The most common mistake in sizing an overland electrical system is also the most expensive: assuming every device runs at its rated wattage, all the time. Do that and your fridge alone — a 45W compressor unit — appears to need 45 × 24 = 1,080 Wh per day. Build a battery bank around that number and you'll buy (and carry, and pay for) roughly twice the battery you actually need. The fix is one concept: duty cycle.
Duty cycle: the fridge isn't always running
A 12V compressor fridge doesn't run continuously. It cycles on to pull the box down to temperature, then shuts off until it drifts back up. Across a day in reasonable ambient temps, a typical fridge runs maybe 30–40% of the time — call it 35%. So the real daily draw of that 45W fridge is:
45 W × 24 h × 0.35 ≈ 378 Wh/day — not 1,080.
That single correction often halves the calculated load, because the fridge is usually the biggest single consumer in the build. Duty cycle rises in hot weather, with a full (warm) load of drinks, or with frequent lid openings — so size with some margin — but starting from 100% is just wrong.
AC loads lose ~15% through the inverter
Anything you run off an inverter — a laptop charger, a small appliance — pays an efficiency tax. A typical inverter is ~85% efficient, so an AC device's draw on the battery is its wattage divided by 0.85. A 65W laptop pulls closer to 76W from the 12V side. DC-native devices (USB, the fridge, lights) skip this penalty, which is a good reason to run as much as possible on DC.
Sizing the battery bank
Once you have an honest daily Wh figure, the bank size is:
Amp-hours = (daily Wh × days of autonomy) ÷ (12 V × depth of discharge)
Depth of discharge (DoD) is how much of the battery you can safely use: about 0.8 for LiFePO4, ~0.5 for AGM or lead-acid. So a fridge-only 378 Wh/day load wanting 3 days of reserve on lithium needs (378 × 3) / (12 × 0.8) ≈ 118 Ah (we'll add the other loads in the worked example below). The same load on AGM needs ~189 Ah — and weighs far more. This is why LiFePO4 wins on any weight-sensitive build: more usable capacity per pound, and it tolerates deep cycling.
Charging: solar and DC-DC
A battery bank is a buffer; you still have to refill it. Two sources:
- Solar replaces draw while you're parked. Useful output is panel watts × peak sun hours × ~0.85 system efficiency. A 200W panel in 4 peak-sun hours yields roughly 680 Wh — comfortably more than our example fridge-led load.
- DC-DC charger tops the bank from the alternator while you drive. Its size depends on how long you drive each day: it has to replace the day's shortfall within that window. Amps ≈ (shortfall Wh ÷ 12 ÷ driving hours) × 1.25. Drive only an hour a day and you need a much bigger charger than someone covering ground for four.
That driving-hours factor is the piece most calculators ignore — ours asks for it directly so the DC-DC recommendation matches how you actually travel.
A worked example
Fridge (45W, 35% duty) 378 Wh + LED lights (15W × 4h) 60 Wh + phone/USB 40 Wh + Starlink (35W × 4h) 140 Wh ≈ ~620 Wh/day. For 3 days on LiFePO4: (620 × 3) / (12 × 0.8) ≈ 190 Ah of battery, or ~65 Ah for a single day's buffer if you're charging daily by solar or alternator. Without the duty-cycle correction the fridge alone would have pushed this past 1,300 Wh/day and doubled the bank.
Bottom line
Honest inputs beat oversizing. Apply duty cycle to the fridge, divide AC loads by inverter efficiency, pick a DoD that matches your chemistry, and size charging to how you travel. Run it all through the Battery Bank Calculator — it handles the duty cycle, inverter loss, and driving-hours DC-DC math for you, and shows the weight trade-off between AGM and lithium.