Watt-Hours vs. Amp-Hours: Why Wh is the Universal Currency
In 12-volt automotive and marine circles, battery capacities are traditionally quoted in Amp-Hours (Ah). However, an Amp-Hour without specifying voltage tells only half the story. 100Ah at 12V is 1,200 Watt-Hours (Wh), whereas 100Ah at 48V is 4,800 Watt-Hours—four times the energy.
To avoid costly sizing mistakes, always convert every device to Watt-Hours per day first:
Energy (Watt-Hours) = Power (Watts) × Runtime (Hours/Day)Once you have total daily Watt-Hours, dividing by your battery bank’s nominal DC voltage yields the exact daily Amp-Hours your battery bank must supply.
The Inverter Efficiency Penalty (10% to 20%)
Inverters convert DC battery voltage (12V, 24V, or 48V) into AC household electricity (120V or 230V). No inverter is 100% efficient. High-quality pure sine wave inverters operate at approximately 85% to 92% efficiency under moderate load.
If you run a 100-Watt laptop charger for 3 hours (300 Wh of AC work), the inverter actually extracts:
300 Wh ÷ 0.85 = 352.9 Wh from the battery bankThe remaining 52.9 Watt-Hours are lost as heat through the inverter cooling fins. Our calculator applies this efficiency penalty automatically to all AC loads.
The Silent Battery Killer: Inverter Standby Idle Draw
A typical 2000W or 3000W pure sine wave inverter consumes between 15 Watts and 40 Watts just staying powered on with no appliances plugged in. Left turned on 24 hours a day, that idle tare consumes:
25 Watts × 24 hours = 600 Watt-Hours per day (50 Amp-Hours at 12V)In many compact campervans or cabins, the inverter’s idle consumption exceeds the energy used by LED lighting and water pumps combined! Whenever possible, run continuous loads (refrigerators, ventilation fans, USB chargers) directly on DC 12V/24V power circuits to keep the inverter switched off when not needed.
Compressor Duty Cycles: Understanding Refrigerators
A portable 12V compressor fridge (such as Dometic, ICECO, or Engel) is rated at approximately 45W to 55W. However, the compressor does not run continuously. In moderate 21°C (70°F) ambient temperatures, a well-insulated fridge runs approximately 20 to 30 minutes out of every hour—a 30% to 40% duty cycle (7 to 9 hours of total compressor runtime per 24-hour day).
In hot summer weather (32°C / 90°F), the duty cycle may climb to 60% (14.4 hours runtime). Always plan for peak summer duty cycles when sizing off-grid refrigeration.