Battery Runtime Calculator: How Long Will It Actually Last?
Battery runtime is one division: usable watt-hours divided by the load in watts. A 500Wh battery running a 60W device gives you about 8.3 hours on paper. In the real world you'll get closer to 6 to 7, because the sticker number and the number you can actually use are two different things.
Here's the honest version of the formula:
Runtime (hours) = (Battery Wh × usable share × inverter efficiency) ÷ load in watts
Use the Battery Runtime Calculator, plug in the battery size and the device watts, and adjust the usable share to match your battery chemistry.
Step 1: turn amp-hours into watt-hours
Deep-cycle batteries are sold in amp-hours. Devices are rated in watts. Those don't talk to each other until you convert:
Watt-hours = amp-hours × battery voltage
A 100Ah 12V lead-acid battery is 1,200Wh. A 100Ah LiFePO4 battery is usually 12.8V nominal, so it's 1,280Wh. Same amp-hours, slightly different energy.
This is also why comparing a 12V and a 24V battery on amp-hours alone is meaningless. A 100Ah 24V battery holds twice the energy of a 100Ah 12V battery.
Step 2: work out how much you can actually use
You never get 100% of the rated capacity out. How much you get depends on the chemistry.
| Battery type | Safe usable share | Why |
|---|---|---|
| Flooded lead-acid | About 50% | Deep discharge destroys cycle life fast |
| AGM / gel | 50% to 60% | Slightly tougher than flooded, still limited |
| LiFePO4 (lithium iron phosphate) | 80% to 100% | Handles deep discharge, BMS protects the cells |
| Portable power station (LiFePO4) | About 85% | Rated Wh minus BMS reserve and inverter overhead |
Manufacturers quote depth of discharge differently. Check your battery's own spec sheet before you plan around these.
That difference is huge and it catches people out constantly. A 100Ah AGM battery and a 100Ah LiFePO4 battery cost different money for a reason. The lithium one gives you roughly twice the usable energy per cycle and lasts several times as many cycles.
Step 3: subtract the inverter
If you're running a 240V appliance off a battery, an inverter converts DC to AC and loses energy doing it. A decent pure sine wave inverter runs at about 85% to 92% efficiency when it's loaded reasonably. At very light loads the efficiency drops off, sometimes badly.
Inverters also draw power just being switched on. Idle draw of 5W to 20W is normal. Leave a 15W idle inverter running overnight with nothing plugged in and you've burned about 180Wh for nothing.
Running a device on 12V DC directly, like a camping fridge with a DC plug, skips the inverter losses entirely. That's why 12V fridges are so much more efficient off a battery than a 240V bar fridge through an inverter.
Two worked examples
100Ah AGM battery, 60W laptop and lights
- 1,200Wh rated (100Ah × 12V)
- 50% usable to protect the battery = 600Wh
- Inverter at 88% = 528Wh delivered
- 528 ÷ 60W = about 8.8 hours
100Ah LiFePO4 battery, same 60W load
- 1,280Wh rated (100Ah × 12.8V)
- 90% usable = 1,152Wh
- Inverter at 88% = 1,014Wh delivered
- 1,014 ÷ 60W = about 16.9 hours
Nearly double the runtime from the same headline amp-hours.
What common devices actually draw
Nameplate ratings are peak, not average. A fridge rated at 45W isn't drawing 45W all the time, because the compressor cycles. Use the average column when you're planning a night or a weekend.
| Device | Rated watts | Average draw over an hour |
|---|---|---|
| LED camp light | 3W to 8W | Same as rated |
| Phone charging | 10W to 25W | Only while charging, roughly 15Wh per full phone |
| Laptop | 45W to 90W | 30W to 60W in normal use |
| 12V camping fridge (40L) | 45W to 60W running | 15W to 25W average, compressor cycles |
| Modem and router | 10W to 20W | Same as rated, runs constantly |
| CPAP without humidifier | 30W to 60W | 30W to 40W typical |
| CPAP with heated humidifier | 60W to 110W | Can more than double the draw |
| Starlink standard dish | 50W to 75W | Roughly the same continuously |
| Electric blanket | 60W to 120W | Cycles with the thermostat |
| Microwave | 1,000W to 1,500W | Short bursts, still brutal on a battery |
| Electric kettle | 1,800W to 2,400W | About 100Wh to boil a litre |
| Fan heater | 2,000W to 2,400W | Do not plan on this working |
Check the compliance plate on your own gear. These are typical ranges for planning, not specifications.
The four things that make real runtime worse than the maths
1. High discharge rates (lead-acid only)
Pull hard from a lead-acid battery and you get less total capacity out of it. This is the Peukert effect. A 100Ah AGM battery rated over a 20 hour discharge might only deliver 70Ah if you drain it in 5 hours. LiFePO4 barely cares about this, which is another reason it wins on heavy loads.
2. Temperature
Cold batteries hold less. A battery in a ute tray on a frosty morning in the Snowy or central Tasmania will deliver noticeably less than the same battery at 25°C. Most LiFePO4 packs also refuse to charge below 0°C unless they have a heater built in, which is a nasty surprise on a winter trip.
3. Age
Capacity fades with cycles. A three-year-old AGM battery that's been regularly run flat might be at 70% of its original capacity. If your setup used to last two nights and now barely does one, the battery is usually the answer.
4. Phantom loads
Inverter idle draw, the battery monitor, the fridge controller, a USB hub, a Bluetooth module. None of them are big. Together they can quietly eat 10W to 25W around the clock, which is 240Wh to 600Wh a day.
Sizing a battery from the other direction
Most of the time you don't want runtime from a battery. You want a battery for a known runtime. Flip the formula:
Battery Wh needed = (load in watts × hours needed) ÷ (usable share × inverter efficiency)
Say you want to run a 45W fridge that averages 16W, plus 10W of lights and charging, for three days off-grid with no solar. That's 26W average, or 624Wh per day, or 1,872Wh over three days. At 90% usable through an 88% efficient inverter you need about 2,364Wh, so roughly a 200Ah LiFePO4 setup. On AGM at 50% usable you'd need well over 4,200Wh of rated capacity to do the same job.
That's the moment most people decide the lithium price tag is fine after all.
Related calculators
- Power Station Runtime Calculator for portable stations rated in Wh
- UPS Runtime Calculator for how long a computer or modem stays up in a blackout
- Power Bank Capacity Calculator for real charges out of a power bank
- Blackout Prep Calculator if you're planning around outages rather than camping
Frequently asked questions
How do I calculate battery runtime?
Convert the battery to watt-hours (amp-hours times voltage), multiply by the share you can safely use (about 50% for lead-acid, 80% to 100% for LiFePO4), multiply by inverter efficiency (about 88%), then divide by the load in watts. A 100Ah 12V LiFePO4 battery running a 60W load gives roughly 17 hours.
How many watt-hours is a 100Ah battery?
A 100Ah 12V lead-acid battery is 1,200Wh. A 100Ah LiFePO4 battery is normally 12.8V nominal, so 1,280Wh. Usable energy is lower than both figures because you should not fully discharge either chemistry.
Why does my battery run out faster than the calculator says?
Usually four reasons: depth of discharge limits, inverter conversion losses and idle draw, high discharge rates hurting lead-acid capacity through the Peukert effect, and capacity fade as the battery ages. Cold weather makes all of it worse.
Can I run a kettle or heater off a battery?
Briefly, and only with a big inverter. A 2,000W fan heater drains a 1,200Wh usable battery in about 36 minutes. Kettles use roughly 100Wh to boil a litre, which is survivable, but resistive heating is the fastest way to flatten any battery bank.