What "clear-sky payback years" on a solar quote actually hide

I had a bloke in Bendigo show me two quotes last month, same rough system size, same rough price, and payback estimates of 4.1 years and 6.8 years. Nobody had lied to him. Both numbers were "correct" under the assumptions each installer chose. That's the part that trips people up: payback years is not a measured outcome, it's a forecast built on a stack of assumptions the sales rep picked, and most homeowners never see the stack, only the headline number at the bottom of the quote.

After six years crawling around roofs before I started writing about this stuff full-time, I can tell you the payback figure on a glossy proposal is one of the least reliable numbers in the document. Not because installers are being dodgy, usually. It's because the calculation depends on things that are genuinely uncertain at the point of sale, and a bit of optimism creeps into each one. Stack four or five optimistic assumptions together and a 7-year payback quietly becomes 4.5 on paper.

The self-consumption assumption is doing most of the work

The single biggest lever in any payback model is what percentage of your solar generation you actually use in the house versus export to the grid. Self-consumed power offsets your retail rate, maybe 28-35c/kWh depending on your plan. Exported power earns a feed-in tariff, often under 10c/kWh these days in most states. That gap is enormous, and it means a quote's payback number is extremely sensitive to whatever self-consumption percentage got typed into the spreadsheet.

A lot of proposal software defaults to something like 40-50% self-consumption for a "typical" household. Fine for a household home most of the day. Rubbish for a household where both adults commute and the kids are at school until 3:30pm, generating almost nothing to use the midday solar peak. I've seen quotes built on assumptions that simply don't match the family's actual daytime occupancy, and nobody asked.

Ask the installer directly what self-consumption percentage they used and why. If they can't answer, or say "the software worked it out," that's a soft spot in the whole proposal. It's worth reading our piece on how to size a home solar system before you get quotes at all, because sizing and self-consumption are joined at the hip — a bigger system doesn't help payback if nobody's home to use the extra generation.

Degradation curves get smoothed into something rosier than reality

Panel warranties typically guarantee something like 90% output at year 10 and 80-85% at year 25, depending on the manufacturer's specific performance warranty terms. That's a real, contractual figure. But a lot of payback calculators apply a flat, gentler degradation curve, or skip degradation in the early years entirely because the difference is small enough to not bother modelling.

It genuinely is small early on, maybe half a percent a year. The problem is compounding assumptions: mild degradation modelling plus optimistic self-consumption plus a flattering electricity price escalation all stack in the same direction, toward a shorter payback. None of them are individually dishonest. Together they're a rounding error factory.

Electricity price escalation assumptions swing the number wildly

Here's one that genuinely surprised me when I started comparing quotes side by side after moving into content work: some proposals assume electricity prices rise 3% a year, others assume 5%, a few conservative ones assume flat pricing. Over a 10-year payback horizon that difference alone can shift the outcome by a year or more, because higher future prices make every kWh of self-consumed solar worth more in the projection, dragging payback forward.

Nobody can actually know future retail tariff movements with precision. The Australian Energy Regulator and the state-based regulators publish default market offer changes annually, and they've moved in both directions over the past decade depending on wholesale costs, network charges and policy settings. A quote that bakes in an aggressive escalation rate isn't fraudulent, it's just picking the optimistic end of a genuinely uncertain range. I'd argue installers should be required to show the escalation assumption on the quote itself, in plain text, not buried in a PDF appendix. Most don't.

Inverter and battery replacement costs rarely make the payback line

A string inverter typically carries a shorter warranty than the panels, often in the 10-12 year range depending on brand and model, sometimes extendable. That means most 25-year solar systems will need at least one inverter replacement, and the payback calculation on your original quote almost never accounts for that cost sitting in year 11 or 12. It's treated as a separate future expense rather than folded into the payback timeline, which understates the true breakeven point.

If you're comparing a string inverter setup against microinverters or power optimisers, this replacement-cost asymmetry is worth weighing properly rather than glossing over, because the long-run cost profile is genuinely different between the three topologies. We cover the mechanics in string inverters vs microinverters vs power optimisers. If a battery is bundled into the quote, the same blind spot applies to battery cycle life and eventual replacement, which our piece on LFP vs NMC battery chemistry gets into in more technical detail.

Roof orientation and shading get averaged, not measured

Plenty of proposal software pulls generation estimates from satellite imagery and a generic irradiance model for your postcode, then applies a simple orientation and tilt correction. That's a reasonable starting point but it's not a shading study. If there's a gum tree two doors down that throws shade across half the array from 2pm in winter, or a neighbour's second-storey addition clips the western string, the software has no idea unless someone walked the roof and looked.

Even partial shading on a small section of a string inverter system can drag down output across the whole string, not just the shaded panels, which is exactly the mechanism covered in how shading wrecks solar payback. If your quote's generation estimate came from a desktop assessment rather than a site visit with someone physically checking sightlines to the horizon in both directions, treat the payback number as a rough guide, not a forecast.

Split-orientation roofs complicate this further, and the industry line on east/west arrays being automatically inferior is, in my view, overstated for households that are home during the day, something I go through properly in split-orientation roofs: is east/west solar actually a bad idea?

What to actually ask for before you sign

The fix here isn't complicated, it's just rarely offered upfront. Ask the installer, in writing, for the self-consumption percentage used, the degradation curve applied, the electricity escalation rate assumed, and whether inverter or battery replacement costs are factored into the payback year or treated as a separate future line item. A good installer will have this at hand because they built the model. A sales-heavy outfit running everyone through the same templated proposal often won't, and that gap tells you something about who you're dealing with.

We've written a broader checklist on what a legitimate quote should itemise line by line in what a real solar or battery quote should itemise, and if you're still shortlisting installers, how to choose a solar installer covers the vetting questions worth asking before quotes even land. For the panels themselves, understanding what the efficiency percentage on the spec sheet actually buys you, covered in our efficiency ratings piece, helps you sanity-check whether the generation assumptions in the payback model are even physically plausible for the panel chosen.

None of this means payback estimates are useless. They're a genuinely useful comparison tool between two proposals from the same installer using the same assumptions. They're a much shakier tool for comparing across installers, or for treating as a guarantee. The Clean Energy Council's consumer guidance is worth a read if you want a plain-language sense of what a compliant quote in Australia should disclose, and the U.S. Department of Energy's solar consumer resources cover similar ground for American readers sizing up proposals. Either way, the number on the bottom of the page is a starting conversation, not a contract.

Get the assumptions in writing, compare like for like, and treat any quote that won't show its working with a healthy dose of scepticism. That Bendigo homeowner, in the end, picked neither installer and got a third quote instead. Fair enough, too — when the first two numbers don't reconcile, the only sensible move is to ask more questions, not split the difference.

— Sam Whitfield, Solar & Batteries