Solar Payback Calculator

Two opposing trends, and why one wins

A solar array gets worse every year and the thing it replaces gets more expensive every year. Payback depends on which effect is larger, and it is not close.

Panels degrade at roughly half a percent annually. After twenty-five years a modern module produces around 87 percent of its original output, which is why performance warranties are typically written near that figure.

Electricity rates in the United States have risen at roughly two to four percent a year over recent decades, though with wide regional variation. At three percent, the price doubles in twenty-four years.

Compounding at three percent against decay at half a percent means the value of each year’s production rises steadily despite the array producing less. A kilowatt-hour generated in year twenty is worth substantially more than one generated in year one, and a flat-rate calculation misses that entirely.

This is why simple payback arithmetic — net cost divided by first-year savings — consistently overstates the payback period. The error is not large in a low-escalation market, but at four or five percent it is a matter of years.

The variable that dominates everything

Of every input on this page, the one carrying the most risk is not in the form at all: whether your utility continues to compensate exported energy at the rate it does today.

Under full retail net metering, every exported kilowatt-hour offsets one you later import, so annual production is what matters and the array’s value is straightforward. Several large markets have moved away from this, replacing it with export rates well below retail.

When that happens the economics change immediately and substantially. Production consumed on site retains full value; production exported earns a fraction. An array sized for 100 percent annual offset in a net metering market can be substantially oversized under net billing.

Existing systems are often grandfathered for a period, but the terms and duration vary and have been shortened. This is a policy risk rather than a technical one, and no calculation can price it.

The practical hedge is to know your current export rate, ask what happens at the end of any grandfathering period, and be cautious about sizing well beyond your own consumption in a market where the rules look unsettled.

What this model leaves out

The projection here is deliberately simple: cash purchase, no maintenance, no resale value. Each of those moves the real answer.

Financing changes the picture substantially. A loan spreads the cost so that monthly savings may exceed the payment from the first month, which feels better than a cash purchase even though total interest adds years to true payback. Compare loan terms against this cash baseline rather than treating them as equivalent.

Inverters are the maintenance item. String inverters typically carry ten to twelve year warranties and are commonly replaced once during a system’s life, at a cost of several thousand dollars. Microinverters and optimisers carry longer warranties and shift the risk profile.

Resale value works in your favour. Studies generally find owned systems add value at sale, while leased systems can complicate one. A system paying back in twelve years may be a better proposition than that figure alone suggests.

Finally, this is a projection based on assumptions you supply, not a forecast. Its usefulness is in comparing scenarios — what happens if rates rise two percent instead of four — rather than in predicting a specific year.