"How many solar panels do I need for off-grid?" is one of the first questions people ask when planning a standalone system. The answer depends on your daily energy use, peak sun hours where you live, and the wattage of the panels you choose. This guide walks you through the math and gives you a clear path to the right panel count.
The Three Numbers You Need
Before you can size your array, you need three inputs:
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Daily energy use in watt-hours (Wh). How much energy do you use per day? This comes from a load list: each appliance’s watts × hours run per day, summed. If you’re not sure, see our guide on how to calculate daily energy use and building a simple load list.
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Peak sun hours. This is the number of hours per day your location receives the equivalent of full sun (1,000 W/m²). It varies by place and season. We explain it in detail in peak sun hours explained. For rough sizing, use a conservative value (e.g. 3–4 for many temperate regions, or your worst month).
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Panel wattage. Common off-grid panels are 100 W to 400 W. The wattage you choose affects how many panels you need: higher-wattage panels mean fewer panels for the same total power.
The Basic Formula
Array size (W) = Daily use (Wh) ÷ Peak sun hours ÷ System efficiency
System efficiency accounts for losses (typically 0.7–0.85): wiring, charge controller, battery round-trip, dust, and temperature. Using 0.75 is a safe default.
Example: 2,000 Wh/day, 4 peak sun hours, 75% efficiency:
- Array size = 2,000 ÷ 4 ÷ 0.75 ≈ 667 W
- With 200 W panels: 667 ÷ 200 ≈ 4 panels (round up to 4, or 800 W for margin)
So in this example you’d need at least four 200 W panels (or the equivalent in other wattages).
Why Round Up and Add Margin
- Round up so you don’t undersize (e.g. 3.4 panels → 4 panels).
- Add 10–20% margin if you want headroom for cloudy days, future loads, or battery charging in limited hours. Oversizing the array a bit is cheaper than running short every winter.
What About Batteries?
Panels only produce when the sun shines. For power at night and on cloudy days, you need a battery bank. Sizing the battery is separate: it depends on days of autonomy and depth of discharge. Our off-grid solar sizing calculator does both panel and battery sizing in one place.
Next Steps
- Get your daily use (Wh) from a load list or our calculator.
- Look up peak sun hours for your location (worst month if you want year-round reliability).
- Apply the formula above, then divide by your chosen panel wattage to get panel count.
- Use WattSizing’s calculator to double-check and get battery, inverter, and charge controller recommendations.
You don’t need to guess—with daily use, sun hours, and panel wattage, you can confidently answer: how many solar panels do I need for off-grid?
