How SolarMath calculates: the method behind every number

SolarMath uses the same arithmetic a good installer uses — then shows it to you. No black boxes, no “up to” savings, no production figures from the sunniest day of the year.

Sunshine: peak sun hours, not vibes

Each state carries an annual-average peak sun hours figure — the equivalent hours of full-strength sun per day, averaged across the whole year including winter and cloudy spells. Arizona’s 6.5 and Washington’s 3.9 are why identical systems produce wildly different savings in different states.

Annual production ≈ kW × sun hours × 365 × 0.80

The 0.80 factor is the honest part: inverter losses, panel temperature, dust, wiring and imperfect roof angles typically cost 15–25% versus laboratory ratings. Quotes that skip it overpromise by exactly that much.

Prices: 2025 US installed costs

Residential systems averaged roughly $2.50–$3.10 per watt installed in 2025 before incentives; we default to $2.85/W. Premium panels, batteries, ground mounts and steep or tiled roofs push above the range; large simple systems in competitive markets land below it.

Every price input is editable — plug in your real quote.
Compare quotes on $/W, never on the monthly payment.
Cash price is the benchmark; financing adds dealer fees.

Rates: your state’s real average

State selects pre-fill with 2025 average residential electricity rates — about 12¢/kWh in Utah and Idaho, around 31¢ in California and Massachusetts, over 40¢ in Hawaii. Your actual rate is on your bill; edit it, because savings scale almost one-for-one with the rate you avoid paying.

Payback and savings tools add rate escalation (default 3%/yr, roughly the long-run US trend) and panel degradation of 0.5%/yr, so year 25 is modelled honestly, not copy-pasted from year 1.

The formulas, tool by tool

System size

Monthly kWh = bill ÷ rate → kW needed = (annual kWh × offset %) ÷ (sun hours × 365 × 0.80)

First your bill is converted to energy: a $180 bill at 20¢/kWh means roughly 900 kWh a month. Then the tool divides the yearly energy you want to produce by your state’s specific yield — the kWh one kilowatt of panels realistically generates in a year where you live. Finally the kW figure is rounded up to whole panels at the wattage you select, because you cannot buy 0.4 of a panel.

Payback

Net cost = (kW × 1,000 × $/W) − incentives · Savings(year n) = production × 0.995ⁿ × rate × (1 + escalation)ⁿ

The tool estimates year-1 production from your state’s sunshine, then walks year by year: production shrinks by the degradation rate, the electricity price grows by your escalation rate, and the year’s bill savings accumulate until they cover the net cost. The break-even point is interpolated within the final year, so “9.4 years” means the system pays for itself about five months into year ten.

Panel count

From roof: panels = ⌊usable sq ft ÷ 21⌋ · From usage: panels = ⌈annual kWh ÷ (panel kW × sun hours × 365 × 0.80 × direction factor)⌉

In roof mode the tool divides your usable area by the realistic per-panel footprint including setbacks. In usage mode it divides your yearly consumption by one panel’s realistic yearly output in your state, adjusted for your roof’s direction — a west-facing panel produces about 15% less than a south-facing one, a north-facing one about 35% less, and the count rises to compensate. Either way, total kW = panels × panel wattage ÷ 1,000.

25-year savings

Value of a kWh = self-used × retail rate + exported × retail rate × export % · Production(year n) = year-1 × 0.995ⁿ

Each year the tool values your production in two parts. Self-consumed electricity avoids a purchase at the full retail rate. Exported electricity earns your export percentage of that rate — 100% under classic net metering, often 20–40% under net billing, near zero where exports are compensated at wholesale avoided cost. Prices escalate yearly; production degrades yearly; upkeep and the year-13 inverter swap are subtracted as they occur.

The SolarMath method, in five lines

Production: state-average peak sun hours × 365 × 0.80 performance factor — losses included, always.
Prices: 2025 US installed average ≈$2.85/W (typical range $2.50–$3.10), editable in every tool.
Incentives: default $0. Federal residential credit ended after 2025; only verified state/local help should be added, by you.
Exports: valued at the rate your tariff actually pays — never silently assumed to be full retail.
Long view: 3% default rate escalation, 0.5%/yr degradation, inverter replacement in year 13 of projections.

Why two calculators give different answers

Run the same house through three solar calculators and you will get three different systems, prices and paybacks. The gap is rarely dishonesty — it is five quiet choices, and knowing them lets you interrogate any estimate, including ours:

The production model. How much a kilowatt makes in a year depends on the sunshine data and the performance factor applied to it. We use state-average peak sun hours × 365 × 0.80, losses included — the full state table is printed on the System Size Calculator page. A tool that skips the losses factor shows about 25% more production from the same roof.
The price per watt assumed. Estimates inherit whatever $/W the tool defaults to. Ours is $2.85/W, the 2025 US average; a tool assuming $3.30/W makes the same system look ≈16% dearer, and one assuming $2.40/W makes it look like a bargain no installer may actually offer.
The rate basis. Savings are production × the price of electricity. Tools differ on whether they use your state’s average rate, a national average, or your real tariff — and on whether they escalate it. We pre-fill your state’s 2025 average and escalate at 3%/yr by default; switching escalation off lengthens every payback on this site.
The incentive rules. This is the 2026 fault line. A calculator still subtracting the 30% federal residential credit — which ended on 31 December 2025 — shows a system about $8,800 cheaper on a $29,300 install, and a payback years shorter, than the rules now allow. Ours subtracts no federal credit, ever.
Panel wattage and rounding. Systems are sold in whole panels. Whether a tool rounds a 9.3 kW need up to 23 × 410 W panels (9.43 kW) or quotes the fractional figure changes the price, the production and the payback slightly — small, but real.

When two answers disagree, ask which of the five differs. The honest tool can always tell you.

What we never assume

No federal residential tax credit (rules changed after 2025).
No 1:1 net metering unless your state’s rules actually offer it — the projector lets you set your real export credit.
No “free” panels, no $0 bills, no 40-year panel life.
No home-value uplift and no carbon-credit income in savings totals.

Estimates only. A site survey and a written quote from a licensed installer are the final word — our job is to make sure you can check their maths.

Rates, prices and incentive rules checked November 2026.