide) cable sizing calculations showing the proposed cable routes are adequate.
Why it matters: Undersized cables cause voltage drop, reducing system efficiency and potentially failing BS7671 compliance.
Real example we found: A Cardiff homeowner's quote specified 4mm² cable for a 32-metre run. Should have been 6mm². Would have reduced output by 8% and cost £2,000 to fix post-installation.
What to ask: “Can I see the cable calculations for my installation?”
Good sign: They provide detailed voltage drop calculations.
Bad sign: “We always use 6mm² cable, it's fine.”
The quote should explicitly state the design is BS7671:2018 compliant or reference British Standards.
Why it matters: BS7671 is the UK electrical safety standard. Non-compliant installations can fail inspection, void insurance, or create safety hazards.
What to ask: “Is this design BS7671:2018 compliant, and will you provide certification?”
Good sign: Clear statement of compliance with documentation included.
Bad sign: No mention of standards or “yeah, we meet all the regulations.”
Your quote should specify exact panel make and model (not just “450W panels”), inverter make and model with specs, cable types and sizes, mounting system details, and protection devices (MCBs, RCDs, SPDs).
Why it matters: Generic specs mean they might substitute cheaper equipment or haven't actually designed your system yet.
What to ask: “Can you provide exact make and model for all major components?”
Good sign: Detailed equipment list with datasheets available.
Bad sign: “We use high-quality panels” or “depends what's available.”
A single line diagram shows how the solar system connects to your existing electrical installation. It's the electrical “blueprint.”
Why it matters: Without an SLD, the installer doesn't have a proper design — they're winging it.
What to ask: “Can I see the single line diagram for my installation?”
Good sign: Professional electrical schematic showing all connections.
Bad sign: “We'll figure that out on the day” or “too technical for homeowners.”
Check if annual generation predictions seem too good to be true.
UK average solar production:
Example: A 4kW system in Manchester should produce 3,200–3,600 kWh/year. If the quote says 5,000 kWh/year, that's a 40% overestimate.
What to ask: “What assumptions are these predictions based on? Can I see the calculation?”
Good sign: Conservative estimates with clear methodology (PVsyst or similar).
Bad sign: Round numbers with no supporting calculations.
Situation: A Kent homeowner received an £8,500 quote for a 6kW solar system with battery.
What we found:
Outcome: The homeowner went back to the installer with our findings. The installer revised the design:
Total cost of corrections before installation: £550
Cost if discovered after installation: £2,000+
“That one review saved us thousands in potential rework. Best £550 I ever spent!” — Homeowner, Kent
Option 1: Ask for Clarification. Sometimes information exists but wasn't included in the quote. Ask politely for missing documents.
Option 2: Get a Second Quote. Compare with another installer who provides detailed specifications.
Option 3: Get Independent Verification. We offer design health checks where we review your quote for BS7671 compliance, cable sizing adequacy, system sizing appropriateness, safety and protection, and value for money.
Send us your solar quote and we'll review the design, compliance, and value for money. No obligation, no sales pressure — just honest advice.