What Size Solar Do You Actually Need?
Undersize it and you're rationing by day three. Oversize it and you've spent good money on panels you'll never fully use. The number that gets it right isn't printed on the panel — it's your daily draw. Here's the arithmetic, with real figures.
Step 1 — work out what you use in a day
Everything starts here, and it's the step most people skip. Add up what your gear pulls over twenty-four hours. Typical figures for a 12V camp setup:
| What's running | Per day | Notes |
|---|---|---|
| 12V fridge — small, ≤40L | 30 Ah | The big one, every time. Climbs steeply in heat and every time the lid opens. |
| 12V fridge — medium, 40–60L | 40 Ah | |
| 12V fridge — large or dual-zone | 55 Ah | |
| LED lights | 4 Ah | Awning light plus interiors, an evening's worth |
| Phone / tablet charging | 3 Ah | Two people's devices |
| Fan | 7 Ah | Running overnight in warm weather |
| Cooking | 8 Ah | |
| Inverter, occasional AC | 8 Ah | Laptop charger, blender, the odd job |
| Hot water | 12 Ah | |
| CPAP | 20 Ah | Without a heated humidifier. One roughly doubles it. |
A common weekend setup — medium fridge, lights, two people's phones — lands around 47 Ah a day. Add a fan and you're around 55; add a CPAP and you're closer to 70.
Somewhere between a third and most of that is drawn after dark — the longer your nights, the bigger the share — which is a separate question worth getting right on its own — running your gear through the night covers what a night actually costs and how much of your battery you can safely spend.
Why the ranges are wide, and why nobody can narrow them for you. A fridge's draw depends on ambient temperature, how full it is, how often it's opened, and whether it's in the sun. The same 50L fridge can pull 30 Ah on a mild coastal night and 70 Ah in the afternoon desert. Anyone quoting you one number for "a fridge" hasn't asked where you're going.
Step 2 — the sizing sum
Your panels have to replace that draw inside the hours the sun's actually working. Three things go into it:
- Your daily draw — from step 1, in amp-hours.
- Peak sun hours — not daylight hours. It's how many hours of full-strength sun your day is worth. Typically 4–6 in the tropics. In a mid-latitude winter — most of Australia, New Zealand and the US — it's about 3–4 for a panel tilted towards the sun, but only half that for one lying flat, and far less again in northern Europe.
- A reality factor of 0.70 — panels don't deliver their rated watts into a battery. Heat, angle, dust and the charge controller all take a cut.
- A bit of headroom — about 15% — so you're sized for an ordinary day, not only a perfect one.
Panel watts ≈ (daily Ah × 12) ÷ (peak sun hours × 0.70) × 1.15
Worked through, for a 47 Ah day with 4 peak sun hours:
| 47 Ah × 12 V | 564 Wh to put back |
| ÷ 4 peak sun hours | 141 W |
| ÷ 0.70 for the real world | 201 W |
| × 1.15 headroom = 232 W, rounded up | 240 W |
So a fridge, lights and phones at 47 Ah a day wants around 240 W of panel. On a 3-sun-hour winter day, the same setup wants 310 W — which is why a rig that coasts through summer starts rationing once winter sets in.
For the battery, the sum works the other way: your daily draw × 1.5 days of autonomy, divided by 0.80 because that's the share of a lithium battery you can safely use. For a 47 Ah day that works out at 88 Ah, so a 100 Ah battery is the size to buy.
How you set it up swings the real-world figure by more than most upgrades do — a well-positioned 200 W beats a badly-placed 300 W on most winter days. Worth knowing before you buy more panel than you need.
Get a ballpark in ten seconds
Tell it where you're headed and what you run. It'll size the solar and battery to a typical day for that spot — then the app shows your real number once you're out there.
Why a 200 W panel isn't 200 W
The rating is a lab figure: square-on to bright sun, cool, clean. Out bush it sees none of those. Angle, heat, haze, dust and time of day all pull it down, and a panel running hot loses output as it warms — which is exactly when the sun is strongest.
70% is a fair working figure for a decent setup. A blanket lying flat in winter can drop below half. That's not a dud panel; it's the difference between the lab and a campsite, and it's why sizing to the sticker leaves you short.
Sanity-check it against the real thing
The sum above gets you a number to buy against. What it can't know is your actual spot, your actual angle, and what the sky is doing — the difference between 3 and 5 peak sun hours is the difference between 310 W and 190 W for the same gear. That one assumption changes the answer by about two-thirds.
Solar Spotter estimates what your panel will really make where you are, so you can check the assumption instead of finding out on day three. Three things it does that a sizing calculator can't:
- It uses your real latitude and date, not a regional average. Peak sun hours aren't a property of a country — they're a property of where you are and what month it is. A chart built for "Australia" or "the US" is averaging across thousands of kilometres and four seasons.
- It keeps working once you've left. The sizing happens at home with signal; the checking happens at camp without it. Sun position runs on the phone itself, and output carries on from the last forecast it saved, so the numbers don't stop when the bars do.
- It works wherever you're going. Any latitude, either hemisphere, any season. With Pro, plan a trip and it checks each stop against the forecast in the days before you get there.
Your data stays on your phone, and it's a one-off purchase rather than a subscription.
Common questions
How many watts of solar do I need for camping?
Work out your daily draw first — a medium fridge is 40 Ah a day, lights and phones another 7. For a typical 47 Ah day, the sum is (47 × 12) ÷ (4 peak sun hours × 0.70) × 1.15 ≈ 240 W. For winter, work it out again with winter's sun hours: at around 3, that's about a third more panel. Further from the equator, or with a panel lying flat, it can be double.
How many watts of solar to run a 12V fridge?
A 40–60 L compressor fridge pulls roughly 40 Ah a day in mild conditions, which needs about 200 W of panel on a good day. In real heat the same fridge can hit 70 Ah, pushing you toward 350 W. Fridges are the one load worth over-sizing for, because heat pushes the fridge's draw up while also taking a cut off your panel.
Is 200W of solar enough?
For a fridge, lights and phone charging in decent conditions — just about, with nothing to spare. It stops being enough when you add a fan or a CPAP, camp somewhere hot, or travel in winter, when sun hours fall to three or well below depending on how far you are from the equator. 200 W covers roughly a 40 Ah day at 4 peak sun hours; work out your own draw and see where you land.
Why isn't my panel making its rated watts?
Because the rating is a lab number — square-on to bright sun, cool and clean. Real conditions cost you: angle, heat, haze, dust, and the panel warming up as the sun gets stronger. About 70% of rated is a fair day for a well-set-up panel. A flat blanket in winter can make under half, which is a setup problem rather than a panel problem.
Size it right the first time
Solar Spotter reads your spot, your angle and today's conditions. Free to download — Pro shows what your own panel really makes and whether your battery keeps up.
