About Solar PV System Sizing Calculator (Panels from Your Bill)
The solar PV sizing calculator answers "how many solar panels do I need" from the number you already have: the kWh on your electricity bill. It converts monthly or annual usage to a daily average, divides by your site's peak sun hours, and applies the NREL PVWatts default system-loss derate of 14% to get the DC array size in kilowatts, then rounds up to whole panels at your panel wattage.
Because the installed array is whole panels, the tool also reports what that array actually does: the installed kW DC, a roof-area estimate at a representative 21.5 ft2 per 400 W-class panel, the annual kWh the array produces at your sun hours, and the share of your usage it covers — typically a few percent above your target because of the round-up. Set the usage offset below 100% for a partial system or above it to bank net-metering credits.
How It Works
- Enter your electricity usage from a bill — monthly kWh (default) or annual kWh via the toggle; the entry converts automatically when you switch.
- Enter your site's peak sun hours per day. Typical US annual averages run roughly 3.5–4 (Pacific Northwest, Northeast), 4–4.5 (Midwest, Mid-Atlantic), 4.5–5.5 (Southeast, Plains), and 5.5–6.5 (Southwest desert); look up your exact site value in NREL solar resource data or run it through PVWatts.
- Keep the PVWatts default 14% system losses unless you have a reason to change them — they cover soiling, shading, mismatch, wiring, and the rest of the DC-to-AC chain — and set the usage offset (100% covers the whole bill).
- Pick your panel wattage (400 W is the common residential class today). The tool computes the target array kW DC, rounds up to whole panels, and reports the installed array, roof-area estimate, annual production, and achieved offset.
- The array size is DC (nameplate) kilowatts, the convention solar quotes use; the losses figure already accounts for converting that DC rating into delivered AC energy.
Worked Example
A home uses 900 kWh per month (10,800 kWh/yr) at a site with 5 peak sun hours. Daily usage is 900 × 12 ÷ 365 = 29.59 kWh. With the PVWatts 14% losses and a 100% offset, the target array is 29.59 ÷ (5 × 0.86) = 6.88 kW DC. At 400 W per panel that is 17.2 panels, rounded up to 18. The installed array is 18 × 400 = 7.2 kW DC, needing roughly 18 × 21.5 = 387 ft2 of roof. It produces 7.2 × 5 × 365 × 0.86 = 11,300 kWh per year — about 105% of the usage, slightly over target because of the whole-panel round-up.
Formulas
- Daily usage
E_daily = E_monthly x 12 / 365 (or E_annual / 365)- Target DC array size (PVWatts inverted)
P_dc = E_daily x (offset/100) / (PSH x (1 - L/100))- Panel count and installed array
n = ceil(P_dc x 1000 / W_panel); P_installed = n x W_panel / 1000- Roof area estimate
A = n x 21.5 ft2- Annual production and achieved offset
E_prod = P_installed x PSH x 365 x (1 - L/100); offset_achieved = E_prod / E_annual x 100
Standards & References
- NREL PVWatts v5 methodology (Dobos, 2014) -- energy model and the 14% default system losses
- NREL solar resource data -- site-specific peak sun hours
Frequently Asked Questions
How many peak sun hours should I use?
Peak sun hours are the day's total solar energy expressed as hours of full 1,000 W/m2 sun. Typical US annual averages by region: Pacific Northwest and Northeast about 3.5–4, Midwest and Mid-Atlantic about 4–4.5, Southeast and the Plains about 4.5–5.5, and the desert Southwest about 5.5–6.5. These are representative ranges — tilt, azimuth, and microclimate move the number, so look up your exact site in NREL solar resource data or run your address through the PVWatts calculator.
What do the 14% system losses cover, and where does the number come from?
It is the NREL PVWatts v5 default: soiling 2%, shading 3%, snow 0%, mismatch 2%, wiring 2%, connections 0.5%, light-induced degradation 1.5%, nameplate rating 1%, age 0%, and availability 3%, combined multiplicatively to 14.08% and presented as 14%. Together with the inverter efficiency PVWatts models separately, this is the whole DC-to-AC chain — so the array size here is DC nameplate kilowatts, the number solar quotes are written in.
Is the array size DC or AC?
DC. The calculator sizes the array in DC nameplate kilowatts — the sum of panel ratings — which is the convention quotes, permits, and $/W pricing use. The losses percentage is what turns that DC rating into delivered AC energy in the production estimate, so you should not derate the array size again for the inverter.
How accurate is the roof-area estimate?
It is representative, not a layout. The tool uses 21.5 ft2 per panel, in line with 400 W-class residential modules (a Q.PEAK DUO BLK ML-G10+ 400 measures 74.0 x 41.1 in, about 21.1 ft2), but products range from roughly 19 to 23 ft2 and a real roof loses usable area to fire setbacks, vents, hips, and shading. Treat the figure as a first check that the array plausibly fits, then confirm with your panel datasheet and a layout.
Should I size for more than 100% of my usage?
Sometimes. An offset above 100% banks surplus generation as net-metering credits or covers planned loads such as an EV or a heat pump. But check your utility first: many tariffs cap system size near 100–120% of historical usage, and where exports are credited below the retail rate, deliberate overproduction pays back poorly. The 200% cap here is headroom for those planned-load cases, not a recommendation.
Why does the achieved offset come out slightly above my target?
Because panels are bought whole. The target array for the worked example is 6.88 kW, but 18 x 400 W panels install 7.2 kW, so production lands near 105% instead of exactly 100%. The gap is largest with big panels on small systems; entering a smaller panel wattage narrows it. The calculator always rounds up, so the installed array meets or exceeds the target.