How Many Solar Panels Do I Need? A Complete Sizing Guide

If you're considering solar, the first question is always the same: "How many panels do I need?" It's a deceptively simple question with a frustrating answer — it depends. But once you understand the three variables that drive the calculation, you can size a system confidently in under five minutes.

The three variables that determine panel count

Solar panel sizing comes down to a simple equation: panels needed = daily kWh needed / per-panel daily kWh output. Each side of that equation has a few inputs.

Your daily kWh needed comes from your electricity bill. Take your average monthly kWh and divide by 30. A typical US household uses 900 kWh/month, so the daily need is 30 kWh. South African households average 600–1,100 kWh/month depending on municipality and geyser type.

Per-panel daily output depends on the panel wattage, your local peak sun hours, and system losses. The formula is (panel watts / 1000) × peak sun hours × (1 − losses). For a 400W panel in a 5-PSH climate with 14% losses: 0.4 × 5 × 0.86 = 1.72 kWh/day per panel.

So a 30 kWh/day household needs 30 / 1.72 = 17.4 panels, rounded up to 18. Total system size: 7.2 kW. That's a typical US residential system.

Peak sun hours, not daylight hours

The most common mistake in solar sizing is using daylight hours instead of peak sun hours (PSH). PSH measures the equivalent number of hours per day when solar irradiance averages 1,000 W/m² — the standard test condition for rating panels. A clear summer day in Phoenix might give 7 PSH even though the sun is up for 14 hours, because morning and evening sunlight is much weaker than midday sun.

Annual PSH averages smooth out seasonal variation. The continental US ranges from 3 PSH (Pacific Northwest, Northeast) to 7 PSH (Southwest desert). South Africa ranges from 4.5 (Cape Town winter) to 6.5 (Northern Cape). Use annual averages for sizing — designing for summer peak will leave you short in winter.

The NREL PVWatts calculator and the South African Solar Radiation Atlas are the authoritative sources for PSH data. Most solar installers will look up your exact location, but having a rough number going in prevents being upsold.

System losses: why the calculator isn't naive

A naïve calculation would multiply panel wattage by sun hours and call it done. Real systems lose 10–20% of theoretical output to inverter conversion, wiring resistance, soiling (dust), thermal derating, and module mismatch. The default 14% in our calculator captures industry-typical losses.

Losses vary by climate. Hot climates lose more to thermal derating (panels lose 0.3–0.4% efficiency per °C above 25°C). Dusty or rainy climates lose more to soiling. Without regular cleaning, desert systems can lose 5–8% to dust alone. Cool, rainy climates like the US Pacific Northwest often get away with 10% losses.

Panel wattage: bigger isn't always better

Modern residential panels range from 400W to 540W. Higher wattage means fewer panels, but the physical area per panel is also larger — so the total roof footprint is roughly the same. The advantage of higher-wattage panels shows up when roof space is constrained: 18 × 400W panels vs 14 × 540W panels produce similar total energy but the second array fits on a smaller roof.

Efficiency (the % of sunlight converted to electricity) matters more than raw wattage when space is tight. Premium panels like SunPower Maxeon (22.6%) and REC Alpha (22.6%) deliver more watts per square meter than commodity panels (20–21%). The premium costs about 30% more per watt but produces 10% more per square meter — worth it for small roofs, not worth it for large roofs.

What about net metering?

If your utility offers 1:1 net metering (full retail credit for exports), size for 100% of annual usage. If they offer reduced net metering (avoided-cost rates of $0.03–0.08/kWh), size for 70–85% of usage — overproducing wastes money. If they offer no net metering at all, you need a battery to use your solar — see the Off-Grid System Designer.

California's NEM 3.0 (April 2023) is the cautionary tale: it cut export rates from ~$0.30/kWh to ~$0.08/kWh overnight, extending payback periods from 6 years to 10–12 years. If your jurisdiction is considering similar reform, size for self-consumption, not export.

Putting it together

The honest answer to "how many panels do I need" requires three numbers from you (monthly kWh, location, panel choice) and one assumption (14% losses). Plug them into our Solar Panel Sizing Calculator and you'll have a defensible number in seconds. Use that number to sanity-check installer quotes — if a salesperson proposes 30 panels when your math says 18, ask why.

For the financial question — is solar worth it over 25 years — see our Solar Savings Calculator. For sizing a system that includes battery backup, see the Off-Grid System Designer.