How to Size Your Solar Panels (Step-by-Step)
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We test solar panels, power stations, and DIY kits in real-world conditions and share practical, step-by-step guidance.
The Panel Sizing Formula
Use this quick formula to estimate how many panels you need:
Panels Needed = Daily kWh ÷ (Panel Watts × Sun Hours × Loss Factor ÷ 1000)Loss Factor: Use 0.75 by default; adjust between 0.7–0.85 based on climate, wiring, and inverter quality. Prefer a conservative value if you have shading or high summer roof temps.
Or skip the math and use our Panel Sizing Calculator for an instant estimate.
Worked Example
Scenario: A small home uses 18 kWh/day. Location receives 5 peak sun hours. You’re considering 400W panels and prudently assume a 0.75 loss factor.
Per‑panel kWh/day ≈ 400 W × 5 h × 0.75 ÷ 1000 = 1.5 kWh
Panels needed ≈ 18 ÷ 1.5 = 12 panels (≈ 4.8 kW array). Add 10–20% for growth or sub‑optimal siting → 13–14 panels.
Key Inputs Explained
- Daily kWh: Read from your utility bill or add up appliance loads. Track seasonal swings.
- Peak Sun Hours: Varies by location; 3–6 is common. Southern states enjoy more, northern fewer.
- Panel Watts: 350–450W is typical for modern modules; higher wattage reduces panel count.
- Loss Factor: Captures wiring, heat, inverter, and soiling losses. Start at 0.75.
RVs & Off‑Grid Considerations
Space and weight constraints matter. Favor high‑efficiency panels and energy‑dense batteries. Pair panels with a portable power station like the Jackery Explorer 1000 Pro for a compact, plug‑and‑play system.
Recommended Products
| Product | Best For | Features | Link |
|---|---|---|---|
| Jackery Explorer 1000 Pro | Overall Pick | 1002Wh, fast charging, portable | Buy on Amazon |
| Renogy 100W Kit | Starter Arrays | Durable, simple wiring | Buy on Amazon |
FAQs
How accurate is the 0.75 loss factor?
It’s a pragmatic average. Cooler climates and premium inverters can justify 0.8–0.85; hot roofs with shading may use 0.7.
How do seasons change my panel count?
Winter has fewer sun hours; consider a buffer (10–20%) or add a few panels if you need consistent winter output.
Do I need a bigger inverter than array size?
Many size inverters at 80–120% of array DC rating depending on clipping tolerance and peak production goals.