About PanelTally

PanelTally answers one question, how many solar panels a given bill and a given sky require, with one formula, run by the same code in the calculators and in the static tables. This page lists every constant in that formula so you can check our work, and says plainly what the site is not.

The formula

Panels = monthly kWh ÷ 30.4 days ÷ (panel kW × peak sun hours × 0.77), rounded up. That is the whole engine. The 30.4 is the average month. The 0.77 is NREL's quick-estimate derate from rated DC watts to delivered AC energy, covering system losses (soiling, wiring, mismatch, about 14%), inverter conversion, and temperature; the output guide itemizes it. Roof area plans at 21 sq ft per 400W-class panel, glass plus racking gaps, setbacks not included.

The sun figures

State peak-sun-hour values are statewide annual averages we compiled from NREL solar resource (GHI) data, rounded to 0.1 hours, shown ranked on the sun-hours page. Statewide averages blur real spread, and each state page carries its largest city's figure and says which way the rest of the state leans. For an address-exact number, NREL's PVWatts is free and better than any table, ours included; we link it everywhere because a site survey should beat a statewide average every time.

The usage assumptions

Where a page needs a household figure, we use EIA's average of about 900 kWh a month. The house-size pages scale that assumption sub-linearly with floor area (our call, stated on each page) because usage grows slower than square footage. Appliance pages state their per-load planning figure and its basis (ENERGY STAR ranges, nameplate draw times duty cycle) inline, next to the number, where distrust can find it.

What this site is not

A quote, an engineering design, or financial advice. Panel counts here are planning numbers for sizing expectations and refereeing bids. Roof direction, pitch, and shade move real output by double-digit percentages (the roof guide has the percentages), interconnection rules vary by utility, and nothing here estimates payback, incentives, or taxes, on purpose. When our number and an installer's site survey disagree, the survey wins, and you should ask them to show the sun figure that drove it.

Who runs this

PanelTally is one of a family of small single-topic calculators. It takes no money from panel makers or installers; if referral links appear, they are marked sponsored and never change the math. Corrections are the most useful mail we get: if a figure here disagrees with your measured data, the contact page is open.