Solar panel sizing is the part most people get wrong. Too few panels and your battery never fully charges. Too many and you've wasted money. Here's how to get it right.
The Formula
Solar Watts = Daily Energy (Wh) ÷ Peak Sun Hours ÷ System Efficiency
1,000 ÷ 5.5 ÷ 0.75 = 242W of solar panels
So you'd need approximately 250W of solar panels (round up to the nearest common panel size).
What Are "Peak Sun Hours"?
Peak sun hours (PSH) is not the same as hours of daylight. It's the number of hours where solar irradiance equals 1,000 W/m² — basically, hours of "full strength" sun.
| Region | Annual Avg PSH |
|---|---|
| US Southwest (AZ, NV) | 6.5h |
| US South (TX, FL, CA) | 5.5h |
| US Midwest (IL, OH) | 4.5h |
| US Northeast (NY, MA) | 4.0h |
| US Northwest (WA, OR) | 3.5h |
| UK | 3.0h |
| Central Europe | 3.5h |
| Southern Europe (Spain) | 5.0h |
| Australia (North) | 6.0h |
| Middle East | 6.5h |
Why 75% System Efficiency?
You lose energy at every stage:
- •Wire losses: 2-5%
- •MPPT controller: 3-5%
- •Battery charge/discharge: 5-10%
- •Temperature derating: 5-15%
- •Panel degradation: 2-5% (over time)
- •Dust/shading: 5-10%
Combined, you typically get about 75% of the panel's rated output into usable battery storage. This is a conservative (safe) estimate.
Sizing Examples
Weekend Camping (500 Wh/day, US South)
500 ÷ 5.5 ÷ 0.75 = 121W → Get a 100-200W folding panel
One 200W panel is plenty. Most camping power stations accept 100-200W solar input.
Full-Time RV (1,500 Wh/day, US South)
1,500 ÷ 5.5 ÷ 0.75 = 364W → Get 400W of panels
Two 200W panels, or one large 400W panel. Check your power station's max solar input.
Off-Grid Cabin (3,000 Wh/day, US Midwest)
3,000 ÷ 4.5 ÷ 0.75 = 889W → Get ~1,000W of panels
Multiple panels required. Consider fixed-mount roof panels for a cabin.
Home Emergency Backup (2,000 Wh/day, US Northeast)
2,000 ÷ 4.0 ÷ 0.75 = 667W → Get 600-800W of panels
Note: for emergency backup, solar is supplemental — you primarily charge from the grid before an outage.
Portable vs Fixed Panels
| Type | Watts Range | Best For |
|---|---|---|
| Folding portable | 60-200W | Camping, van life (store when not in use) |
| Rigid portable | 100-400W | RV roof, semi-permanent |
| Fixed mount | 200-600W per panel | Cabin roof, permanent install |
Max Solar Input: The Bottleneck
Your power station has a maximum solar input wattage. You cannot exceed this regardless of how many panels you connect.
Examples:
- •EcoFlow RIVER 2: 110W max → one 100W panel maximum
- •EcoFlow DELTA 2: 500W max → up to 500W of panels
- •BLUETTI AC200L: 1,200W max → you can go big
- •Jackery 2000 Plus: 1,200W max → multiple panels
Common Mistakes
- Using hours of daylight instead of peak sun hours — 12h of sun ≠ 12h of peak generation
- Ignoring system losses — Raw panel watts ≠ usable battery watts
- Exceeding max solar input — The excess is wasted (and may damage the unit)
- Not accounting for winter — If you need year-round power, size for your worst month
- Forgetting shading — Even partial shade on one cell can cut output by 50%+
Use Our Calculator
Our Power Calculator does all of this automatically. Enter your appliances, select your region, and it recommends the exact solar wattage based on your location's peak sun hours.
→ Get your solar panel recommendation