LiFePO4 Battery Runtime Calculator: How to Estimate Hours from Ah, Voltage and Load

A battery label gives you voltage and amp-hours, but it does not directly tell you how many hours a real load will run. Runtime depends on the energy available at the battery boundary, the energy consumed by the load, conversion losses, the load's duty cycle and the point at which the battery or BMS stops discharge.

The practical answer is:

Estimated runtime (hours) ≈ usable battery energy (Wh) ÷ average load (W)

For an AC load supplied through an inverter, the input side must also include inverter efficiency and idle consumption. For a DC load, the calculation must use the correct DC voltage, current and protection boundary.

This guide answers one customer question:

> How do I estimate LiFePO4 battery runtime from Ah, voltage and load without treating a nominal label as a guaranteed result?

The examples are illustrative planning calculations. Use the exact battery datasheet, BMS limits, inverter data, temperature conditions, measured load and installation requirements before making a purchase or promising runtime.

The quick runtime formula

Start with four values:

1. Nominal battery voltage in volts.

2. Rated capacity in amp-hours.

3. The fraction of nominal energy that is allowed to be used.

4. The load energy consumed at the relevant battery or inverter boundary.

The first-pass nominal energy calculation is:

Nominal energy (Wh) = nominal voltage (V) × rated capacity (Ah)

A more useful planning estimate is:

Usable energy (Wh) = nominal voltage × rated Ah × usable-energy fraction

Then:

Runtime (h) ≈ usable energy (Wh) ÷ average load (W)

The usable-energy fraction is not a universal LiFePO4 constant. It may be limited by the manufacturer's stated usable capacity, discharge floor, BMS settings, inverter cut-off, temperature, age, current, reserve policy or the specific operating objective.

Why Ah alone is not enough

Amp-hours measure charge, not energy. A 100Ah battery at one nominal voltage does not store the same nominal energy as a 100Ah battery at another voltage.

The U.S. Department of Energy's explanation of solar integration and storage separates energy capacity in kWh from power capacity in kW and notes that storage and conversion are not perfectly efficient. Keep those two dimensions separate throughout the calculation.

Step 1: Calculate nominal energy from voltage and Ah

Suppose an illustrative battery is described as 12.8V and 100Ah:

12.8V × 100Ah = 1,280Wh

That is a nominal energy figure, not a promise that 1,280Wh will reach the load. The number may be based on a particular temperature, discharge current, cut-off voltage, test method and end-of-test condition.

For a 51.2V and 100Ah illustrative battery:

51.2V × 100Ah = 5,120Wh

The second example has four times the nominal energy because the nominal voltage is four times higher at the same Ah rating. It does not automatically have four times the output power: power is also constrained by the battery, BMS, current path and inverter.

Nominal voltage is a calculation reference

A LiFePO4 battery's voltage changes during operation. The nominal voltage is useful for an initial energy or current calculation, while the actual voltage at a particular state of charge and load can be different.

For a final design, ask for:

• the manufacturer's nominal voltage definition;
• the operating-voltage range;
• the test voltage and capacity conditions;
• the low-voltage or BMS cut-off boundary;
• the permitted charge and discharge current;
• the conditions under which usable energy is stated.

Do not replace an exact product datasheet with a nominal value copied from a product title.

Step 2: Convert nominal energy into usable energy

A runtime calculator needs a clear answer to “usable for what purpose?” A system that must preserve emergency reserve should not use the same assumption as a test that deliberately discharges to a laboratory cut-off.

A simple planning expression is:

Usable energy = nominal energy × permitted usable fraction

The permitted usable fraction may be set by:

• the battery manufacturer's usable-energy specification;
• a BMS discharge floor;
• an inverter low-voltage cut-off;
• an owner's reserve policy;
• temperature and installation constraints;
• an end-of-life or degradation allowance;
• a project requirement to stop before a protection event.

Illustrative usable-energy calculation

Using the illustrative 12.8V / 100Ah battery:

Nominal energy = 12.8 × 100 = 1,280Wh

If a planning worksheet assumes that 80% of nominal energy is available before the chosen reserve or cut-off:

Planning energy = 1,280 × 0.80 = 1,024Wh

The 80% is an explicit planning assumption, not a claim about every LiFePO4 battery. Replace it with the exact product or project value when available.

The Victron BMV-712 operating documentation explains that effective capacity depends on discharge rate and temperature, and that a monitor integrates measured current over time. That is why a runtime estimate should show its assumptions instead of presenting one fixed number.

Step 3: Define the load at the correct boundary

The load can be measured in different places:

• an appliance's AC input;
• the inverter's AC output;
• the DC bus;
• the battery terminals;
• a dedicated DC load circuit.

Do not divide battery Wh by an AC nameplate wattage until you have accounted for the path between the battery and the load.

Direct DC load

For a direct DC load, a first-pass calculation can use:

Runtime (h) ≈ usable battery energy (Wh) ÷ average DC load (W)

If the load is known as current rather than power:

Runtime (h) ≈ usable capacity (Ah) ÷ average DC current (A)

The current-based version is only meaningful when the voltage, current, cut-off and capacity test conditions are compatible. A DC converter, cable loss or variable-current load can make the simple version optimistic.

AC load through an inverter

For an AC load supplied by an inverter, estimate the battery-side input power as:

Battery input power ≈ AC load power ÷ inverter efficiency + inverter idle power

Then:

Runtime (h) ≈ usable battery energy ÷ battery input power

Inverter efficiency changes with load and operating mode. Idle draw matters when the AC load is small or cycles on and off. Use the inverter manufacturer's curve or measured data where the result matters.

The Victron Phoenix battery-capacity guidance illustrates the same design principle: build the load's watt-hours, account for inverter losses and no-load consumption, then translate the result to the battery boundary.

Step 4: Use average load, not only the largest label

Runtime is an energy question, so the load profile matters. A refrigerator, pump, compressor, heater or communications system may not draw its nameplate power continuously.

For a load with several operating blocks:

Total load energy (Wh) = Σ(load power × time in hours)

Then:

Average load (W) = total load energy ÷ observation period

The table does not claim that those loads behave that way in a particular home. It shows why a 180W refrigerator nameplate cannot be treated as a continuous 180W load for every hour, and why its starting power still needs a separate inverter check.

Duty cycle is not the same as starting power

A duty cycle reduces the energy estimate when a device is off or operating at lower power for part of the period. It does not remove the need to check starting demand.

For example:

• a compressor may consume moderate energy over a day but require a short starting surge;
• a pump may run for only a few minutes but draw high power while starting;
• an electric heater may run at a stable high power for a long period;
• a communications load may be small but continuous.

Use an energy worksheet for runtime and a power worksheet for startup and continuous output.

Step 5: Include inverter efficiency and idle consumption

For an AC system, an initial estimate can be built in two stages.

Illustrative example:

• AC load: 500W;
• assumed inverter efficiency: 90%;
• assumed inverter idle draw: 40W;
• assumed usable battery energy: 1,024Wh.

Battery input estimate:

500 ÷ 0.90 + 40 ≈ 596W

Runtime estimate:

1,024 ÷ 596 ≈ 1.72h

The calculation is only as good as the assumptions. If the inverter's actual efficiency at 500W is different, if its idle consumption changes by operating mode or if the load cycles, the result changes.

Do not copy the 90% and 40W values into a product recommendation. They are illustrative values for showing the method.

Efficiency is a path property

A battery system can have losses in:

• the battery's charge and discharge behavior;
• BMS and contactors;
• cables, fuses, breakers and connections;
• DC-DC converters;
• the inverter;
• AC distribution;
• the appliance's own power supply.

A round-trip efficiency number is not always the same as discharge-path efficiency from the battery to an AC load. State which boundary the estimate uses.

Step 6: Check power separately from runtime

A battery can have enough energy for a long runtime and still be unable to start or carry the load.

Use a separate power screen:

A first-pass battery-side current estimate is:

Battery current (A) ≈ battery input power (W) ÷ actual battery voltage (V)

At 800W battery input and an illustrative 51.2V voltage:

800 ÷ 51.2 ≈ 15.6A

This is arithmetic, not a product rating. Actual current can rise as voltage falls, and the BMS or current path may impose a lower limit than the arithmetic suggests.

For a known inverter rating, use the existing 5kW, 8kW and 10kW LiFePO4 battery current guide. It owns the fixed-inverter current-screen question; this article owns the general runtime-calculation method.

Step 7: Account for low-voltage cut-off and BMS behavior

A runtime estimate ends when the load stops receiving power, not necessarily when every joule in a theoretical battery model has been removed.

The stopping boundary may be:

• inverter low-voltage cut-off;
• BMS low-cell or pack-voltage protection;
• a configured discharge floor;
• a manual reserve target;
• a temperature limit;
• a communication or enable signal;
• a fuse, breaker or contactor event.

If the inverter stops first, the battery may still show voltage. If the BMS disconnects first, the inverter may report a low-voltage or battery fault. The two events should not be treated as the same.

For projects that require a visible state-of-charge record, a properly installed shunt or battery monitor can help compare measured current and energy with the estimate. See Do You Need a Shunt or Battery Monitor for a LiFePO4 Battery?. A monitor improves evidence; it does not change the battery's usable energy or make an incompatible load safe.

Step 8: Make the estimate match the application

The same formula serves different applications, but the inputs are not interchangeable.

Home backup

For a home backup system, define:

• which circuits are protected;
• whether the load runs during an outage;
• the outage duration to plan for;
• refrigerator, pump, HVAC or heater cycling;
• overnight communications and standby loads;
• the reserve left for the next day;
• the solar or grid recovery window.

Use the home battery critical-load planning guide for the broader question of how much storage the home objective needs. The 16kWh home runtime article remains the focused owner for that fixed-capacity home-runtime question.

RV or marine service loads

For an RV or boat, include:

• house or service loads rather than engine-start loads;
• inverter idle draw;
• refrigerator mode;
• lighting and communications;
• pumps and intermittent motors;
• solar, alternator, shore or generator recovery;
• compartment fit, ventilation and service access.

The 100Ah versus 200Ah RV comparison owns that capacity comparison, while this calculator explains the transferable math behind it.

Solar plus battery systems

For a solar-plus-battery system, runtime is only one part of the operating plan. Also check:

• when PV is available;
• whether the load is served directly from PV;
• whether the battery is charging or discharging;
• charge-controller or inverter power limits;
• reserve for overnight or outage operation;
• weather and seasonal recovery.

The solar battery charging schedule guide covers schedule policy and reserve decisions. Do not treat a runtime calculator as a prediction of tomorrow's solar production.

Two complete illustrative examples

Example A: direct DC load

Assume, purely for illustration:

• nominal battery voltage: 12.8V;
• rated capacity: 100Ah;
• permitted usable fraction: 80%;
• average DC load: 256W;
• DC path efficiency: 95%.

Nominal energy:

12.8 × 100 = 1,280Wh

Usable energy after the stated planning fraction:

1,280 × 0.80 = 1,024Wh

After the illustrative DC path allowance:

1,024 × 0.95 = 972.8Wh

Estimated runtime:

972.8 ÷ 256 ≈ 3.80h

This result is not a promise that a 12.8V / 100Ah product will run a 256W load for 3.8 hours. Replace the illustrative values with the exact battery, converter, wiring, temperature, current and cut-off data.

Example B: AC load through an inverter

Assume, purely for illustration:

• nominal battery voltage: 51.2V;
• rated capacity: 100Ah;
• permitted usable fraction: 85%;
• AC load: 800W;
• inverter efficiency at that load: 92%;
• inverter idle draw: 40W.

Nominal energy:

51.2 × 100 = 5,120Wh

Usable energy:

5,120 × 0.85 = 4,352Wh

Estimated battery input power:

800 ÷ 0.92 + 40 ≈ 909W

Estimated runtime:

4,352 ÷ 909 ≈ 4.79h

Battery-side current at the illustrative 51.2V reference:

909 ÷ 51.2 ≈ 17.8A

The current is not constant throughout the entire discharge if battery voltage changes. The BMS, inverter, cable, fuse and battery specifications must all be checked separately.

A practical runtime worksheet

Before comparing products or asking for a quotation, record the following:

The home battery quote information checklist helps turn these fields into a supplier-ready brief. A complete brief is more useful than sending only “How many hours will a 16kWh battery last?”

Common runtime-calculation mistakes

Dividing Ah by watts

Ah and W are different units. Convert Ah to Wh with a voltage assumption, then account for the load boundary.

Treating nominal kWh as delivered AC energy

Nominal battery energy is not the same as AC energy delivered through an inverter. Include the usable-energy rule, conversion path and idle draw.

Using the appliance's maximum label for every hour

Maximum power may be needed for the inverter check but may overstate energy if the device cycles. Build the actual profile where possible.

Ignoring a small continuous load

Inverter idle power, network equipment, control electronics and monitoring can consume energy for the whole outage. Small loads matter over long periods.

Using a single efficiency percentage for every condition

Efficiency changes with load, temperature, voltage and operating mode. State whether the number is measured, manufacturer-stated or illustrative.

Treating runtime as a power rating

Runtime math cannot prove that a battery, BMS, fuse, cable or inverter can start a motor or carry a high continuous load.

Assuming the BMS will deliver the last calculated watt-hour

The BMS protects the battery according to its own limits and signals. A protection event can end the load before a simple energy equation predicts.

Combining two batteries without recalculating the system

Parallel or series expansion changes voltage, capacity, current, BMS communication, protection and usable-energy assumptions. Recalculate the complete bank and verify the exact configuration.

When a runtime estimate is good enough—and when it is not

A rough estimate can be useful for:

• comparing two capacity ranges;
• deciding whether a load is obviously too large;
• preparing a first inquiry;
• identifying which measurement is missing;
• comparing energy recovery options.

Use a measured or manufacturer-supported model when:

• the load is a motor, compressor, heater or medical/critical system;
• the outage objective is safety- or business-critical;
• temperature is low or highly variable;
• the system is near a BMS, inverter, fuse or cable limit;
• the battery is old or has a known degradation history;
• the result determines an installation or purchase commitment.

For capacity evidence after commissioning, see How to Test a LiFePO4 Battery's Real Capacity. A capacity test can improve confidence in the battery's measured output, but it does not replace the inverter, power and installation checks.

Frequently asked questions

What is the simplest LiFePO4 battery runtime formula?

A useful first pass is usable battery energy in Wh divided by average load in W. Calculate nominal energy as voltage multiplied by Ah, then apply the permitted usable-energy fraction and the losses between the battery and load.

How do I convert LiFePO4 battery Ah into runtime?

First convert Ah to Wh with a voltage assumption. Then divide usable Wh by average load W. If the load is AC, include inverter efficiency and idle consumption; if it is DC, include the converter and wiring boundary where relevant.

Is LiFePO4 battery runtime equal to Ah divided by amps?

It can be a first-pass calculation for a direct DC load when usable Ah, current, voltage, temperature and cut-off conditions match. It is not a universal guarantee, especially for AC loads, variable loads or systems with conversion losses.

Does a higher-voltage battery always last longer?

Not automatically. At the same Ah rating, higher nominal voltage creates more nominal Wh, but runtime depends on capacity, load, usable fraction, power limits and the complete system. A higher-voltage battery may also require a different inverter and installation architecture.

Should I use nominal or usable capacity?

Use usable capacity for the runtime estimate. Use nominal capacity only as the starting arithmetic, then replace the assumed usable fraction with the exact manufacturer's, BMS, inverter or project value.

Do I include inverter efficiency in the calculation?

Yes, when the load is AC and the battery supplies it through an inverter. Include the inverter's operating efficiency and no-load or idle consumption at the relevant operating point.

Why is my measured runtime shorter than the formula?

Possible causes include a higher real load, inverter idle draw, a different efficiency curve, lower temperature, a lower BMS or inverter cut-off, capacity aging, cable losses, inaccurate capacity assumptions or a load profile that was not measured.

Can a battery with enough Wh still fail to run my appliance?

Yes. The inverter may not support the appliance's continuous or starting power, or the battery-side current path or BMS may reach its limit. Energy and power are separate screens.

How do motor loads change runtime?

The motor's energy depends on how long and how often it runs, while the inverter must also survive its starting demand. Measure or obtain both the running profile and starting requirement.

Does a battery monitor make the runtime estimate accurate?

A correctly installed and configured monitor can improve measured current, energy and SOC evidence. It cannot correct a wrong capacity setting, an incorrect current path, an unmeasured load or an incompatible inverter.

Can I compare runtime estimates from two suppliers?

Only if the voltage, capacity test, usable-energy rule, load boundary, efficiency, idle draw, temperature, cut-off and reserve policy are stated on the same basis. Otherwise the numbers may not be comparable.

What should I send AmpBird for a runtime-based quotation?

Send the application, location, protected loads, average and peak power, duty cycle, desired hours, voltage preference, inverter model, charging sources, temperature, reserve policy and any product or document requirements. A load worksheet and the supplier's assumptions make the review much more useful.

The decision rule

Use the runtime estimate to narrow the design, then stop and verify the boundaries before ordering:

• Continue to technical review when the load profile, usable-energy assumption and power limits are documented.
• Hold when the energy calculation is plausible but the inverter, BMS, temperature, cut-off or current-path assumptions are missing.
• Stop when a supplier promises runtime from Ah alone, hides the load boundary, ignores starting power or cannot identify the conditions behind the capacity number.

AmpBird's Home Battery Systems collection is a starting point for complete storage-system routes. The correct product path still depends on the measured load, voltage, inverter, installation and evidence requirements.

Final checklist

Before accepting a runtime number, confirm:

• [ ] Voltage and Ah are from the exact product or clearly marked as illustrative.
• [ ] Nominal Wh is calculated from the same voltage and capacity basis.
• [ ] Usable energy is stated separately from nominal energy.
• [ ] The load is defined at the AC or DC boundary being calculated.
• [ ] Average load and duty cycle are shown.
• [ ] Continuous and starting power are checked separately.
• [ ] Inverter efficiency and idle draw are included for AC loads.
• [ ] Battery-side current is checked at the actual voltage range.
• [ ] BMS, inverter and low-voltage cut-off boundaries are recorded.
• [ ] Temperature, age, reserve and recovery assumptions are visible.
• [ ] The estimate is not presented as a guaranteed product runtime.
• [ ] The same assumptions are used when comparing quotations.

A runtime calculator is valuable because it makes assumptions visible. It should lead to better measurements and a better product brief, not a false promise based on one Ah number.

Need help building a usable battery brief?

Send AmpBird the application, load worksheet, desired runtime, voltage, inverter, installation location and supplier documents. AmpBird can help organize the energy, power, current and product-route questions around the actual project. Final compatibility, engineering and installation decisions remain with the responsible system designer and installer.

Review the Home Battery Systems collection and contact AmpBird when you have a specific battery-runtime question.

Technical references

• U.S. Department of Energy — Solar Integration: Solar Energy and Storage Basics
• Victron Energy — BMV-712 Smart: Operation
• Victron Energy — Phoenix Inverter: Battery Capacity
• Victron Energy — Lynx Smart BMS: Features