How to Calculate Battery Runtime

The battery runtime formula tells you how many hours a battery will power your devices before it needs recharging. It works for any battery (lithium, lead-acid, AGM, even a laptop battery) and uses five inputs you can find on any spec sheet.

Runtime (hours) = (Capacity × Voltage × DoD × Efficiency) / Load

That is the entire formula. The rest of this guide explains each variable, walks through three real scenarios, and flags the mistakes that cause the biggest errors in practice.

The Formula Broken Down

Each variable in the runtime formula represents a real physical property of your battery and system. Understanding what each one does helps you make better estimates and spot when a result does not look right.

Capacity (Ah): the total charge a fully charged battery can deliver, measured in amp-hours. A 100Ah battery can theoretically supply 1 amp for 100 hours, or 10 amps for 10 hours. In practice, higher discharge rates slightly reduce effective capacity through the Peukert effect, and the reduction is larger in lead-acid batteries than in lithium ones.

Voltage (V): the battery's nominal voltage, the figure on its label, such as 6V, 12V, 24V or 48V. Multiply Ah by V to get watt-hours (Wh), the true measure of stored energy. The Wh-to-Ah converter handles this conversion instantly for any voltage. A 50Ah 24V battery stores the same energy as a 100Ah 12V battery: both are 1,200Wh (50 × 24 and 100 × 12).

Depth of Discharge (DoD): the fraction of total capacity you plan to use before recharging. This guide uses 0.50 for flooded lead-acid, Trojan's recommended limit: its battery maintenance guide prints 50% or less as recommended and 80% as the maximum safe discharge. It uses the same 0.50 for AGM, because Concorde's Lifeline technical manual recommends an average depth of discharge no greater than 50%. For LiFePO4 it uses 0.80: Victron's Smart LiFePO4 datasheet prints its cycle life at 80, 70 and 50% depth of discharge (2,500, 3,000 and 5,000 cycles) and a capacity loss under 1% per 100 cycles even at 100%. Trojan also prints that shallow discharges give a longer battery life. See our depth of discharge guide for chemistry-specific recommendations.

Efficiency: accounts for energy lost as heat in wiring, connections, and the inverter (if used). Pure DC systems: 1.00 (no inverter, so nothing in the formula to lose it in). Systems with an inverter: this guide's examples use 0.90. Inverter makers print their best case: Victron's Phoenix Inverter Smart datasheet lists a maximum efficiency of 92 to 96% by model and battery voltage, and its 120V MultiPlus datasheet lists 92 to 94%, so even at its best an inverter turns 4 to 8% of what passes through it into heat (100 minus the printed figure). Renogy lists its 3000W 12V pure sine wave inverter at above 90%. Those are maximums, and the guide's 0.90 sits below all of them; your own inverter's datasheet governs, so use its figure when you have it. The same Victron sheets print a zero-load power of 8 to 20W, which the inverter draws whenever it is switched on, even with nothing plugged in.

Load (W): the total power draw of all connected devices, in watts. For constant loads (a light, a heater), this is the nameplate wattage. For cycling loads (a fridge, an air compressor), use the average power draw over a full cycle, not the peak draw when the compressor is running.

Walking Through the Variables Step by Step

Gather these five inputs and plug them into the formula to get your runtime estimate.

1. Read the battery label. Find capacity (Ah) and voltage (V). If the battery lists Wh instead (common on laptop batteries and power banks), divide by voltage to get Ah. Say a laptop battery is 72Wh at 11.1V: 72 / 11.1 = 6.5Ah. Use the battery capacity calculator for conversions. The label voltage is a name rather than a measurement, so check what the watt-hour total comes to at 12.0V for lead-acid or 12.8V for LiFePO4 before you go further.
2. Determine your DoD. This guide uses 0.80 for LiFePO4 and 0.50 for lead-acid and AGM, the figures set out above. Trojan names 80% as the maximum safe discharge for its deep-cycle batteries and warns that discharging a flooded battery 80% or more will damage it, so never plan past that. For other lithium-ion chemistries, use the depth of discharge the maker prints. If you do not know the chemistry, use 0.50, the lowest figure here, so the estimate comes out short rather than long.
3. Estimate system efficiency. Direct DC loads: this guide's examples use 1.00, since no inverter sits in the path and the cable loss depends on your own run, which the voltage drop calculator below gives you. Running AC loads through an inverter: this guide uses 0.90, as set out above, unless your inverter's datasheet prints its own figure. Cable loses power too, and our voltage drop calculator shows how much: its Outdoor Lighting 12V preset, 10 AWG copper carrying 8A over a 100 ft run (200 ft there and back), drops 1.99V, 16.57% of 12V. The wire turns the current times those lost volts into heat, so the same share of the battery's power never reaches the load. Run your own length, gauge and current through it, then multiply your efficiency figure by the share that gets through: 0.83 for the preset run, since 100 minus 16.57 is 83.43%.
4. Measure or look up your load. Check the device label for wattage. If only amps are listed, multiply by voltage: for example, if a 12V fridge's label lists 3.5A, it uses 12 × 3.5 = 42W. If only volts and amps are listed on an AC device, multiply them for apparent power (VA), then multiply by the device's power factor (PF) to get true watts: W = VA × PF. A power factor below 1 means the inverter supplies more volt-amps than the device turns into watts, since VA = W / PF and dividing by a number below 1 gives a bigger result. Check the label on your device, or its manual, for the power factor.
5. Divide and get your answer. Plug everything into the formula, or use the battery runtime calculator: on flooded, AGM and gel it also corrects for discharge rate, which the bare formula does not.

Three Worked Examples

Example 1: Camping LED lights on a portable battery

The example's assumed setup: a 100Ah 12V LiFePO4 battery and five 10W LED string lights (50W total), with no inverter - the lights run directly on 12V DC.

Runtime = (100 × 12.8 × 0.80 × 1.00) / 50

= (1,280 × 0.80) / 50

= 1,024 / 50

= 20.5 hours

At six hours of lighting an evening, the example's assumption, that is three full evenings from a single charge (3 × 6 = 18 hours, inside the 20.5). There is no efficiency factor because there is nothing to lose it in: the lights sit straight on the battery, so the only derate is the depth of discharge you stop at. A LiFePO4 battery delivers more usable runtime than lead-acid from the same Ah rating - our LiFePO4 vs lead-acid comparison explains why.

Example 2: RV refrigerator on a battery bank

Battery: the example's assumed bank of two 100Ah 12V AGM batteries in parallel (200Ah total). Load: a Dometic CFX5 35 12V compressor fridge, the example's chosen fridge and the model our fridge power outage guide names. Dometic prints its rated input current at 12V as 7.6A, but the compressor cycles on and off, so the average is far lower: its printed energy consumption is 1.11 amp-hours per hour on 12V with 90°F outside and 39°F inside, an average of 1.11 × 12 = 13.32W. No inverter.

Runtime = (200 × 12 × 0.50 × 1.00) / 13.32

= 1,200 / 13.32

= 90.1 hours

There is no inverter, so the efficiency is 1.00, and Concorde recommends stopping at 50% depth of discharge. At 13.32W the bank draws 1.11A, 0.0056C on 200Ah, slower than the slowest rate Victron prints for its AGM range, the 20-hour row itself, and below that row our calculators hold the capacity at the printed label rather than crediting more, so the formula and the calculator agree on this one. Flooded batteries are different: Trojan prints a 100-hour row for the T-105, 250Ah against the 225Ah label (100-Hr Rate / 250 Ah on its product page), so between the 20-hour and 100-hour rows the flooded calculator credits up to 11% over the label, which is why the quick reference below shows 22h 48m for 200Ah flooded at 50W where the plain formula gives 21h 36m.

That is 3 days 18 hours, the same figure the calculator prints. If Marcus from our solar battery bank sizing guide arrives at his campsite Friday at 6 PM, the fridge alone runs until about noon on Tuesday: 6 PM Friday plus 90 hours, rounding the 90.1 down. Add lights, phone charging and a water pump and the bank is carrying more than the fridge's draw, so the whole system runs out sooner.

Example 3: Home UPS running a computer and monitor

The example's assumed setup: a UPS with one internal 12V 9Ah high-rate sealed lead-acid battery, carrying a desktop computer (200W) + monitor (40W) + router (12W) = 252W total through the UPS inverter.

The amp-hour formula above breaks down here. A single 12V block feeding 252W through the inverter is delivering 23.33 amps, what our solar watts to amps calculator returns for a power of 280W at a voltage of 12V (the 280W is worked out below), and no 9Ah block holds its rated capacity anywhere near that current. Battery makers publish constant-power discharge tables for exactly this case, and our UPS battery backup time calculator reads them instead:

Battery draw = 252 / 0.90 (this guide's assumed inverter efficiency) = 280W from the one 12V block

Power-Sonic's high-rate sheet prints 46.6W per cell at 10 minutes for an 8.5Ah block; six cells and the step up to 9Ah make that 296W (46.6 × 6 × 9 / 8.5), a little more than the 280W this block carries, so it runs a little past 10 minutes, and the calculator reads between that row and the sheet's next longer one

= about 10 minutes: the table gives 10.9, which the calculator shows as 11, and a shutdown plan rounds down

This is why consumer UPS units are designed for graceful shutdown, not extended operation. The small internal battery provides just enough time to save your work and shut down. For longer backup runtime, external battery packs or a dedicated UPS battery backup system with larger batteries is necessary.

Common Calculation Mistakes

Using Ah as if it were Wh. A 100Ah battery does not store 100 watt-hours. It stores 100Ah × voltage. At 12V, that is 1,200Wh. At 24V, it is 2,400Wh. Always multiply Ah by voltage before comparing batteries or calculating runtime. Two batteries with the same Ah but different voltages store very different amounts of energy.

Ignoring DoD. Treating a 100Ah lead-acid battery as if you can use all 100Ah is an easy mistake to make. At this guide's 0.50 DoD, you have 100 × 0.50 = 50Ah. Using more still works on the day, but it shortens the battery's life.

Ignoring the inverter. At this guide's assumed 0.90 inverter efficiency, every watt of AC load costs you 1 / 0.90 = 1.11 watts from the battery. A 1,000W AC load draws about 1,000 / 0.90 = 1,111W from the battery. Forget this factor and your runtime estimate comes out longer than the battery can deliver.

Using peak watts instead of average watts. Say a refrigerator nameplate reads 150W, this example's figure: it only draws 150W while the compressor runs, and the compressor runs only part of the time. The US Department of Energy lists 404 kWh a year for an 18.1 cu ft top-freezer in the column it labels less efficient, an average of about 46W (404,000 Wh divided by 8,760 hours), and 363.5 kWh a year for an ENERGY STAR model, about 41W. Using 150W in your calculation gives you a runtime about 70% shorter than reality, because 46 divided by 150 is 0.31. Check for "average consumption" in the device manual, or measure with a kilowatt-hour meter.

Forgetting temperature effects. Cold batteries deliver less capacity. Battery University explains that cold raises a battery's internal resistance and lowers its capacity, and that the size of the drop differs with chemistry. If you are running a battery system in an unheated space during winter, reduce your capacity estimate accordingly.

When the Simple Formula Falls Short

The runtime formula assumes a constant load and a linear discharge curve. Real-world systems deviate from both assumptions, and understanding when the formula breaks down helps you plan for the gap.

The Peukert effect. Lead-acid batteries deliver less total energy at higher discharge rates, and the makers print the size of it. Trojan rates its T-105 at 225Ah on the 20-hour rate, and the same data sheet prints 207Ah on the 10-hour rate and 185Ah on the 5-hour rate: 92% and 82% of the label. Internal resistance and chemical kinetics limit how fast a lead-acid plate can give up its charge, and the label is measured at the gentlest rate of the three.

LiFePO4 is far less affected: Victron footnotes its nominal-capacity row "Discharge current 1C", so the label is already measured at a one-hour rate. You no longer need to knock a guessed percentage off by hand. Our runtime calculators read the correction from those printed discharge tables, interpolating between the printed rates, so the figure you get for a flooded, AGM or gel battery is already rate-adjusted.

Non-linear and intermittent loads. The formula works cleanly for constant loads like lights or heaters. For intermittent loads - refrigerators, air compressors, pumps - the compressor cycling pattern means your battery alternates between heavy draw and zero draw.

The average power determines total runtime, but the peak current during compressor-on periods causes greater instantaneous voltage sag and wiring losses than the average would suggest. For mixed loads where some devices cycle while others run continuously, calculate runtime based on the combined average draw, then plan on less runtime than that figure, because the peaks lose more in the wiring than the average shows.

Quick Reference Table

Runtimes assume system efficiency of 90%, this guide's assumption for a system with an inverter. For direct DC loads there is no inverter to lose anything in, so add back whatever efficiency figure you entered for it. For exact runtime with your specific setup, use the 12V battery runtime calculator.