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 month-by-month figures on the state pages

Each state page carries a twelve-month curve. Its shape comes from NASA's POWER climatology (parameter ALLSKY_SFC_SW_DWN, the SYN1DEG twenty-year monthly climatology covering January 2001 to December 2020, fetched 2026-08-08), sampled on the one-degree grid POWER publishes. For each state we took every grid cell at least half inside the Census cartographic boundary (cb_2023_us_state_20m, 2023 vintage), capped at 20 cells picked farthest-point-first so a large state keeps its geographic spread, and combined them weighted by the cosine of latitude. States too small to fill four such cells use their four best-covered cells; Rhode Island fits inside one cell and its page says so. 689 cells across the fifty states.

Two limits, both load-bearing. POWER measures irradiance on a horizontal surface, which is not the same quantity as a tilted-array peak sun hour, so we publish the monthly figures as percentages of each state's own annual mean and apply that shape to our statewide peak-sun-hours number. The shape transfers; the absolute level does not, and we never mix them. Second, a fixed array tilted toward the equator flattens the seasonal curve compared with horizontal, because winter sun arrives at a better angle to a tilted panel than to flat ground. That means the winter dip shown on every state page is deeper than a real tilted array sees. We would rather state that plainly than quietly shrink the numbers by a fudge factor we cannot source.

The within-state spread on each state page is the gap between the sunniest and cloudiest sampled cell, in percent, with the compass position of each. It is the spread the sampling can resolve, which is a floor on the real one: a one-degree cell is about 55 miles of latitude and smooths whatever happens inside it.

Electricity prices and household use

The price, consumption and bill figures on the state pages come from EIA's Electric Power Annual, Table 5.A, "2024 Average Monthly Bill - Residential", built from forms EIA-861, schedules 4A-D, EIA-861S and EIA-861U. That is the 2024 calendar year, the most recent complete year EIA publishes, fetched 2026-08-08. Each figure is a state-wide average across every residential customer, so it blurs utility-by-utility differences, and on price those are large: two neighbouring territories in one state can differ by more than two states differ from each other. Treat the state number as the middle of a range, not as your tariff.

Where a page turns those into money, the sum is one 400-watt panel's yearly output times the state's average residential rate, and nothing else. No install cost, no incentive, no financing, no assumption about how a utility settles exported energy. It is the value of electricity not bought. It is not payback, and we do not calculate payback anywhere on this site.

The usage assumptions

Where a page needs a household figure, we use EIA's average of about 900 kWh a month. EIA's measured 2024 figure is 863 kWh; 900 is the rounded planning number this site has used from the start, and the two appear side by side on the state pages rather than being quietly reconciled. 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.