The two numbers
1. Daily harvest, not panel rating. A panel's wattage is measured under standard test conditions — 1,000W per square meter at 25°C cell temperature — which is a laboratory, not a backyard. In real deployment expect meaningfully less: heat reduces output, the angle is rarely optimal, and the charge controller takes a share.
Our working figure, given as a band because the honest answer is a band: a 100W panel delivers roughly 400–500Wh across a good summer day, well placed. That is about 50–80% of what a naive rating-times-daylight calculation would suggest.
2. The station's solar input ceiling. Every station publishes a maximum solar input in watts. Exceed it and the extra panel does nothing. This is the hard cap on any array, and it is the first thing to check.
The arithmetic
Panels needed = battery capacity ÷ harvest per panel per day, then check the total against the station's input ceiling.
A 1,000Wh station at 450Wh of harvest per 100W panel needs 1,000 ÷ 450 = 2.2 panels to refill fully in a day. Round up: three 100W panels, or 300W of array. Two panels roughly refill it in a day and a half, which for most outages is enough.
Sizing table
| Battery capacity | Daily top-up (slows the decline) | Full refill in a day | Check the input ceiling is at least |
|---|
| 300 Wh | 100 W | 100 W | 100 W |
|---|
| 500 Wh | 100 W | 100–200 W | 200 W |
|---|
| 1,000 Wh | 100–200 W | 200–300 W | 300 W |
|---|
| 1,500 Wh | 200 W | 300–400 W | 400 W |
|---|
| 2,000 Wh | 200–300 W | 400–600 W | 600 W |
|---|
| 3,000 Wh | 400 W | 600–800 W | 800 W |
|---|
Computed at 400–500Wh of harvest per 100W of panel per good summer day. Winter, cloud and poor placement reduce all of these substantially — how much, and why. These are estimates from a stated assumption, not measurements; we have not tested any panel.
Top-up or refill? Decide which you are buying
A top-up array adds meaningfully to the battery each day without refilling it. It turns a one-day battery into a two- or three-day one, which for most outages is the whole requirement, and it is the cheaper and lighter option.
A refill array replaces a full day's consumption, making the system indefinitely sustainable in summer. It is roughly double the panel, and it is what you want for a genuine multi-day regional outage where nothing can be bought.
Most households are buying the first and pricing the second. Be clear which one your outage history justifies — how to find that out.
The four things that reduce your real number
- Angle. A flat panel loses a great deal against one tilted toward the sun, especially in winter when the sun is low. Most folding panels have adjustable legs — use them, and reposition once or twice through the day.
- Shade. Worse than it looks. Cells wired in series mean one shaded section can drag down the whole panel, so partial shade costs more than the shaded fraction.
- Heat. Panels lose output as cell temperature rises above 25°C, which is exactly what happens on the sunny days you are counting on. Leaving an air gap underneath helps.
- Dirt and snow. A dusting of snow takes output effectively to zero. Dust and pollen cost a few percent.
Bigger panels or more panels?
For the same total wattage, fewer larger panels are lighter in total, faster to set out, and involve fewer connections. More smaller panels are easier to carry individually, easier to position around obstructions, and cheaper to add incrementally.
For a fixed home setup, fewer larger panels. For something you carry to a campsite, several 100W panels. The camping scenario weighs that differently from a home one.
Check compatibility before you buy across brands
Three things have to line up, and a bundle exists partly to answer them for you:
- Voltage inside the station's charge controller window. Too low and it will not charge; too high and it can damage the controller.
- Current not exceeding the input maximum.
- Connector matching, or adapting — usually MC4 or a proprietary plug.
Manufacturers publish an accepted input range for exactly this reason. Whether to buy a bundle or the parts separately.
