Watt-Hours Explained: How to Calculate Power Station Runtime
Last updated: April 2026
Watt-hours (Wh) measure how much total energy a battery can deliver over time. To calculate how long a portable power station will run a device, divide the battery's watt-hour capacity by the device's wattage: Wh ÷ W = hours of runtime. A 1,000Wh power station running a 100W laptop lasts approximately 8.5 hours after accounting for 10-15% inverter efficiency losses.
What Are Watt-Hours?
A watt-hour is a unit of energy equal to one watt of power sustained for one hour. It tells you how much "fuel" a battery holds, just like gallons tell you how much fuel a gas tank holds.
If you see a portable power station rated at 2,000Wh, that means it can deliver 2,000 watts for one hour, or 200 watts for ten hours, or 100 watts for twenty hours. The math is straightforward multiplication: power (watts) multiplied by time (hours) equals energy (watt-hours).
You will also see batteries rated in amp-hours (Ah). To convert: Ah × voltage = Wh. A 100Ah battery at 12.8V holds 1,280Wh. Watt-hours are a more useful comparison metric because they account for voltage differences between battery systems.
How to Calculate Runtime
The basic formula is simple:
Runtime (hours) = Battery Capacity (Wh) ÷ Device Power (W)
Example: You have a 1,500Wh power station and want to run a 60W mini fridge. The theoretical runtime is 1,500 ÷ 60 = 25 hours.
But this is the theoretical maximum. Real-world runtime is always lower due to efficiency losses. Read on to understand why.
Why Real Runtime Is Lower: Efficiency Losses
When you plug an AC device into a power station, the internal inverter converts DC battery power to AC household power. This conversion is not 100% efficient -- typically 85-90% of the energy makes it through. The rest is lost as heat.
To get a realistic runtime estimate, multiply the battery capacity by 0.85 before dividing:
Realistic Runtime = (Wh × 0.85) ÷ Device Watts
Using the same example: (1,500 × 0.85) ÷ 60 = 21.25 hours -- about 4 hours less than the theoretical calculation.
Additional factors that reduce runtime:
- • Cold temperatures reduce battery capacity by 10-30%, depending on chemistry and severity.
- • BMS reserves -- the battery management system may reserve 5-10% of capacity to protect cell health.
- • DC devices are more efficient -- plugging into 12V or USB-C ports bypasses the inverter entirely, giving you closer to the full rated capacity.
- • Battery age -- capacity degrades over time. A LiFePO4 battery retains 80% capacity after 3,000-5,000 cycles; NMC degrades faster.
Common Device Runtime Table
Estimated runtimes for common off-grid devices. All AC figures include a 15% inverter efficiency loss. Actual results vary by specific device and conditions.
| Device | Typical Draw | 1,000Wh | 2,000Wh | Notes |
|---|---|---|---|---|
| Smartphone charge | 10-15W | 57-85 charges* | 114-170 charges* | ~12Wh per full charge |
| Laptop | 50-100W | 8.5-17h | 17-34h | Varies by workload |
| LED lights (string) | 10-20W | 42-85h | 85-170h | Very efficient |
| Mini fridge (12V) | 40-60W | 14-21h | 28-42h | Compressor cycles on/off |
| CPAP machine | 30-60W | 14-28h | 28-57h | 2-3 nights typical |
| TV (32") | 30-55W | 15-28h | 31-57h | LED models |
| Electric blanket | 50-100W | 8.5-17h | 17-34h | Low setting uses less |
| Coffee maker | 600-1,200W | 0.7-1.4h | 1.4-2.8h | High draw, short use |
| Microwave | 700-1,200W | 0.7-1.2h | 1.4-2.4h | High draw, short use |
| Hair dryer | 1,000-1,800W | 0.5-0.85h | 0.9-1.7h | Very high draw |
* Smartphone charges calculated based on ~12Wh per full charge, not continuous draw.
Running Multiple Devices Simultaneously
When running multiple devices, add up their wattages and divide the battery capacity by the total. Also confirm the power station's continuous output wattage can handle the combined load.
Example scenario:
- Mini fridge: 50W (continuous)
- LED lights: 15W
- Phone charging: 15W
- Total: 80W
With a 2,000Wh power station: (2,000 × 0.85) ÷ 80 = ~21 hours
Keep in mind that some devices (fridges, CPAP machines) cycle on and off, so their average draw is lower than peak draw. Check the device's average wattage if available for more accurate estimates.
How to Size Your Power Station
Follow these steps to determine how many watt-hours you need:
- 1. List every device you plan to run and its wattage (check the label or manual).
- 2. Estimate hours of daily use for each device.
- 3. Multiply watts × hours for each device to get daily Wh consumption.
- 4. Add up all daily Wh totals.
- 5. Divide by 0.85 to account for inverter losses.
- 6. Add 20% headroom for comfort and battery longevity.
Browse our portable power station reviews to find a unit that matches your calculated needs, or use the sizing recommendations in our RV living guide and camping guide.