Battery Life Calculator
Runtime from capacity, load, and usable depth of discharge — lead-acid 50%, lithium 80%.
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You need
5.0 hoursruntime
50 Ah usable at a 10 A draw
- Usable capacity
- 50 Ah
- Runtime
- 5.0 hours
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In short
How long will a 100 Ah battery last?
Runtime = capacity × depth of discharge ÷ load. A 100 Ah battery at a 50 percent depth of discharge gives 50 usable amp-hours, which at a 10 A draw is 5.0 hours. Move to a LiFePO4 battery designed around 80 percent and the same 100 Ah runs 8.0 hours; halve the load to 5 A and the lead-acid figure doubles to 10 hours.
This is the linear model, so it reads as an optimistic ceiling at heavy draws, where a lead-acid battery delivers measurably less than its rated amp-hours.
How to use the battery life calculator
Enter the amp-hour capacity, the steady current the battery is supplying, and the usable depth of discharge, and the tool returns the runtime in hours along with the usable amp-hours it worked from. The arithmetic is deliberately short: usable capacity is the rating multiplied by the fraction you are willing to draw, and runtime is that usable capacity divided by the load.
There is no electricity rate anywhere on the page, because this answers how long the battery lasts rather than what it costs. The two inputs that move the answer most are the load, which is inversely proportional to runtime, and the depth of discharge, which is the setting most people get wrong by simply leaving it at the maximum.
5.0 hours
Lead-acid at 50 percent
100 Ah battery, 10 A draw
8.0 hours
LiFePO4 at 80 percent
same 100 Ah, same 10 A
10 hours
Halve the load to 5 A
lead-acid figure doubles
Make the load figure realistic, because an optimistic load produces an optimistic runtime and the error compounds through everything downstream. The number the tool wants is the steady current of everything drawing at once, which is rarely the sum of every device you own. Add the amps directly if your equipment is labelled that way, or divide each device wattage by the system voltage and sum the results, which is what the volts-to-watts tool does in reverse.
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Where a load cycles on and off — a fridge compressor, a water pump, a fan on a thermostat — use the average current over an hour rather than the peak, since the peak only exists for part of the time and sizing to it wastes a lot of battery.
Comparing two batteries at different voltages?
Amp-hours cannot rank batteries across system voltages. The watt-hours tool multiplies capacity by nominal voltage so a 12 V and a 48 V bank land on one scale.
Open the battery watt-hours calculator →Do
- Enter the nameplate amp-hour rating, not a guess at how full the battery is.
- Set the depth of discharge to match the chemistry: 50 percent lead-acid, 80 percent LiFePO4.
- Divide each device wattage by the system voltage before summing the amps.
- Use the average current over an hour for anything that cycles on and off.
Don't
- Size a lead-acid bank around the full nameplate, which ages it fast.
- Trust the figure at heavy draws, where the linear model reads as a ceiling.
- Expect the amp-hours to run out before an inverter low-voltage cutout does.
- Ignore cold, which cuts available capacity on every chemistry.
What the depth-of-discharge setting is really doing, chemistry by chemistry. Every runtime figure is the same 100 Ah nameplate battery under a steady 10 A draw, so the only thing changing down the table is how much of that nameplate you allow yourself to use — which is the single decision that separates a bank lasting years from one lasting months.
| Chemistry or setting | Depth of discharge commonly designed to | Usable Ah from a 100 Ah nameplate | Runtime at a steady 10 A | What the cycle life looks like at that depth |
|---|---|---|---|---|
| Flooded lead-acid, automotive starting battery | 10 to 20 percent | 10 to 20 Ah | 1.0 to 2.0 hours | Built for short high-current bursts, not repeated deep cycling at all |
| Flooded lead-acid deep-cycle, Trojan recommendation | 20 to 50 percent | 20 to 50 Ah | 2.0 to 5.0 hours | Trojan advises 20 to 50 percent for optimum life even though the battery can go to 80 |
| Flooded lead-acid deep-cycle, conventional design point | 50 percent | 50 Ah | 5.0 hours | The figure most off-grid sizing guides and this tool default to |
| Flooded lead-acid pushed hard | 80 percent | 80 Ah | 8.0 hours | Trojan says the battery is capable of 80 percent but does not recommend it as a routine |
| AGM sealed lead-acid, long-service sizing | 50 percent | 50 Ah | 5.0 hours | Broadly matches a flooded battery of the same rating at the same depth |
| AGM sealed lead-acid, deeper cycling | 80 percent | 80 Ah | 8.0 hours | Industry references quote AGM at 80 percent for the life a flooded battery gets at 50 |
| Gel sealed lead-acid | 50 percent | 50 Ah | 5.0 hours | Similar to AGM, and noticeably happier with slow deep discharges than fast ones |
| LiFePO4, common design figure | 80 percent | 80 Ah | 8.0 hours | Battle Born publishes a cycle life of 3,000 to 5,000 for its LiFePO4 packs |
| LiFePO4, full rated capacity | 100 percent of rated Ah | 100 Ah | 10.0 hours | The battery management system cuts off first, so rated 100 percent is not cell-level empty |
| Lithium NMC, as used in tool and e-bike packs | 80 percent, managed by the pack electronics | 80 Ah | 8.0 hours | Higher energy density than LiFePO4 and generally fewer cycles for the same depth |
| Any chemistry, very shallow cycling | 20 percent | 20 Ah | 2.0 hours | The gentlest routine there is, and the top of the Trojan recommended window |
| Any chemistry, shallow cycling | 30 percent | 30 Ah | 3.0 hours | Buys a lot of cycle life at the cost of carrying capacity you never use |
| Any chemistry, run flat | 100 percent | 100 Ah | 10.0 hours | A theoretical figure; as a routine on lead-acid it is the fastest way to kill a bank |
How much of the nameplate can you actually use?
The idea to internalise is that you never get to use a battery whole nameplate. A lead-acid battery cycled routinely past half empty ages fast, which is why 50 percent is the conventional sizing figure across off-grid design, and why Trojan itself recommends discharging its deep-cycle batteries only 20 to 50 percent of rated capacity even though the battery is capable of going to 80 percent.
LiFePO4 is a different animal: Battle Born publishes a cycle life of 3,000 to 5,000 for its LiFePO4 packs and rates them for 100 percent usable depth of discharge, which no flooded battery offers. The practical consequence is that swapping chemistry at the same amp-hour rating changes your runtime by roughly 60 percent without changing a single physical dimension.
Read it: The battery never changes; the only thing moving down the chart is the fraction of the nameplate you are willing to draw.
Computed as capacity × depth of discharge ÷ load, the same linear model as the tool.
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Full guide
How Long Will a Battery Run My House? The Two Tests It Has to Pass
A 100 Ah battery is 1,200 Wh, 600 Wh of it usable — and none of that says whether your fridge will start.
Read the full guide →The formula, worked line by line
Runtime is usable capacity divided by load, and usable capacity is the rating multiplied by the fraction of it you are prepared to draw. Both steps are ordinary division, which is exactly why the depth-of-discharge term carries so much weight: it is a straight multiplier on the answer, so a battery rated at 50 percent usable produces precisely five-eighths the runtime of the same battery rated at 80 percent.
Everything here stays in amp-hours and amps rather than watt-hours and watts, because that is how batteries and DC loads are labelled and because the system voltage cancels out of both sides of the division. If your loads are given in watts, convert them to amps at the system voltage first — a 120 W load on a 12 V system is 10 A — and the arithmetic below then applies unchanged.
usable Ah = capacity × depth of discharge
hours = usable Ah ÷ load
load in amps = load in watts ÷ system voltage
100 Ah × 0.50 = 50 Ah; 50 ÷ 10 A = 5.0 hours- Nameplate capacity
- 100 Ah
- Depth of discharge
- × 0.50
- Usable capacity
- 100 × 0.50 = 50 Ah
- Steady load
- ÷ 10 A
- Runtime
- 5.0 hours
Change one input at a time to see which lever matters. Lift the depth of discharge to 80 percent for LiFePO4 and the same battery under the same load runs 80 ÷ 10 = 8.0 hours, a 60 percent gain. Halve the load to 5 A instead and the lead-acid battery runs 50 ÷ 5 = 10 hours. Double it to 20 A and the runtime collapses to 2.5 hours.
Where the arithmetic starts to bend
That last case is where the linear model starts to overstate things. A deep-cycle lead-acid capacity is conventionally rated at the twenty-hour rate, which for a 100 Ah battery means 5 A for 20 hours. A 20 A draw is four times that rate, and Peukert law — with an exponent industry references put around 1.1 to 1.3 for true deep-cycle batteries — says the delivered capacity falls as the rate rises.
So the 2.5 hours the formula returns at 20 A is a ceiling, not a forecast. LiFePO4 is far less affected, which is another reason lithium banks behave closer to their arithmetic than lead-acid ones do.
Two things this calculation deliberately does not model. It does not track voltage sag, so it will not tell you when an inverter with a low-voltage cutout will shut down before the amp-hours run out, and on lead-acid that cutout often arrives first.
And it does not account for temperature, which reduces available capacity in the cold on every chemistry and can be severe on lithium, where charging below freezing is restricted by the battery management system. Use the result to plan and to compare options, and have a licensed electrician or installer size anything that will be permanently wired in.
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Questions people ask
How long will a 100 Ah battery last at a 10 A load?
About 5.0 hours on a lead-acid battery sized at the conventional 50 percent depth of discharge, because 100 Ah × 0.50 gives 50 usable amp-hours and 50 divided by 10 A is 5 hours. A LiFePO4 battery of the same rating designed around 80 percent gives 80 usable amp-hours and runs the identical load for 8.0 hours. Drop the load to 5 A and the lead-acid figure doubles to 10 hours, since runtime scales inversely with current.
Covered in depth in How Long Will a Battery Run My House? The Two Tests It Has to Pass →
Why should I not use the full rated capacity?
Because depth of discharge is the main thing that decides how many cycles a battery survives. Trojan recommends discharging its deep-cycle flooded batteries only 20 to 50 percent of rated capacity for optimum life, even though the battery is physically capable of 80 percent. LiFePO4 tolerates far deeper cycling, with Battle Born publishing a cycle life of 3,000 to 5,000 and rating its packs for 100 percent usable depth of discharge. Sizing around the usable slice rather than the nameplate is what turns a bank that lasts months into one that lasts years.
Lead-acid or lithium — which one actually runs longer?
For the same amp-hour rating, lithium runs longer because more of the rating is usable. At the conventional 50 percent for lead-acid against 80 percent for LiFePO4, a 100 Ah lithium battery delivers 80 usable amp-hours to a lead-acid battery 50, which is 60 percent more runtime from an identical label. Lithium also holds its voltage far better under load and loses much less capacity at high discharge rates, so the arithmetic on this page describes a lithium bank more accurately than it describes a lead-acid one.
What exactly should I enter as the load in amps?
The steady current the battery is supplying with everything on at once. Add the amps of each device directly if they are labelled that way. If they are labelled in watts, divide each wattage by the system voltage first, so a 120 W load on a 12 V system is 10 A. For anything that cycles on and off, such as a fridge compressor or a thermostat-controlled fan, use its average current over an hour rather than its peak draw, or the runtime will come out far too short.
Does a heavy load really reduce the usable capacity?
Yes, and the effect is well documented. Deep-cycle lead-acid capacity is conventionally rated at the twenty-hour rate, so a 100 Ah battery means 5 A sustained for 20 hours. Discharge that same battery in two hours and it delivers noticeably less than 100 Ah, a relationship captured by Peukert law with an exponent commonly given as 1.1 to 1.3 for deep-cycle batteries. This tool uses the straightforward linear model, which is accurate enough at moderate draws and optimistic at heavy ones relative to capacity.
Sources
Where the constants and formulas on this page come from. Each line names the figure it backs.
That Trojan recommends discharging its deep-cycle batteries only 20 to 50 percent of rated capacity for optimum life, even though the battery is capable of being cycled to 80 percent.
Frequently Asked Questions — Trojan Battery Company
The published cycle life of 3,000 to 5,000 and the 100 percent usable depth of discharge quoted for LiFePO4.
12V 100Ah LiFePO4 Deep Cycle Battery — specifications — Battle Born Batteries, August 2026
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