The ten most common solar mistakes — and how to avoid them

We've reviewed enough solar projects (ours and other suppliers') to see the same mistakes recurring. These are the ten that cost buyers the most money, the most production, or the most headache. Avoid them and you're already ahead of most of the market.

1. Sizing the system wrong

The most common mistake is sizing based on "how much roof space we have" or "how big a system will fit the budget" rather than on consumption pattern and export economics.

In markets with poor export compensation, oversized systems produce energy you effectively give away. In markets with good net-metering, undersized systems leave value on the table. The right size depends on your load curve and your utility's export rules — not your roof size.

Fix: size from 12 months of electricity bills and the applicable net-metering regime.

2. Accepting optimistic yield estimates

Yield numbers in quotes are often optimistic. Sometimes through assumption inflation (higher peak sun hours than your location actually delivers); sometimes through omission (no shading loss, no soiling loss); sometimes simply through not modelling your specific site.

A system sold on "15,000 kWh per year" that actually delivers 12,500 is a 17% shortfall in savings — and by the time you notice, the installer has moved on.

Fix: ask for the model assumptions. Compare against independent tools. Ask for a production guarantee.

3. Ignoring the inverter replacement

Inverters typically last 10–12 years. A "25-year payback" calculation that assumes zero replacement cost is fiction. At year 11 or so, you'll spend 5–10% of the original system cost on a new inverter.

Fix: insist on a 25-year cash flow model that includes inverter replacement at year 10–12. The quote's ROI number should reflect that line item.

4. Skimping on mounting hardware

Mounting hardware is the most-hidden part of a solar system. Buyers don't usually ask about it; some suppliers use it as the margin cushion.

Cheap mounting hardware causes:

• Corrosion in coastal / humid climates
• Fastener failures in high-wind events
• Water ingress at poorly-designed roof penetrations
• Panel damage from excessive flex

Fixing any of these in year 7 is expensive. Paying an extra 10–15% for tier-1 mounting hardware on day one prevents all of it.

Fix: ask specifically what mounting system is being used and look it up.

5. Skipping monitoring

Without production monitoring, you won't know for years if the system is underperforming. And "the system works" (the lights are on) is not the same as "the system produces what it should."

A system producing 15% below model for 3 years before you notice costs you real money. Monitoring is cheap (often free with modern inverters) but only works if you actually check it.

Fix: insist on monitoring, verify it's installed and working at commissioning, and check it at least monthly for the first year.

6. Not reading the warranty fine print

"25-year warranty" sounds robust. The fine print often reveals:

• Performance warranty is an average curve, not a hard floor
• Labour for warranty replacement isn't covered — you pay to remove and reinstall
• Warranty claims go to the manufacturer, not the installer (and often require extensive documentation)
• Workmanship warranty from the installer is only 1–2 years

A 25-year hardware warranty with no labour cover and a 1-year workmanship warranty is a lot weaker than it sounds.

Fix: read the warranty section before signing. Ask for specific examples of how claims have been handled for past customers.

7. Accepting a quote without a proper site survey

Satellite imagery shows the roof outline, not the orientation error, the shading from the neighbour's tree, the old roof structure that can't handle the load, or the electrical panel that needs upgrading.

Quotes issued without a site visit miss real-world constraints. Often they work out fine; sometimes they lead to expensive change orders during installation.

Fix: require a site visit before final quote. Any serious supplier does this anyway.

8. Heavy upfront payments

A payment schedule demanding 50–70% upfront before any hardware is on site puts all the risk on you. If the supplier goes out of business between your deposit and your installation, you're in a difficult position.

Fix: insist on a staged payment schedule tied to deliverables, not dates.

9. Not replacing the old roof first

Solar is designed for 25+ years. If your roof has less than 15 years of life remaining, install solar on the new roof, not the old one. Otherwise, you'll pay to remove and reinstall the entire solar system when the roof gets replaced — typically 15–25% of the original installation cost.

Fix: have the roof assessed before committing. If it's close to end of life, schedule roof replacement before (or concurrent with) solar installation.

10. Optimising purely for lowest capex

The cheapest quote is rarely the cheapest over 25 years. Cheaper systems tend to have:

• Shorter-lived components
• Weaker warranties
• Less thorough design
• Lower-quality installation
• Suppliers less likely to exist at year 8

Add up the lifetime costs — including early component replacement, weaker warranty recovery, and occasional service calls — and the cheap quote often ends up costing more than a mid-tier one.

Fix: compare total 25-year cost of ownership, not sticker price. The metric that matters is effective cost per kWh over the system's life.

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