Runtime comes from usable watt hours divided by actual draw, and both halves of that get misread constantly. A battery runtime calculator has to account for voltage, depth of discharge and inverter losses, because a 100Ah label describes none of them. Our guide on how many watt hours you need covers sizing from the load side.
The calculator below works out what your bank actually holds, what your load actually draws, and how long one covers the other.
Amp Hours Are Half a Number
| Bank | Total energy | Usable, LiFePO4 | Usable, lead acid |
|---|---|---|---|
| 100Ah at 12V | 1,200Wh | 960Wh | 600Wh |
| 100Ah at 24V | 2,400Wh | 1,920Wh | 1,200Wh |
| 100Ah at 48V | 4,800Wh | 3,840Wh | 2,400Wh |
| 200Ah at 12V | 2,400Wh | 1,920Wh | 1,200Wh |
| 400Ah at 12V | 4,800Wh | 3,840Wh | 2,400Wh |
The same 100Ah figure describes four times the energy at forty eight volts that it does at twelve. Comparing banks by amp hours alone is meaningless unless the voltages match.
Watt hours are the honest unit. Multiply amp hours by nominal voltage and compare those numbers instead.
Depth of Discharge Removes a Chunk Before You Start
Lead acid batteries are damaged by deep cycling, so about half the rated capacity is genuinely available. Draining a flooded or AGM bank to twenty percent repeatedly will cost you most of its service life.
Lithium iron phosphate tolerates eighty percent routinely and ninety percent occasionally. That difference means a 100Ah lithium bank delivers more usable energy than a 150Ah lead acid bank.
Price per usable watt hour is the comparison that matters, not price per amp hour. Our comparison of LiFePO4 versus lead acid covers where each one wins once that is accounted for.
Our guide on discharge depth covers the tradeoff between usable capacity and cycle life.
The Inverter Takes Its Cut
Converting DC to AC costs energy, and that loss leaves as heat regardless of how good the inverter is. Good units run around ninety percent under load, cheaper ones closer to eighty five.
The practical effect is that delivering 150W of AC pulls roughly 170W from the bank. Over a long run that difference is substantial.
DC loads skip the conversion entirely. Running lighting, fans and refrigeration on DC where possible is one of the few free efficiency gains available. Our guide to DC to AC conversion covers the maths in more depth.
Idle Draw Matters on Long Runs
An inverter consumes power whenever it is switched on, typically a few watts to twenty depending on size. On a heavy load that is negligible.
On an overnight standby run it dominates. An inverter idling at 15W for twelve hours consumes 180Wh, which can exceed what the actual load used.
Search settings help. Most inverters can sleep until they detect a load, and enabling that on a lightly used circuit changes overnight consumption dramatically.
Parasitic draws add up elsewhere too. Our guide on what drains a solar battery covers the ones people miss.
Why Real Runtime Falls Short of Calculated
Cold reduces capacity
Battery chemistry slows in cold, and available capacity drops with it. Lithium is affected less than lead acid but still loses meaningfully below freezing, and charging a cold lithium cell causes permanent damage.
High discharge rates cost extra on lead acid
The Peukert effect means a lead acid battery discharged quickly delivers less total energy than the same battery discharged slowly. Lithium is far less sensitive to this.
Age erodes it steadily
Every bank loses capacity over its life. A three year old bank does not hold what its label says, and runtime calculations based on nameplate figures will overshoot.
Voltage sag under load
Terminal voltage falls as current rises, so the inverter draws more amps to deliver the same watts. The effect is small on a healthy bank and pronounced on a tired one.
Runtime Against Autonomy
Runtime answers how long a bank lasts on a given load right now. Autonomy answers how many days a system carries you with no meaningful charging at all.
They use the same energy figure and different denominators. Runtime divides usable watt hours by an instantaneous load, while autonomy divides them by total daily consumption.
Off grid sizing generally works from autonomy, because the binding case is a run of overcast days rather than a single evening. Two to three days is a common target, and five is normal where winter cloud is persistent.
Runtime is the more useful figure for backup and portable use, where the question is whether a specific load survives a specific outage.
Temperature Changes the Answer
Cold slows the chemistry and reduces available capacity in every battery type. Lead acid loses more than lithium, and both lose more than their datasheets suggest at the extremes.
A bank rated at room temperature can deliver noticeably less on a cold night in an unheated space, which is exactly when a heating load is likely to be running.
Charging a lithium battery below freezing causes permanent plating damage rather than a temporary reduction. Most quality lithium batteries include low temperature charge protection, and it is worth confirming yours does.
Heat is the slower problem. High temperatures increase available capacity slightly while shortening total service life considerably, so a hot equipment cupboard costs you years rather than hours.
Treat the Result as a Ceiling
The calculation assumes a healthy bank at room temperature delivering steady current. Real conditions are worse than that in every direction.
Build in margin rather than planning to the calculated figure. A bank sized so that a normal night uses half its usable capacity will still work on a cold night three years from now.
Measuring beats estimating. Our roundup of battery monitors covers shunt based meters that count actual amp hours in and out rather than guessing from voltage.
For sizing a bank from scratch rather than checking one you have, our battery bank sizing calculator works from daily consumption and days of autonomy instead.
Battery Runtime FAQ
How long will a 100Ah battery run a 150W load?
A 100Ah 12V lithium bank holds 1,200Wh total and about 960Wh usable at 80% depth of discharge. Through an inverter at 87% efficiency, a 150W load pulls around 172W, giving roughly five and a half hours. The same bank in lead acid gives about half that.
Why does voltage matter if the amp hours are the same?
Because energy is amp hours multiplied by voltage. A 100Ah bank at 48V holds four times the energy of a 100Ah bank at 12V. Comparing banks by amp hours alone only works when the voltages match, which is why watt hours is the more useful unit.
What depth of discharge should I use?
Around 50% for flooded or AGM lead acid, and 80% for lithium iron phosphate in normal use. Lithium tolerates 90% occasionally without much penalty. Going deeper than these figures on lead acid shortens service life sharply rather than causing immediate failure.
How much does the inverter cost me?
Typically ten to fifteen percent. A load drawing 150W of AC pulls roughly 170W from the battery, and the difference becomes heat. Running loads directly on DC where the appliance allows it avoids this entirely.
Does inverter idle draw matter?
On long low load runs, considerably. An inverter idling at 15W consumes 180Wh over twelve hours, which can exceed the load it was left on for. Enabling the search or sleep mode so it wakes on demand is the usual fix.
Why is my real runtime shorter than calculated?
Several effects stack. Cold reduces available capacity, high discharge rates cost extra on lead acid through the Peukert effect, and every bank loses capacity as it ages. Treat the calculated number as an optimistic ceiling and size with margin.
Can I mix batteries of different ages or brands?
It is a poor idea. Batteries wired together equalize toward the weakest member, so an older or smaller unit drags the whole bank down to its own behavior. Mixing chemistries is worse still, since they need different charge voltages that no single controller can satisfy.
How do I know what my bank is actually doing?
A shunt based battery monitor counts amp hours in and out rather than inferring state of charge from voltage, which is unreliable under load. It is the only way to see real usable capacity as a bank ages, and it costs a fraction of the bank it protects.
