MEASURING WATT-HOURS
The most common mistake when entering the off-grid power space is confusing Watts (W) with Watt-Hours (Wh). One measures the speed of power delivery; the other measures the size of the gas tank.
Surge vs. Running Watts
Every appliance has a running wattage. A refrigerator might use 150W while running. However, when the compressor kicks on, it requires a massive, instantaneous spike of power—perhaps 600W. This is the Surge Wattage. If your power station or generator cannot handle that momentary surge, the inverter will overload and shut down, even if the running wattage is well within limits.
| Appliance | Avg. Running Watts | Est. Surge Watts |
|---|---|---|
| Smartphone Charger | 15W | 15W |
| CPAP Machine | 40W | 60W |
| Full-Size Refrigerator | 150W - 400W | 600W - 1200W |
| Space Heater | 1500W | 1500W |
Calculating Battery Depletion
Watt-Hours (Wh) dictate how long a battery will last. A 1000Wh battery powering a 100W device will theoretically run for 10 hours. However, inverters are not 100% efficient. Converting DC battery power to AC wall power loses about 10-15% of the energy as heat.
"Always multiply your theoretical run time by 0.85 to account for inverter inefficiency. The math must reflect reality, not laboratory perfection."
The AC vs DC Tax
To maximize capacity, power devices via DC (like USB or 12V car ports) whenever possible. Bypassing the AC inverter saves that 15% energy tax, extending the lifespan of your battery bank significantly during an extended outage.