I sized my camping battery wrong three times before I built a calculator for it

The first time I killed a battery at 2am, it was my own fault. I'd added up the watts on the stickers — fridge 45W, lights 10W, phone 12W — and rounded up to a 100Ah battery. What I'd missed is that "watts on a label" and "amp-hours in a box" aren't the same unit, and a compressor fridge spikes to 3–4× its running draw the second it kicks on.

The second time, I'd learned about watt-hours but forgot the inverter efficiency loss. The third time, I trusted the "up to 8 hours" claim on a product page that was measured at the fridge's minimum setting with nothing else running.

So I sat down and actually worked the math. The unit that matters is watt-hours — watts × hours. A 45W fridge running 24h isn't 45Wh, it's ~1,080Wh, and that's before the surge tax. Here's the tiny function I kept reaching for:

// watts pulled, hours run per day, plus a safety buffer
function dailyWh(devices, buffer = 0.2) {
  const base = devices.reduce((sum, d) => sum + d.watts * d.hours, 0);
  return Math.ceil(base * (1 + buffer));
}

const rig = [
  { watts: 45, hours: 24 },  // compressor fridge, runs near-constant
  { watts: 10, hours: 5 },   // LED lights
  { watts: 60, hours: 0.5 }, // phone + laptop top-up
];

console.log(dailyWh(rig)); // ~1354 Wh needed per day

That 1,354Wh is the number you actually size against — not the 67W sum of the stickers. Then you divide by your battery's nominal voltage to get amp-hours, and shave off ~20% so you're not parking a lithium cell at 100% discharge every night (which quietly halves its lifespan).

Once I'd done this by hand one too many times, I put it into a camping power calculator so I'd stop redoing the arithmetic on the back of a receipt. It walks the same path — list the devices, get the real Wh, then the battery and solar size that actually hold — and it's the thing I now send to friends instead of a paragraph of text.

Solar is the part people get backwards. A panel's "100W" is a lab number at perfect noon sun; you realistically collect 4–6 sun-hours a day, and only in summer. So a 100W panel is closer to 400–600Wh/day, not 2,400. If your rig eats 1,354Wh, one panel won't keep up — you need two, or a smaller fridge. The physics here is worth reading up on; watt-hour and photovoltaic system are the two pages I wish I'd opened before trip one.

The takeaway I keep repeating to myself: size for the peak and the real daily draw, not the sticker sum. A 20% buffer isn't optional padding, it's the difference between a battery that lasts the season and one you're replacing in the spring.