Battery Runtime Calculator
A 1,000Wh battery running a 150W load — a standard refrigerator — lasts about 5 hr 40 min once you account for 85% inverter efficiency. Enter your own battery capacity and load below for an exact number.
Advanced: check a surge/starting load
Motors and compressors (fridges, pumps, power tools) briefly draw far more than their running watts when they start. Leave this blank if your load doesn't have a motor.
Estimated runtime
5 hr 40 min
5.67 hours
How the formula works
Runtime in hours equals the battery's usable watt-hours (Wh), times its inverter efficiency, divided by the load in watts: hours = (usable Wh × efficiency) ÷ load W.
Usable Wh matters because it's not always the same as the number printed on the box. Some lithium batteries limit how deeply they discharge to protect the cells, so their usable capacity is lower than their total rated capacity. Use the usable figure from the spec sheet when you have one.
Inverter efficiency matters because a battery stores energy as DC, but most household devices run on AC. Getting from DC to AC requires an inverter, and no inverter is 100% efficient — some energy is lost as heat in the conversion. This calculator defaults to 85%, a reasonable estimate for a battery or power station's built-in inverter, but you can edit it if your device's spec sheet lists an exact figure. Skipping this step and dividing Wh straight by watts overstates real-world runtime by roughly 15-20%.
Worked example
Say you have a power station rated at 1,200Wh usable capacity, and you want to run a CPAP machine that draws 40W continuously overnight. At 85% inverter efficiency: 1,200 × 0.85 ÷ 40 = 25.5 hours — more than two full nights on a single charge.
Now add a second load: a Wi-Fi router drawing 15W around the clock alongside the CPAP. Combined continuous load is 55W, so runtime drops to 1,200 × 0.85 ÷ 55 ≈ 18.5 hours — about 18 hr 33 min. Adding loads together before dividing is the right approach any time you're running more than one device at once.
Common mistakes
Ignoring surge watts for motors. A refrigerator, sump pump, or well pump needs a short burst of extra power — often 2-3x its running watts — just to start the compressor or motor. A battery with enough continuous output to run the appliance can still fail to start it if the surge rating is too low. Check the "Advanced" section of the calculator above to compare your device's surge watts against your battery's rated surge output.
Using rated Wh instead of usable Wh. If your battery's spec sheet lists both a total and a usable capacity, always calculate with the usable number — it already accounts for the depth-of-discharge limit the manufacturer built in.
Forgetting the compressor duty cycle. Appliances like refrigerators and freezers don't run continuously — the compressor cycles on and off, so the running-watts figure only applies while it's actually on. A whole-day runtime estimate for a fridge should use the appliance's typical daily run hours, not 24 hours of continuous draw.
Want a specific real power station's per-appliance runtimes instead of a generic estimate? See our EcoFlow vs. Jackery comparison for actual product runtime tables, or our best solar battery guide for home battery backup. If your battery is rated in amp-hours rather than Wh, convert it first with our amp-hours to watt-hours calculator.
Common appliance running watts
These are published running-watt and surge-watt specs for common household loads, so you can plug realistic numbers into the calculator above. Based on published specs — actual draw varies by model. For appliances that cycle on and off (fridges, ACs), use the average watts column for whole-day runtime math, not running watts — see "Forgetting the compressor duty cycle" above. Curious what running one of these costs in electricity? Try our electricity cost calculator, or size a backup generator for a full set of loads with our generator size calculator.
| Appliance | Running watts | Average watts | Surge watts |
|---|---|---|---|
| Refrigerator (standard, 18-20 cu ft, frost-free) | 150W | 50W | 800W |
| CPAP Machine (no heated humidifier attachment) | 40W | — | — |
| Wi-Fi Router + Modem (combo unit) | 15W | — | — |
| Window Air Conditioner (8,000-10,000 BTU) | 1,200W | 400W | 3,600W |
| Sump Pump (1/3 HP, submersible) | 800W | — | 1,300W |
| Well Pump (submersible, 1/2 HP) | 1,000W | — | 2,100W |
| Microwave Oven (countertop, ~1,000-1,200W cooking power) | 1,000W | — | — |
| Portable Electric Space Heater | 1,500W | — | — |
"—" under average watts means the appliance is an on-demand load (sump pump, well pump) with no sourced daily-average duty cycle, or doesn't cycle — its running watts already is its draw while in use.
FAQ
How do you calculate battery runtime?
Multiply the battery's usable watt-hours (Wh) by its inverter efficiency (typically about 85%), then divide by the load in watts. A 1,000Wh battery running a 150W load at 85% efficiency lasts 1,000 × 0.85 ÷ 150 ≈ 5.7 hours (5 hr 40 min). Use the calculator above to plug in your own numbers.
How long will a portable power station run a refrigerator?
It depends on the station's usable Wh and the fridge's running watts. A standard frost-free refrigerator draws about 150W running (per published appliance specs), so a 1,000Wh power station lasts roughly 5 hr 40 min on that load alone. A fridge's compressor also cycles on and off rather than running continuously, so real-world runtime on a full day is usually longer than this continuous-load estimate.
Why don't you get 100% of a battery's rated Wh as usable power?
Two things eat into it. First, some batteries cap depth of discharge below 100% to protect the cells, so their usable Wh is lower than the rated/nameplate Wh — use the usable figure if the spec sheet lists one. Second, converting stored DC energy to AC power through an inverter isn't lossless; roughly 10-15% is lost as heat, which is why this calculator applies an inverter efficiency (85% by default) to the Wh figure before dividing by your load.
What is the difference between running watts and surge watts?
Running watts (also called continuous or rated watts) is what a device draws once it's operating normally. Surge watts (starting or peak watts) is the brief spike a motor or compressor needs to start — often 2-3x the running watts. A battery can have enough continuous output to run a fridge all day but still fail to start it if its surge rating is too low. Check both numbers, not just running watts, for any motor-driven load.
How many watt-hours do I need to power a device overnight?
Multiply the device's running watts by the hours you need to cover, then divide by your inverter efficiency to find the Wh you need from the battery. For example, a 40W CPAP machine run for 8 hours needs 40 × 8 ÷ 0.85 ≈ 376Wh of usable battery capacity. Our amp-hours to watt-hours calculator can help if your battery is rated in Ah instead of Wh.
Data sources
Last updated 2026-09-22
- EcoFlow — Appliance Wattage Chart & Energy Usage Calculator — accessed 2026-09-22
- Generatorist — Power Consumption of Household Appliances — accessed 2026-09-22
- Unbound Solar — Power Table — accessed 2026-09-22
Written by GoSolarIndex Team