You've got three quotes. They're all different prices, different panel counts, different promises. How do you actually compare them?
Price per kWp isn't enough. Panel brand isn't enough. Here's what to actually compare, in order of importance.
Step 1: Normalise the system sizes
Quotes often have different kWp totals. Before comparing prices, convert everything to cost per kWp:
Total quoted price ÷ system size in kWp = cost per kWp. Now all three quotes are in the same unit.
Prices that vary by more than about 20% per kWp usually mean one of these: the cheaper quote is cutting corners on hardware, the cheaper quote is cutting corners on installation, or the more expensive quote is padding margin. All three are worth investigating.
Step 2: Compare panel specs, not just brands
Two tier-1 panel brands can have very different models. Look for:
- Wattage per panel — a 450W panel is different from a 550W panel even from the same manufacturer.
- Efficiency rating — the percentage of sunlight converted. Modern mono is 20–23%.
- Performance warranty — typically 80–87% of rated output at year 25. Higher is better.
- Temperature coefficient — how much output drops as panels heat up. Lower absolute value is better.
Step 3: Line up the inverters
Inverters are often where quotes differ most invisibly. Compare:
- Brand and model — specific part numbers.
- Inverter capacity vs panel capacity — inverters slightly undersized for the panel array ("oversizing the DC side") is good practice up to a point; aggressively undersized inverters clip output.
- Warranty length — standard is 10–12 years, extensions available to 25.
- Type — string inverter (cheap, efficient, vulnerable to shading), microinverter (higher cost, handles shading well), hybrid (battery-ready).
Step 4: Compare production estimates on the same basis
If three quotes predict different annual production for systems of similar size in the same location, somebody's assumptions are wrong. Ask each vendor:
- What peak sun hours did you assume?
- What shading losses did you include?
- What system losses did you model?
If two quotes agree and one is much higher, the optimistic one is usually wrong.
Step 5: Read the warranty section carefully
This is where quotes diverge most in ways buyers often miss. Compare:
- Product warranty on panels — 25 years is standard. Less is a downgrade.
- Performance warranty — what percentage of rated output is guaranteed at year 25.
- Inverter warranty — years covered, whether extensions are offered, who handles claims.
- Workmanship warranty — from the installer on their own work. 10 years is serious; 1–2 years is cosmetic.
- Labour for warranty work — often overlooked. If a panel fails in year 12, does the installer cover labour to remove and replace it, or just the new panel?
Step 6: Calculate effective cost, not sticker cost
A 5% lower sticker price on a quote with worse warranties, an unknown installer, and optimistic production figures is almost always more expensive over 25 years than a slightly higher-priced quote with proper backing. The effective cost is:
(Upfront price) + (likely inverter replacement at year 10–12) − (realistic 25-year production value) = effective cost. Whichever quote has the lowest number here is the actual cheapest.
Step 7: The intangibles
Equal spec sheets don't mean equal outcomes. Consider:
- Who you actually spoke to. The engineer or the salesperson?
- Response time. Did they reply to your questions in hours or days?
- References. Can you speak to clients served 5+ years ago?
- Gut check. Did they answer questions honestly or deflect?
The right quote isn't always the cheapest. It's the one that, when you walk through the above seven steps, leaves you confident the system will still be producing and serviced in year 20. That usually takes a bit more than a one-page PDF.