Solar Savings Calculator

How it works

Gather system costs, generation estimates, and tariff details

Enter the installed system price and any verified incentive that actually reduces the upfront cost. A later tax credit or uncertain rebate should not be entered as immediate cash without adjusting the scenario. System capacity is in kilowatts, while annual generation yield is kilowatt-hours per installed kilowatt per year. Use a site-specific estimate for yield rather than assuming the example represents your roof.

The annual-use input represents household electricity consumption. The self-use percentage is the share of solar generation you expect to use on site, subject to the annual consumption limit. Electricity you use on site is valued at the entered import price, while the remainder is valued at the export price. These rates may differ substantially. The model does not automatically establish your utility’s net-metering or billing arrangement.

Solar example: a 5 kW system with 7,000 kWh generation

A 5 kW system with a yield of 1,400 kWh per kW produces 7,000 kWh in its first modeled year. At 60% self-use and sufficient household demand, 4,200 kWh offsets imports and 2,800 kWh is exported. With an import price of 0.20 and an export credit of 0.05 per kWh, gross annual benefit is 840 + 140 = 980.

Subtracting maintenance of 150 leaves a first-year benefit of 830. For a net installed cost of 15,000, a constant 830 annual benefit would imply a simple payback of about 18.07 years. The actual projection in this tool also applies the degradation and tariff changes you enter, so its payback can differ from that fixed-benefit shortcut. Set both annual changes to zero to reproduce the shortcut.

Separate operating benefit from loan-funded cash flow

If part of the installation is financed, upfront cash equals net system cost minus the amount borrowed. A fixed monthly payment is calculated from the loan rate and term. Each year’s cash flow subtracts that year’s loan payments from the operating benefit. The cumulative cash series starts with the upfront outflow at year zero and then adds each annual net movement.

A loan can reduce the initial out-of-pocket cost while adding interest and future obligations. The result therefore shows remaining loan balance separately, especially when the projection ends before the loan is fully repaid. Cumulative cash is not the same as net wealth. The unfinanced payback metric compares operating benefits with the full net system cost and does not use the financing arrangement to make payback appear artificially immediate.

Interpret solar payback as an assumption-based projection

Generation degradation reduces output each year after year one. The tariff-change assumption adjusts both import and export prices annually. Maintenance is held constant. Unfinanced payback is interpolated within the first year when cumulative operating benefit recovers net system cost. A later period of negative benefit can reverse that recovery, so inspect the full schedule as well as the headline payback number.

Annual self-use is a simplified estimate rather than an hourly simulation of sunlight and household demand. Battery dispatch, seasonal billing, fixed utility charges, equipment replacement, tax deductions, and resale value are not automatically modeled. Use the linked generation and energy references to develop the inputs, then compare conservative and optimistic scenarios. The calculator’s example is a transparent arithmetic demonstration, not an installation quote or a savings guarantee.

Solar savings & financing: formula and assumptions

Generation = capacity × annual yield. Annual benefit = on-site kWh × import tariff + exported kWh × export tariff − maintenance. Cumulative cash also subtracts upfront cash and loan payments.

On-site generation is limited to the smaller of the entered self-use share and annual household consumption. Remaining generation is exported at the entered rate. Degradation and tariff changes compound annually. Incentives reduce cost immediately; later tax credits should not be treated as upfront cash. Financing uses fixed monthly amortization. No battery dispatch, hourly net-metering, tax benefit, or resale value is modeled.

Worked example

Reference example currency: USD. These fixed examples do not change with your calculator preferences.

Using the Solar savings & financing example:

Installed system cost ($)

15000

Verified upfront incentive or discount ($)

0

System capacity (kW)

5

First-year generation per kW installed (kWh/kW/year)

1400

Annual household electricity use (kWh)

9000

Generated electricity used on site (%)

60

Import electricity price per kWh ($)

0.2

Export credit per kWh ($)

0.05

Annual maintenance and other costs ($)

150

Annual import/export price change (%)

0

Annual generation degradation (%)

0.5

Projection length (years)

25

Amount financed ($)

0

Loan annual interest rate (%)

6

Loan term (years)

10

Generation = capacity × annual yield. Annual benefit = on-site kWh × import tariff + exported kWh × export tariff − maintenance. Cumulative cash also subtracts upfront cash and loan payments.

Example result

• Year zero shows your upfront cash. Unfinanced payback interpolates within the first recovery year using annual benefits and excludes financing. Later negative benefits can reverse a recovery.
• Cumulative cash is not net wealth: any remaining loan balance is shown separately. No resale value, later tax credits, battery replacement, or equipment replacement is assumed.

These are illustrative inputs. Change the values to explore a different scenario.

What to keep in mind

Use one currency throughout. Currency selection changes labels, not exchange rates. Calculate, then save a named scenario in My workspace to keep it in this browser; export a workspace backup to move it to another device.

Formula references

• US Department of Energy: homeowner’s guide to solar (opens in a new tab)
• PVWatts: location-specific PV generation estimates (opens in a new tab)

References explain the underlying methods; they do not imply endorsement or individual professional review of Finflo.

Common questions