What is being compared
Simple payback asks how many equal years of bill savings recover the net upfront system cost. It is not an internal rate of return and it does not pretend that electricity prices, output, or repairs will stay unchanged. It is a useful first screen because every input has a physical or bill-based meaning: dollars installed, dollars credited, kilowatt-hours produced, and the marginal retail price avoided.
Formula and timing
Annual savings equal annual kWh times dollars per kWh. Net cost equals installed cost minus a confirmed credit or rebate. Payback years equal net cost divided by annual savings. The calculation values every generated kilowatt-hour at the entered rate, so the rate should represent the portion of generation that actually offsets purchases or is paid under an export tariff. It does not compound savings or discount future dollars.
Worked example with the displayed units
At 8,500 kWh and $0.18 per kWh, first-year avoided electricity cost is $1,530. Subtracting a $500 credit from $22,000 gives $21,500. Dividing $21,500 by $1,530 produces about 14.1 years. If only half the energy is used on site and exported power earns less, the effective rate is lower and the recovery period becomes longer; that is a change in the input, not a contradiction in the formula.
When to use this result
Use it when comparing installer proposals or checking whether a quoted production estimate is large enough to matter to a household bill. Obtain generation from a shade and orientation study and obtain the electricity value from the relevant bill tariff. Run a cautious case with lower output or a lower export price. The resulting range is usually more honest than one payback date presented as certain.
Costs and conditions outside the fields
Financing interest, inverter replacement, panel degradation, roof work, insurance, permitting, battery cost, tax treatment, time-of-use tariffs, and annual rate changes are excluded. A credit belongs in the form only when eligibility is known; a headline maximum credit may depend on tax liability or location. Net metering rules and export compensation can change, so do not value all output at the retail rate without checking the agreement.
Mistakes that change the answer
Do not enter watts as annual kWh, or a percentage as a dollar rate. The $0.18 field means eighteen cents, not 18. Do not subtract a loan payment from generation savings; financing should be evaluated as a separate cash-flow layer. A zero electricity rate correctly cannot produce a payback period because there is no modeled saving to recover the cost.
Scope of this page
This page estimates recovery of a solar installation’s upfront cost. It does not size a system, calculate a battery’s autonomy, or compare an investment portfolio. It runs locally and is an educational estimate, not an installation, tax, or financial recommendation.
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