How to Calculate Power Station Runtime (Free Calculator)
How to calculate power station runtime: the one-line formula, why we count 85% of capacity, how duty cycles stretch fridge runtime, and a free calculator.
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To calculate power station runtime, multiply the station’s rated watt-hours by 0.85 for usable energy, then divide by your appliance’s average draw in watts. For cycling appliances like fridges, the average draw is running watts times duty cycle. A 1,024 Wh station gives about 870 usable Wh, so a 100 W load runs about 8.7 hours.
That’s the formula behind every runtime on this site and inside our runtime calculator. Here’s each piece, why it’s there and where it can mislead you.
The runtime formula in one line
Runtime (hours) = rated capacity (Wh) × 0.85 ÷ (running watts × duty cycle)
| Term | Where it comes from | Example |
|---|---|---|
| Rated capacity | Station spec sheet | 1,024 Wh (EcoFlow DELTA 3 Plus) |
| 0.85 | Our usable-energy assumption | 1,024 × 0.85 = 870 Wh |
| Running watts | Appliance label or our wattage table | ENERGY STAR fridge, 160 W |
| Duty cycle | Share of time a cycling appliance actually runs | 0.32 for that fridge |
Worked through: 870 ÷ (160 × 0.32) = 870 ÷ 51.2 = 17 hours.
For anything that runs constantly, like a laptop, router or CPAP, the duty cycle is 1 and you divide by the running watts directly. A ResMed AirSense 11 at its 56.1 W typical draw: 870 ÷ 56.1 = about 15.5 hours.
Before any of this, the station has to be able to run the appliance at all. Running watts must fit the inverter’s continuous rating, and any start-up surge must fit its surge rating. Our watts vs watt-hours explainer covers that half of the problem.
Why we assume about 85% usable capacity
A station’s rated capacity is the energy stored in its cells, measured at the battery. Your appliance sees less, for three main reasons:
- Inverter losses. Turning battery DC into 120 V AC generates heat. Losses are proportionally larger at very light loads, because the inverter’s own standby draw becomes a bigger share of the total.
- DC conversion. USB-C, USB-A and 12 V outputs step the battery voltage up or down, which also costs a little.
- Battery management reserve. Stations hold back a small buffer at the top and bottom of the pack so the cells aren’t pushed to their absolute limits.
We fold all of that into one number, 85%, rather than pretending to know each station’s inverter efficiency curve, which manufacturers rarely publish. It’s deliberately conservative for AC loads. DC loads, such as a 12 V fridge on the car socket or a laptop on USB-C, skip the inverter and will often beat our numbers.
The same assumption applies to every station, so comparisons stay fair: if two stations have the same capacity, they get the same runtime on the same load.
Duty cycle: why fridges run longer than the math suggests
Fridges, freezers and furnace fans run on thermostats. The compressor or blower switches on, runs until it hits its target, then switches off. Using running watts for the whole period overstates their energy use several times over.
Our appliance data carries a duty cycle for each thermostat-driven appliance. For household fridges and freezers it’s derived from EWEB’s published monthly run hours. For example, 230 run hours in a 720-hour month gives 0.32. For 12 V fridges it’s the ratio of a measured steady-state average to the cooling draw.
Here’s what that does on a 1,024 Wh station (870 usable Wh):
| Appliance | Running watts | Duty cycle | Average watts | Runtime | Runtime if it ran nonstop |
|---|---|---|---|---|---|
| ENERGY STAR fridge | 160 W | 0.32 | 51.2 W | 17 h | 5.4 h |
| Top-freezer fridge | 195 W | 0.34 | 66.3 W | 13 h | 4.5 h |
| Side-by-side fridge | 230 W | 0.34 | 78.2 W | 11 h | 3.8 h |
| Compact chest freezer | 180 W | 0.31 | 55.8 W | 16 h | 4.8 h |
| Upright freezer | 220 W | 0.32 | 70.4 W | 12 h | 4.0 h |
| Variable-speed furnace fan | 200 W | 0.24 | 48 W | 18 h | 4.4 h |
12 V compressor fridges cycle even less once they’re cold. On the 286 Wh EcoFlow RIVER 3 Plus, with 243 usable Wh:
| 12 V fridge | Cooling draw | Duty cycle | Average | Runtime |
|---|---|---|---|---|
| Anker EverFrost 40 | 50.1 W | 0.12 | 6.0 W | 40 h |
| Dometic CFX5 45 | 58.2 W | 0.12 | 7.0 W | 35 h |
| Engel MT45 | 47.6 W | 0.23 | 10.9 W | 22 h |
Note the Engel: lower cooling draw than the Dometic, but it cycles about twice as often, so it runs fewer hours. Average draw is what counts.
Duty cycles aren’t fixed. A fridge in a hot garage, a freezer that’s just been loaded with warm food or a fridge whose door keeps opening will all run more. Treat these as typical-conditions figures, and plan a margin if your conditions are harsh.
Running several appliances at once
With more than one appliance, three numbers matter, and our calculator computes all three:
- Running watts: every appliance’s running watts × quantity, added up. This must fit the inverter’s continuous rating.
- Peak watts: running watts plus the single largest start-up surge, since motors rarely start at the same instant. This must fit the surge rating.
- Average watts: every appliance’s running watts × duty cycle × quantity, added up. This sets the runtime.
A worked evening on the DELTA 3 Plus (1,800 W continuous, 3,600 W surge):
| Appliance | Qty | Running W | Average W |
|---|---|---|---|
| ENERGY STAR fridge | 1 | 160 | 51.2 |
| 45-inch LED TV | 1 | 162 | 162 |
| Wi-Fi router | 1 | 6.5 | 6.5 |
| Cable modem | 1 | 6 | 6 |
| LED bulb (60 W equivalent) | 4 | 36 | 36 |
| Phone charger | 2 | 20 | 20 |
| Total | 390.5 | 281.7 |
Runtime: 870 ÷ 281.7 = about 3.1 hours with everything on. That’s the honest result of leaving the TV and four lights on continuously. Turn the TV off and the average falls to about 120 W, stretching runtime past 7 hours.
Now add a 1,500 W microwave. Running watts jump to 1,890.5 W, over the 1,800 W continuous rating, so the station would shut off even though the battery is nearly full. Switch off the TV while the microwave runs and you’re at 1,728.5 W, which fits. Capacity is rarely what trips people up with mixed loads; the inverter limit is.
Surge works the same way. A fridge plus a 13,000 BTU window AC (1,800 W running, 2,800 W start) has a 2,960 W peak, inside the 3,600 W surge rating, but 1,960 W of running load, over the continuous limit. That combination fails on running watts alone.
Try the runtime calculator
Pick a station, add appliances with quantities, and the calculator shows running watts, peak, average draw and hours, along with the smallest stations that can carry your load. Every appliance in it comes from our sourced appliance wattage table.
What you'll plug in
- Full-size refrigerator (ENERGY STAR)160 W, cycles 32% of the time
- Wi-Fi router6.5 W
- LED bulb (60 W equivalent)9 W
- Smartphone charger10 W
Runs all of it for about 8.3 h on a full charge.
- Runtime
- 8.3 h
- Running watts
- 214of 1,800 W inverter
- Start-up peak
- 214of 3,600 W surge
- Average draw
- 105W after cycling
Runtime = 85% of the battery's rated Wh (usable after inverter and DC losses) ÷ the average draw. The start-up peak assumes one motor starts at a time.
Smallest stations that run this load
| Station | Battery | Inverter | Runtime | Buy |
|---|---|---|---|---|
| EcoFlow RIVER 3 | 245 Wh | 300 W | 2.0 h | Amazon |
| EcoFlow RIVER 3 Plus | 286 Wh | 600 W | 2.3 h | EcoFlow |
| Anker SOLIX C300 | 288 Wh | 300 W | 2.3 h | Anker SOLIX |
| BLUETTI Elite 30 V2 | 288 Wh | 600 W | 2.3 h | BLUETTI |
| Jackery Explorer 300 Plus | 288 Wh | 300 W | 2.3 h | Amazon |
| BLUETTI AC50B | 448 Wh | 700 W | 3.6 h | BLUETTI |
| Goal Zero Yeti 500 | 499 Wh | 500 W | 4.1 h | Goal Zero |
| DJI Power 500 | 512 Wh | 1,000 W | 4.2 h | DJI |
The calculator assumes everything in the list runs all the time. For loads you’ll only use part of the day, like a microwave for 15 minutes or lights for 5 hours, a daily watt-hour total is more realistic. Multiply each appliance’s average watts by its hours of use, add them up, and divide by 0.85 to get the rated capacity you need. Our guide on how to size a power station works through full examples that way.
Here’s the same math applied to one station across our default appliance set:
| Appliance | Draw | Inverter | Runtime | Runtime as a share of 24 hours |
|---|---|---|---|---|
| ResMed AirSense 11 CPAP | 56 W | Runs | 16 h | |
| Wi-Fi router | 6.5 W | Runs | 5.6 days | |
| Smartphone charger | 10 W | Runs | 3.6 days | |
| Laptop computer | 50 W | Runs | 17 h | |
| LED bulb (60 W equivalent) | 9 W | Runs | 4.0 days | |
| Full-size refrigerator (ENERGY STAR) | 160 Wavg 51 W cycling | Runs | 17 h | |
| 12V portable fridge (Dometic CFX5 45) | 58 Wavg 7 W cycling | Runs | 5.2 days | |
| LED/LCD TV (about 45-inch) | 162 W | Runs | 5.4 h | |
| Window fan | 153 W | Runs | 5.7 h | |
| Drip coffee maker | 1,300 W | Runs | 40 min | |
| Microwave oven (1.25 cu ft) | 1,500 W | Runs | 35 min | |
| Submersible sump pump (1/2 HP) | 1,000 W3,000 W to start | Start surge too high | — |
Runtime = usable energy ÷ average draw. We count 85% of the rated 1,024 Wh as usable (870 Wh) to cover inverter and DC conversion losses. Fridges and freezers are averaged over their compressor cycle. An appliance runs only if its running watts fit the 2,600 W inverter and its start-up surge fits the 2,600 W surge rating. Wattages come from our appliance wattage table; try your own mix in the runtime calculator.
How accurate are runtime estimates?
Good enough to choose a station, not precise enough to promise an exact hour. The main sources of error:
- Appliance variation. Published wattages are often ranges. EWEB lists computer monitors anywhere from 20 W to 150 W; our table uses the 85 W midpoint. Your model may sit at either end.
- Duty cycle conditions. Room temperature, door openings and how full a fridge is all change how often it runs.
- Charger ratings aren’t draws. A 140 W laptop adapter is a maximum. A laptop that’s already charged draws far less.
- Inverter efficiency. Very small AC loads, such as a single phone charger on the AC outlet, waste proportionally more energy than our 85% assumes. Use USB ports for small devices.
- Temperature and age. Cold batteries deliver less until they warm, and capacity drops over the station’s life toward the 80% figure its cycle rating describes.
For the most accurate numbers, measure. Most stations we track have app monitoring that shows real-time draw, which is one reason our scoring rubric gives credit for apps. A plug-in watt meter on the wall does the same for any appliance before an outage. Plug your measured watts into the calculator and the formula does the rest.
When you’re ready to shop, our rankings of small, mid-size and home backup power stations show these runtimes for every pick.
Frequently asked questions
How do you calculate how long a power station will last?
Multiply its rated watt-hours by 0.85 to get usable energy, then divide by your appliance's average draw in watts. A 1,024 Wh station running a 100 W load lasts about 8.7 hours.
Why is the runtime less than the battery capacity divided by watts?
Converting battery power to 120 V AC and to USB or 12 V outputs loses energy, and stations keep a small reserve to protect the cells. We count 85% of rated capacity as usable to cover those losses.
How do I calculate runtime for a refrigerator?
Use the average draw, not the running watts. An ENERGY STAR fridge runs at about 160 W with a 0.32 duty cycle, so it averages about 51 W. Divide usable watt-hours by 51.
How do I calculate runtime with multiple appliances?
Add each appliance's average draw (running watts × duty cycle × quantity) and divide usable watt-hours by the total. Also check that the combined running watts fit the inverter's continuous rating.
How accurate are power station runtime calculators?
Usually within a reasonable margin for planning, but individual appliances vary. Published wattages are often ranges, and fridges cycle more in hot rooms. Measure your own devices if a few hours matter.