How the calculation works
Runtime is stored energy divided by the rate you draw it, with two adjustments that most quick estimates skip: the portion of the battery you can safely use, and the losses of converting DC to AC.
A 100 Ah 12 V battery holds 1,200 Wh on paper. In lithium you may use roughly 960 Wh of that; in lead-acid closer to 600 Wh, because draining lead-acid past half destroys its cycle life. Run the load through an inverter and another 10 to 15% disappears as heat.
Worked example
A 100 Ah LiFePO4 battery at 12 V running a 100 W AC load:
The same battery in AGM would deliver about 5.2 hours, and running the load on DC instead of through an inverter would stretch the lithium figure to roughly 9.6 hours. Those two choices, chemistry and DC versus AC, change runtime more than anything else you can adjust.
Why real runtime is often shorter
Published capacities are measured at a slow, gentle discharge. Draw current hard and lead-acid batteries deliver noticeably less than their rating, an effect described by Peukert's law; a battery rated 100 Ah over 20 hours may yield the equivalent of 70 Ah when emptied in four. Lithium is far less sensitive to this, which is another reason its usable figure holds up in practice.
Cold weather also reduces available capacity, and inverter idle draw runs continuously whether or not your appliance is on. For planning, treat the calculated figure as an optimistic ceiling and keep a margin.
Common mistakes
- Using the full nameplate capacity. Only lithium gets close to it, and even then 100% discharge is not something to do daily.
- Forgetting the inverter. AC loads always cost more than their label, and the inverter itself consumes 10 to 25 W simply being switched on.
- Assuming a constant load. Fridges and pumps cycle, so their average draw is much lower than nameplate. Use average watts for cycling appliances.
- Planning to empty the bank routinely. Repeated deep discharge shortens life in every chemistry. Size so that normal use takes you part way down, not to the floor.
Frequently asked questions
How long will a 100 Ah battery run a fridge?
A compact 12 V fridge averaging about 45 W of actual consumption will run roughly 20 hours from a 100 Ah lithium battery, less in hot weather when the compressor cycles more. A full-size AC fridge through an inverter drains the same battery in well under a day, which is why cabins with mains-style refrigeration need substantially larger banks.
Does a bigger inverter shorten runtime?
Only through idle draw. The inverter supplies whatever the appliance needs, so a 3,000 W unit powering a 100 W load delivers the same 100 W. Its standby consumption, however, may be double that of a small inverter, which matters on long light-load runs.
Can I connect batteries to extend runtime?
Yes. Wiring in parallel adds amp-hours at the same voltage and multiplies runtime accordingly. Use identical batteries of the same age and chemistry, and keep cable lengths equal so all units share current evenly.