The Solar Savings Calculator estimates what a solar installation generates and saves, and how long it takes to pay for itself, in either of two ways: start from a system you are quoted (its size in kW and your average peak sun hours), or start from your monthly electricity bill and let it size the system for you. Either way it reports annual generation, annual savings, gross cost, simple payback, panel count, and roof area.
How It Works
In direct mode the production model is deliberately simple:
Savings value that generation at your electricity rate, and simple payback divides the installation cost (entered, or estimated at $2.95 per watt) by the annual savings. In bill mode the calculator works backwards: annual usage is the bill divided by the rate times twelve, the system is sized to produce your chosen offset share of that usage at the production factor (default 1,300 kWh per kW per year), and cost follows from size at the cost per watt.
Parameters
| Input | Required | Meaning | |---|---|---| | System size (kW) + sun hours | Direct mode | The quoted system and your solar resource | | Monthly bill + rate | Bill mode | Sizes the system from your usage | | Electricity rate | Yes | 2.95/W | | Offset % / production factor | Bill mode | Default 100% and 1,300 kWh/kW/yr |
Worked examples
Direct mode: a 5 kW system under 4.5 peak sun hours generates 5 x 4.5 x 365 = 8,213 kWh a year; at 2,463.90 a year**, and against a 180 monthly bill at 16,225** at 2,145 a year** for a payback of about 7.56 years. The two modes' different paybacks for similar hardware are the lesson: value depends on your rate and how much generation you actually use, not on the panels alone. The companion Solar Panel Cost Calculator approaches the same decision from the bill side with its own defaults; running both is a good check that you understand which assumptions are moving your answer.
Peak sun hours: the input that carries the estimate
A peak sun hour is not an hour of daylight: it is one hour's worth of full-strength (1,000 W/m2) irradiation, accumulated across the day. Phoenix averages around 6 or more; Seattle around 3.5 to 4; the figure already absorbs season, latitude, and typical weather into a single daily average. Using daylight hours (10 to 14) instead inflates generation two to threefold and manufactures paybacks that will never occur, the single most common error in home solar arithmetic. Get your figure from an installer's site assessment or a recognized irradiance source for your postcode, and remember it describes an unshaded, well-oriented array: your roof's actual azimuth, pitch, and shading are then handled by installers as derating on top of this first-pass model, which is why this calculator's output is a screening estimate, and why two honest quotes for the same house can differ by 15% while both are defensible.
What simple payback leaves out, in both directions
Against the homeowner: exports are usually credited below the retail rate, so generation you do not self-consume is worth less than the savings formula assumes; panels degrade about 0.5% a year; an inverter replacement typically lands in years 12 to 15; and financing interest, where used, is a real cost. In the homeowner's favour: retail electricity prices historically rise, improving savings every year against a fixed installation cost; incentives (where they currently exist, noting the calculator models none) shorten payback directly; and the system's life of 25-plus years means most of its output arrives after payback, as near-free electricity. The balanced reading: treat the payback figure as a screening threshold (under roughly 8 to 10 years is generally attractive at typical rates, over 15 needs the non-financial motives to carry it), and let a site-specific quote with a stated production guarantee make the final case.
Frequently Asked Questions
Why is my payback blank?
Payback only exists when both installation cost and annual savings are positive. A zero rate, zero sun hours, or a zero cost leaves nothing meaningful to divide, so the calculator reports the other figures and leaves payback empty rather than printing a nonsense number.
Should I enter my installer's cost or use the $2.95 per watt estimate?
Your installer's figure, whenever you have one: it captures your roof's complexity, panel tier, and local labour. The per-watt default is a national-average placeholder for the screening stage, useful for comparing system sizes before quotes exist.
Does the calculator include feed-in tariffs or net metering credits?
No. Savings value all generation at the single rate you enter. If your tariff pays less for exports, the honest workaround is to lower the effective rate or the offset so the estimate reflects the self-consumed share, or model exports separately in your own spreadsheet.