By 
A 12V 100 Ah LiFePO4 battery powers a 100W load for about 10 hours. It does this through an inverter. Also, it runs for about 11 hours on DC. However, a 12V 100 Ah AGM battery runs the same 100W load for only about 5.5 hours.
Battery runtime calculator
Estimates use the formulas explained in this guide. Plan a complete system with the all-in-one calculator →
The runtime formula
Runtime (hours) = Battery Ah × Battery voltage × Depth of discharge × Inverter efficiency ÷ Load watts
- LiFePO4: 100 Ah × 12.8 V × 0.85 = about 1,090 usable Wh
- AGM: 100 Ah × 12 V × 0.50 = about 600 usable Wh
- Inverter efficiency: use 0.90 for AC loads, 1.0 for DC loads
Runtime table (AC loads through an inverter)
| Load | Example | LiFePO4 100 Ah | AGM 100 Ah |
|---|---|---|---|
| 25 W | LED lights, router | 39 hours | 21 hours |
| 50 W | Starlink, laptop | 20 hours | 11 hours |
| 100 W | TV + lights | 10 hours | 5.4 hours |
| 200 W | Several devices | 4.9 hours | 2.7 hours |
| 500 W | Small microwave on low, power tools | 2.0 hours | about 1 hour* |
| 1,000 W | Microwave, kettle | about 1 hour | 25–30 minutes* |
At high loads, AGM delivers less than its rating. This is due to the Peukert effect. So, real runtimes are shorter than the formula suggests.
Also check the battery’s maximum continuous current. A 100 A BMS limits a 12 V lithium battery to about 1,200 W, so a 1,000 W load is close to its limit. See battery C-rating.
How long will a 100ah battery last running a refrigerator?
A full-size fridge uses about 1 to 1.5 kWh per day. This is roughly 40 to 60 W on average. A 100 Ah LiFePO4 battery offers about 980 Wh usable through an inverter. It runs the fridge for 16 to 24 hours. An AGM battery manages about 9 to 13 hours. Also, a 12 V compressor fridge using 300 Wh per day can run for about three days from one lithium battery. See how many solar panels you need to run a refrigerator.
How long will a 100 Ah battery run a CPAP?
A CPAP without a heated humidifier typically draws 30 to 60 W. On DC with a 12 V adapter, a 100 Ah lithium battery covers roughly 2 to 3 nights. With heated humidification, consumption can double. Our CPAP battery runtime guide covers the details.
How long to recharge a 100 Ah battery with solar?
Replacing 1,090 Wh with solar at 4.5 peak sun hours needs about 1,090 ÷ (4.5 × 0.80) = 300 W of panels to refill it in one day. See how long a solar panel takes to charge a battery.
Things that shorten runtime
- Inverter idle draw. A 2,000 W inverter can use 15 to 40 W while doing nothing. This alone drains a 100 Ah battery in a day or two. See inverter idle power draw.
- Cold temperatures, especially for lead-acid.
- Battery age. Capacity fades over time. For example, an AGM battery after 300 deep cycles may hold 70 to 80% of its original capacity.
- Motor start-up surges. These do not use much energy but can trip a small battery’s BMS.
Calculate it for your setup
First, add up your daily watt-hours. Next, enter them in the solar calculator. It will tell you the amp-hours you need. Then you can see if one 100 Ah battery is enough. You can also see if you need a larger bank.
Worked example: an overnight load list
A van owner runs these from a 12 V 100 Ah LiFePO4 battery between sunset and morning (12 hours):
| Load | Power | Hours | Wh |
|---|---|---|---|
| 12 V fridge (average) | 30 W | 12 | 360 |
| LED lights | 15 W | 4 | 60 |
| Roof fan | 20 W | 8 | 160 |
| Laptop (via 12 V USB-C) | 45 W | 3 | 135 |
| Phone charging | 15 W | 2 | 30 |
| Water pump | 50 W | 0.2 | 10 |
| Total | 755 Wh |
Usable battery: about 1,090 Wh. Without solar the next day, the fridge alone would empty it by the following afternoon.
Runtime cheat sheet (DC loads, LiFePO4 100 Ah)
| Continuous load | Runtime |
|---|---|
| 5 W | 9 days |
| 20 W | 54 hours |
| 40 W | 27 hours |
| 75 W | 14.5 hours |
| 150 W | 7.3 hours |
| 300 W | 3.6 hours |
Runtime = 1,090 Wh ÷ load watts. For AC loads through an inverter, multiply by 0.90.
How to make a 100 Ah battery last longer
- Run DC loads directly rather than through an inverter.
- Turn the inverter off when not using AC.
- Pre-cool the fridge while the sun is shining. Also, avoid opening it often at night.
- Use LED lighting and efficient fans.
- Add 200 to 300 W of solar so the battery refills each day.
Common mistakes
- Planning with 100% of capacity for AGM. A 100 Ah AGM gives only about 50 Ah of healthy use.
- Forgetting standby loads such as propane detectors, stereo memory and USB sockets with indicator lights.
- Running high-power AC loads that exceed the BMS current limit.
FAQ
How many watts is a 100 Ah battery?
Amp-hours measure energy, not power. A 12 V 100 Ah battery stores about 1.2 kWh (1,200 Wh), or 1,280 Wh for a 12.8 V lithium battery. See amp hours to kWh.
Will a 100 Ah battery run a 1,000 W inverter?
Yes, for roughly an hour at full load with lithium, provided its BMS supports at least 100 A continuous. AGM will struggle and its voltage may sag below the inverter’s cutoff. See battery size for a 1,000 W inverter.
Is two 50 Ah batteries the same as one 100 Ah?
When wired in parallel, two 12 V 50 Ah batteries make a 12 V 100 Ah bank. So, yes, this works.
How long will a 100 Ah battery run a 12 V fridge?
A 12 V compressor fridge averaging 30 to 45 W runs about 24 to 36 hours on 100 Ah LiFePO4.
How long will a 100 Ah battery run a 500 W load?
About 2 hours with LiFePO4 through an inverter, or about 1 hour with AGM.
Related guides
- How Long Do Solar Batteries Last? (Years and Cycles)
- Portable Power Station Runtime Calculator (How Long Will It Last?)
- What Is a Battery Management System (BMS) and Why It Matters
- What Size Battery Backup Do I Need for My Home?
Sources and further reading
- Battery University: BU-402 What Is C-rate?: charge and discharge rate versus capacity and runtime.
- Battery University: BU-205 Types of Lithium-ion: how lithium chemistries, including LiFePO4, compare.
- Battery University: BU-409b Charging Lithium Iron Phosphate: LiFePO4 charging voltages and behaviour.