The Honest Answer First
The number of solar panels you need depends on three things: how much energy you use each day (in Wh), how much sun your location gets (peak sun hours), and the wattage of the panels you buy. Once you have those three numbers, the math is straightforward.
Step 1: Calculate Your Daily Energy Use
List every device you want to power, multiply its wattage by the hours it runs daily, and sum the results. For example: a 150 W fridge cycling 8 hours a day = 1,200 Wh; a 60 W laptop running 4 hours = 240 Wh; LED lighting at 20 W for 5 hours = 100 Wh; phone charging = 20 Wh. Total: roughly 1,560 Wh/day.
| Load | Watts | Hours/day | Daily Wh |
|---|---|---|---|
| Refrigerator (cycling) | 150 | 8 | 1,200 |
| Laptop | 60 | 4 | 240 |
| LED lights (4) | 20 | 5 | 100 |
| Phone | 10 | 2 | 20 |
| Router | 10 | 24 | 240 |
| Total | 1,800 Wh |
Step 2: Find Your Peak Sun Hours
Peak sun hours (PSH) is the number of hours per day when sunlight intensity equals 1,000 W/m². The National Renewable Energy Laboratory publishes PSH maps; most of the southern U.S. gets 4.5–6 PSH daily, while the Pacific Northwest and Northeast average 3–4 PSH.
| Region | Avg PSH/day |
|---|---|
| Southwest (AZ, NM, NV) | 5.5–6.5 |
| Southeast (FL, TX, GA) | 4.5–5.5 |
| Midwest (OH, IL, MO) | 3.5–4.5 |
| Pacific Northwest (WA, OR) | 3.0–3.5 |
Step 3: Size the Array
Divide daily Wh by PSH, then divide by panel wattage, then add 25% for system losses (inverter inefficiency, wiring, dust, heat):
Formula: Panels = (Daily Wh ÷ PSH ÷ Panel W) × 1.25
Example: 1,800 Wh ÷ 5 PSH ÷ 400 W × 1.25 = 1.125 panels → round up to 2 panels of 400 W.
Don’t Forget Battery Storage
Panels only produce power during the day; your loads run around the clock. For every kWh of daily use, plan on 1–1.5 kWh of battery storage so you can run devices overnight and through cloudy stretches. A typical grid-tied home with battery backup targets 10–15 kWh of LiFePO4 storage; an off-grid cabin needs 2–3 days of autonomy (3–4× daily Wh). Lead-acid batteries are cheaper upfront but only deliver 50% of rated capacity versus 80–90% for LFP, so the per-usable-Wh cost gap is much smaller than sticker prices suggest.
Real-World Sizing Examples
| Daily Need | PSH 6 | PSH 4 | PSH 3 |
|---|---|---|---|
| 1,000 Wh | 1 × 200 W | 1 × 300 W | 2 × 200 W |
| 3,000 Wh | 2 × 300 W | 3 × 300 W | 4 × 300 W |
| 5,000 Wh | 3 × 300 W | 2 × 400 W | 5 × 400 W |
| 10,000 Wh | 2 × 400 W + 2 × 300 W | 4 × 400 W | 9 × 400 W |
Important Caveats
- Winter reduces output by 20–50%. Add 1–2 extra panels if your system must perform year-round.
- Shade kills output. Even 10% shade on one panel can drop a series string’s output by 30%+. Use micro-inverters or optimizers if shade is unavoidable.
- Angle matters. A panel tilted at your latitude ± 15° produces 10–15% more than a flat-mounted panel.
- Panel temperature hurts. Output drops 0.3–0.5% per °C above 25°C. Hot roofs underperform cool ones.
Editor’s Picks: Recommended Accessories
- citicr Portable Charger 10000mAh PD20W — small USB backup so phones never drop to zero.
- INIU Laptop Power Bank 20000mAh 65W — laptop power for monitoring a freshly installed array.
- Anker Laptop Power Bank 25000mAh 165W — premium high-output USB-C bank.
- WOLFBOX MegaFlow Air Duster — removes dust and pollen from panels to keep output near rated.
Summary
Calculate daily Wh, look up your peak sun hours, then divide and round up with a 25% loss margin. A typical 2,000–3,000 Wh/day home runs on 4–6 panels of 300–400 W in the sun belt, and 6–10 panels in the north. Size one or two panels larger than the math suggests, and your system will keep producing when dust, clouds, and winter reduce output.