Number of panels, system size, roof area, and seasonal production curve to cover monthly consumption.
Seasonal factors are northern-hemisphere defaults; flip the months mentally for the southern hemisphere. Real production also depends on tilt, orientation, shading, and panel temperature.
Sizing a residential solar PV system is the gating decision for any rooftop install. Get it right and the array offsets the household's electricity bill predictably for 25–30 years; oversize it and you have wasted capital on capacity that gets clipped by the inverter or net-metered back at retail-minus-export-fee rates; undersize and the household keeps paying retail electricity for the missing kilowatt-hours. Installers sell their own quotes, but homeowners who want to sanity-check the proposal — or who are early in the research phase — need an independent estimate. This calculator does that estimate from the four pieces of information any homeowner can supply in two minutes: monthly consumption, panel wattage, peak-sun-hours at the location, and a system-loss assumption.
The output is the number of panels needed to match annual consumption, the system DC kilowattage, the rough roof area required at typical panel dimensions, the annual energy yield, the approximate installed cost at a current per-watt installed cost, and a 12-month production curve overlaid against the constant monthly consumption target. The seasonal curve is the most informative of these — it makes the summer-overproduction-winter-shortfall shape of solar production visible, which is what drives net-metering economics and battery sizing.
The estimator uses the standard "PVWatts-style" simple model:
Peak sun hours (PSH) is the daily insolation expressed as the equivalent number of hours at 1 000 W/m² — the standard test condition for panel ratings. Typical values: 4.5–5.5 PSH for sunny mid-latitude locations (US Southwest, southern Spain), 3.5–4.5 for moderate climates (most of France, Italy, US Midwest), 2.5–3.5 for cloudy northern Europe, < 2.5 for very cloudy or far-north locations. Numbers come from databases like NREL NSRDB or PVGIS.
System losses combine module mismatch, wiring resistance, inverter efficiency (~96 %), soiling (2–5 %), shading (variable), temperature derating (panels run hotter than STC and lose ~0.4 %/°C above 25 °C), and module degradation (~0.5 %/year). 14–20 % is typical for new installs; 25 %+ for shaded or hot-climate roofs.
Seasonal factor curve (Northern hemisphere): production peaks May–July at ~1.4× annual mean, dips November–January at ~0.5×. Southern hemisphere mirrors. The curve is empirical; real arrays vary depending on tilt, orientation, and altitude.
Enter your monthly consumption in kWh — pick a winter month if you want a conservative size, an annual average for net-metering balance, or a summer month for a self-consumption-only system. Enter the panel wattage (300–450 W typical for residential modules in 2024–2026; 600 W+ for newer commercial-size modules). Enter the average peak sun hours per day for your location (PVGIS or local installer estimates). Enter the system loss percentage. Enter the panel area in m² (roughly 1.7–2.1 m² per residential panel). Enter the installed cost per watt in your local currency (EU 1.2–2.0 €/W, US 2.5–3.5 $/W including labor and inverter, regional variation huge).
The result panel shows panels needed, system DC size, roof area, annual production, and approximate installed cost. The chart is a 12-bar monthly production curve compared against the flat monthly consumption target — bars in green over-produce, bars in orange under-produce, with a dashed red target line.
Suburban house, 800 kWh/month consumption, 400 W panels, 4.5 PSH, 20 % losses, 1.95 m² per panel, 1.6 €/W installed.
Seasonal curve at this location: - January: 19 × 43.78 × 0.55 = 458 kWh — short of 800 by 342 kWh. - July: 19 × 43.78 × 1.40 = 1 165 kWh — surplus of 365 kWh. - Annual surplus/shortfall roughly cancels — this is what net-metering is built on.
Apartment with 300 kWh/month and lower PSH (3.8): 8 panels of 380 W work out to 3.0 kW DC, 14.8 m², about € 4 560.
Sunny large home: 1 500 kWh/month at 5.2 PSH and 18 % losses with 450 W panels: 27 panels = 12.15 kW DC, 56.7 m² roof, ~€ 18 225.
PSH is annual average, not constant. Sizing on summer PSH oversizes; on winter PSH undersizes. The calculator uses one number — pick a yearly average for net-metering, monthly minimum for off-grid.
Tilt and orientation. PSH is for an optimally-tilted south-facing array (Northern hemisphere). East-west, flat, or shaded roofs need a derate (usually 5–20 % below optimal).
Net-metering rules vary. Some jurisdictions credit exports at retail rate (1:1 net metering); others at wholesale (avoided cost, ~1/3 of retail); some have export caps. The "annual production = annual consumption" sizing assumption is meaningful only with full net metering.
Inverter clipping. Oversizing DC against inverter AC capacity (DC/AC ratio > 1.2) is intentional in cloudy climates to capture more morning/evening energy, but clips peak sun. The calculator doesn't model clipping.
Battery sizing is separate. If you want self-consumption / blackout backup, batteries are sized from the daily-shortfall integral, not from monthly totals.
Degradation and temperature. The calculator's loss factor is a static average. Real systems degrade ~0.5 % per year (so a 20-year-old panel produces 90 % of new). Hot rooftops in tropical climates can exceed 25 % losses.
Panel datasheet "STC" rating. Panels are rated at 25 °C; rooftop temperatures hit 50–70 °C in summer, knocking 10–15 % off the rated power. This is folded into "system losses" but worth knowing.
Roof structural and shading constraints. Sizing the panels by area ignores skylights, dormers, plumbing vents, chimneys, and shading from trees or adjacent buildings. Real installable panel count is often 70–90 % of the geometric maximum.
Permit and interconnection costs. The cost-per-watt input is for the equipment + labor; permit, interconnection, and engineering can add 5–15 %. Do not budget at exactly the calculator's number.
Currency and incentive blindness. The calculator gives a sticker price. Federal/state/regional rebates, tax credits, and feed-in tariffs can drop net cost by 20–50 %. Layer those on top.