How to Calculate Battery Run Time: The Formula, the Inputs Calculators Miss, and Two Worked Examples
The calculator said eight hours, but your battery quit after five. It's a common surprise, and the math usually isn't the problem. Under the hood, nearly every battery run time calculator is one division problem, and it trusts whatever numbers you feed it. They need usable watt-hours, the real running watts, conversion losses, standby drain, and the way appliances cycle on and off. Below, you'll find the formula in plain terms, a checklist of inputs, and two worked examples on portable power stations, a light evening setup and a fridge-plus-cooking day.
The Battery Run Time Formula
Peel back the screen of a battery runtime calculator and there's a single fraction inside: watt-hours on top, watts on the bottom. A believable answer depends on putting both in matching units and knocking off the energy lost in conversion. If the units feel fuzzy, a quick primer on what a watt-hour measures clears it up.
Three steps
A spreadsheet, a phone app, and any run time calculator online all work through the same three moves. Skip one and the answer drifts:
Convert capacity to watt-hours. Labels often list amp-hours, so multiply by the battery's voltage. Twelve volts times 100Ah equals 1,200Wh.
Find usable energy. An AC outlet routes power through the inverter, and some of it warms the inverter instead of your devices. Our working assumption is 85%, so roughly 870 of a 1,024Wh pack's watt-hours do useful work.
Divide by the load. Usable watt-hours ÷ total running watts ≈ the hours you can expect.
Term | What it means | Where to find it |
Wh (watt-hours) | Energy the battery stores | Battery or power station spec sheet |
Ah (amp hours) | Charge at a stated voltage; multiply by V to get Wh | Battery label |
W (watts) | Power a device draws right now | Nameplate sticker, charger brick, or a plug-in meter |
Rated output (W) | The ceiling a power station holds steadily, hour after hour | Power station spec sheet |
Surge / peak (W) | Short burst for starting loads | Power station spec sheet |
That 0.85 is a planning assumption, not an EcoFlow figure, so every result here is an estimate. Real efficiency moves with the load.
Daily energy: watts × hours
For loads that cycle, think in energy per day. A DOE energy use fact sheet multiplies wattage by daily hours of use, then divides by 1,000 for kWh. Skip the ÷ 1,000 for watt-hours. A 10W reading lamp left on from 6 to 11 p.m. spends 50Wh.
Keep capacity and efficiency apart. A bigger battery runs longer but wastes no less, and a battery capacity calculator answers "how much is in the tank," not "how much reaches the outlet." Most calculators go wrong on the inputs, not the formula.
What a Battery Run Time Calculator Needs From You
A calculator can't account for what you leave out. Feed it measured running watts, set surges aside, allow for appliances that cycle, and add inverter and idle drain. Have these numbers handy before a battery usage calculator sees them.
Input | Why it matters | Where to get it |
Running watts | Sets the real load | Plug-in meter, adapter, or device label |
Starting surge | Decides if a device can start | Device manual; compare with the station's surge rating |
Duty cycle | Fridges and pumps don't run nonstop | EnergyGuide label (kWh per year) |
Hours of use | Turns watts into watt-hours | Your own plan for the outage |
Standby drain | Uses energy with nothing running | Power station manual |
Temperature | Cold cuts usable energy | Where the unit will sit |
Running watts versus the nameplate
Ever notice a three-speed fan's label shows one number? The DOE fact sheet notes that actual use depends on the setting, so the label is a ceiling, not the number to divide by. Some labels list only amps. The fact sheet suggests amps times volts as a stand-in, with 120V standard for most American appliances, but treat that as a worst case. Measuring the device gives a much safer number.
Starting surges and duty cycle
Motors and compressors pull extra power for a moment when they start. Check that the surge fits the station's peak rating, but leave it out of run time, since surge lasts seconds.
Your fridge never unplugs, but its compressor takes frequent breaks. The fact sheet divides plugged-in hours by three to estimate time at full wattage. The yearly kWh on the yellow label is easier, and the FTC's EnergyGuide label guide lists refrigerators among labeled appliances.
Standby drain, cold and battery age
An inverter left on with nothing plugged in still uses energy, and the fact sheet flags "phantom loads" from devices switched off. Check your manual for the idle figure. Cold and an older battery also cut usable energy; the EcoFlow DELTA 3 spec pages list an optimal range of 68°F to 86°F.
The biggest mistake is dividing full capacity by a device's maximum label watts and calling it exact. Our walkthrough on how to calculate watts helps. Here's how real inputs change things when you calculate battery run time.
Two Worked Examples on Portable Power Stations
Keeping lamps and Wi-Fi alive until bedtime is a small ask next to a day of fridge duty plus a hot meal. The same formula covers both, but first, the shortcut that trips people up.
The wrong shortcut
Some people divide capacity by rated output: 1,024Wh ÷ 1,800W ≈ 0.57 hours, or about 34 minutes. You'd only see that if the station were pinned at its maximum. Real loads rarely pull that much.
Lights, router and laptop
Picture a summer thunderstorm that cuts power after dinner. You want lamps, Wi-Fi, and a laptop:
Load | Watts | Source |
Wi-Fi router (TP-Link Archer AX55) | Up to 24W (12V ⎓ 2A adapter) | TP-Link spec |
LED lamps | 20W (example value) | Reader's label |
Laptop | 65W (example value) | Reader's adapter |
Total | 109W |
According to TP-Link's Archer AX55 specs, the router ships with a 12V ⎓ 2A adapter, capping it near 24W. Run that 109W bundle on an EcoFlow DELTA 3 Classic (1,024Wh) and you get 1,024Wh × 0.85 ÷ 109W ≈ 8.0 hours, as an estimate. Adapters are rated for peak demand, so real draw is often lower and the evening stretches further.
Fridge, cooking and router over a day
Picture an ice storm that keeps the lines down into the next afternoon. The list now covers the fridge, one toaster oven meal and all-day Wi-Fi. Over a full day, watt-hours beat hours, which is how a battery duration calculator frames it.
Load | Calculation | Energy per day |
Refrigerator rated 400 kWh/year on its EnergyGuide label (example value) | 400,000Wh ÷ 365 | About 1,096Wh |
Toaster oven 1,225W (DOE fact sheet typical wattage) for 30 minutes | 1,225W × 0.5h | About 613Wh |
Wi-Fi router up to 24W for 24 hours | 24W × 24h | 576Wh |
Total | About 2,285Wh |
Now load the same day onto an EcoFlow DELTA 3 Ultra Plus (3,072Wh). The 0.85 factor leaves about 2,611Wh to spend, and a 2,285Wh day uses that up in roughly 1.1 days, close to 27 hours, as an estimate. Use your own fridge label. Skip the toaster oven and the day needs about 1,672Wh, so the pack lasts nearer 1.6 days.
Energy isn't the only limit; output is too. If the toaster oven (1,225W), the fridge's startup kick and the router run together, their sum must stay below the 3,600W rated output. Look up your fridge's starting draw before relying on it.
Matching each setup to an EcoFlow DELTA 3
The evening setup is a lighter job: a few lamps, Wi-Fi and a laptop. The EcoFlow DELTA 3 Classic is sized for exactly that, and its quick switchover helps the router stay online the moment grid power drops.
The all-day setup stretches from hours to days. With a fridge, cooking and Wi-Fi stacked on 120V, the EcoFlow DELTA 3 Ultra Plus has the headroom, and its app picks which circuits keep power.
Pick by two numbers: watts running at once and the watt-hours the job needs. More output runs more at once; it doesn't add hours. Then test your power station's runtime at home before an outage tests it for you.
How to Check a Run Time Estimate in Real Life
Paper math only goes so far. A battery consumption calculator sharpens fast once measured watts replace guesses.
Measure with a watt meter
A plug-in watt meter shows live watts and a running energy total. Berkeley Lab's watt-meter list cautions that most consumer-grade meters can't safely measure more than about 1,300W, so keep space heaters off them.
Read the station's own numbers
The station's screen and app report output too. Log the steady reading, not brief peaks, and treat the time-left estimate loosely; it jumps with every load change.
Do a dry run at home
Charge to 100%, run your usual outage gear a few hours, then scale up the percentage drop. Repeat on a chilly day for winter. Planning a bigger kit? This guide to power needs for any setup totals several devices.
Ways to Get More Run Time From the Same Battery
Shrink the load and runtime grows. A battery length calculator asks how long the pack lasts; a better question is what can wait.
Shut off the AC outlets when they're idle, since an idle inverter still draws power.
Use USB-C for phones and laptops that accept it, bypassing the inverter.
Stagger power-hungry appliances, one at a time.
Pre-chill the fridge and keep the door shut before a storm. FoodSafety.gov's outage food tips note that food stays cold for about 4 hours with the door closed.
Top up by day from solar or a vehicle.
Conclusion
Battery run time comes down to usable watt-hours divided by real running watts. Convert to watt-hours, subtract losses, use running watts and duty cycle, and check surges separately. Total daily watt-hours, compare with usable energy, and test. For lamps, a router and a laptop, the EcoFlow DELTA 3 Classic covers it; add a fridge, cooking or 120V tools and the EcoFlow DELTA 3 Ultra Plus fits better.
FAQs
How do I calculate battery run time?
Turn capacity into watt-hours (volts × amp hours), multiply by 0.85 for AC inverter loss, then divide by total running watts. A short home test shows how close that lands.
How long will a 100Ah battery last?
Voltage and load both matter. At 12V, 100Ah equals 1,200Wh, so a steady 100W load through an inverter lasts roughly 1,200 × 0.85 ÷ 100 ≈ 10 hours.
Why does my battery run out sooner than the calculator says?
The math was fed rosy numbers. Real devices often pull more, and inverter loss, idle drain, cold and battery age each take a share.
Should I use surge watts in a run time calculator?
No. Base run time on running watts; surge only decides whether the station can get a fridge or well pump started.
How do I calculate run time for a refrigerator?
Multiply the annual kWh on its yellow EnergyGuide label by 1,000, then divide by 365 for daily watt-hours. Compare that with the battery's usable watt-hours.
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