Payback periods: what the number means and how it's manipulated

What payback actually is

Payback is the time taken for cumulative savings to equal the system's cost. Simple in principle. The manipulation happens entirely in the assumptions underneath it, which are rarely shown.

A payback figure without its assumptions is not information. It's marketing.

The five assumptions that move the number

Self-consumption rate. The share of generation you use rather than export. Assume 70% instead of a realistic 40% and payback shortens dramatically - on paper.

Electricity price inflation. Assume prices rise 5% annually forever and future savings balloon. Assume 0% and they don't. Both are guesses; only one flatters the quote.

Generation yield. Optimistic kWh-per-kW figures that ignore your roof's actual orientation, pitch and shading.

Degradation. Panels lose output slowly over time. Ignoring it overstates late-year savings.

System cost. Whether the figure includes the inverter replacement you'll likely need, or ongoing costs.

What a credible payback looks like

It shows its inputs. It uses a self-consumption figure derived from your actual load profile, not a default. It states the electricity price assumption explicitly. It presents a range rather than a false-precision single number, because the underlying variables are uncertain.

Most well-designed residential systems in NSW land in a 3-to-7 year payback range. If someone quotes you 2 years, ask which assumption is doing the work.

What payback misses

Payback treats the system as an investment recovering its cost, and then implicitly stops. But a system with a 25-year panel warranty keeps generating for well over a decade after payback. The lifetime return matters more than the payback date - a system that pays back in 6 years and runs well for 25 beats one that pays back in 5 and degrades.

Payback is a useful sanity check, not the whole decision.