How this calculation works
Electricity used onsite avoids an import purchase; exported electricity earns the entered export payment. Those two values need not match. The same generation cannot be counted in both categories, so the two shares always sum to one.
Annual benefit = generation × [self-use share × import tariff + export share × export tariff] − maintenanceA worked example
5,000 kWh with 60 percent used onsite at 8 and the balance exported at 3 produces 30,000 gross benefit. After 2,000 maintenance the first-year saving is 28,000.
Good to know
Each year’s production falls by the entered degradation rate, while this model keeps tariffs and maintenance constant. Payback occurs only when accumulated savings cover the initial net outlay. Upfront support is an assumption you supply, not a statement of eligibility or a live government scheme.
Reference: US DOE — solar savings and payback (opens in a new tab) ↗
Value electricity according to where it goes
Electricity used onsite avoids an import purchase; exported electricity earns the entered export payment.
Those two values need not match. The same generation cannot be counted in both categories, so the two shares always sum to one.
The Heat pump vs boiler running cost calculator compares a separate electricity demand. If solar supplies that demand, avoid counting the same self-consumed kilowatt-hour as a saving in both project totals.
Read cumulative cash rather than one attractive year
Each year’s production falls by the entered degradation rate, while this model keeps tariffs and maintenance constant.
Payback occurs only when accumulated savings cover the initial net outlay. Upfront support is an assumption you supply, not a statement of eligibility or a live government scheme.
Test export value and maintenance separately
Set the self-use share to zero to value all production at the export tariff, then set it to one hundred to value it at the avoided import tariff. The generation input is unchanged. When the two tariffs are equal, changing self-use share should not change the calculated benefit.
If annual benefit is zero or negative, there is no positive cash saving to recover an initial outlay. The page reports payback as not reached within the selected horizon instead of presenting an infinite or negative number as a useful repayment date. Discounted project value requires a separate discount-rate model.
Work through the entered example
5,000 kWh with 60 percent used onsite at 8 and the balance exported at 3 produces 30,000 gross benefit. After 2,000 maintenance the first-year saving is 28,000.
Start with this case, then change one input and calculate again. Keep the other assumptions fixed so the result has an interpretable cause. The result table exposes intermediate quantities and units rather than hiding them behind a single headline number.
Use the output in a real plan
Each year’s production falls by the entered degradation rate, while this model keeps tariffs and maintenance constant. Payback occurs only when accumulated savings cover the initial net outlay. Upfront support is an assumption you supply, not a statement of eligibility or a live government scheme.
Before purchasing material or comparing alternatives, check the measurements at the point where they enter the calculation. A mathematically precise answer can still be a poor plan when the measured size, loss allowance or price basis is wrong. Save the input assumptions with any result you intend to reuse.
A few questions, answered
What is the calculation method?
Annual benefit = generation × [self-use share × import tariff + export share × export tariff] − maintenance
Does this use live prices or automatic measurements?
No. All measurements, prices and allowances come from your entries. The example values are illustrative and are not a recommendation for your situation.
Can I reuse the result for a different case?
Change the inputs and calculate again. Read the stated units and limits; the assumptions must still apply to the new case.