Battery Runtime Calculator: Estimate Backup Time From Ah, Voltage, Load, DoD & Efficiency

Result

Formulas

\(C_{\text{battery}}=\text{nominal battery capacity in Ah},\quad V_{\text{system}}=\text{nominal system voltage},\quad P_{\text{load}}=\text{load power}\)
\(\mathrm{DoD}=\text{usable depth of discharge percent},\quad \eta=\text{system efficiency percent}\)
\(E_{\text{nominal}} = C_{\text{battery}} \times V_{\text{system}}\)
\(E_{\text{usable}} = E_{\text{nominal}} \times \frac{\mathrm{DoD}}{100} \times \frac{\eta}{100}\)
\(t_{\text{runtime}} = \frac{E_{\text{usable}}}{P_{\text{load}}}\)

Battery runtime depends on usable stored energy and the connected load; this worksheet reports the resulting time in hours. The primary result is Runtime estimate, expressed in hours.

The calculation begins with nominal battery energy in watt-hours, then applies the usable depth of discharge and system efficiency assumptions. The resulting usable battery energy is divided by the connected load in watts.

This estimate is commonly used during backup-power planning for inverter systems, emergency loads, communications equipment, control panels, small off-grid loads, and battery-supported branch circuits. It can also support preliminary load review before selecting batteries, inverters, DC overcurrent protection, conductors, raceway routes, and equipment locations.

Runtime does not establish conductor ampacity, AWG or kcmil size, voltage-drop compliance, inverter listing, battery disconnect requirements, or NEC compliance. Those are separate design and field-verification decisions.

Battery Energy Inputs

The calculator uses five electrical assumptions.

Input	Unit	Electrical purpose
Battery capacity	Ah	Nominal stored charge capacity of the battery bank
Battery system voltage	V	Nominal DC voltage of the complete battery system
Load	W	Power demand supported by the battery
Usable depth of discharge	%	Portion of nominal battery capacity intended for use
System efficiency	%	Allowance for inverter, conversion, wiring, and system losses

Battery capacity is entered in amp-hours rather than watt-hours. Amp-hours describe stored electrical charge, but runtime depends on both charge and voltage. A 200 Ah battery system at 24 V stores more nominal energy than a 200 Ah battery system at 12 V.

Battery system voltage must represent the nominal voltage of the full battery bank, not the voltage of one individual battery when batteries are connected in series. Series-connected batteries increase voltage; parallel-connected batteries increase available amp-hour capacity.

Load is the wattage expected during battery operation. Use the actual supported load rather than the nameplate rating of an entire panel or inverter when the full connected load will not operate at the same time. If the load changes during an outage, calculate separate operating conditions or use a reasonable demand assumption.

Usable depth of discharge limits how much of the nominal battery energy is treated as available. An 80% usable depth of discharge applies a factor of 0.80.

System efficiency applies a separate factor for energy lost through conversion and associated system losses. A 90% efficiency assumption applies a factor of 0.90.

Runtime Formula

The calculator first converts battery capacity and system voltage into nominal stored energy:

\(\displaystyle \text{Nominal battery energy (Wh)} = \text{Battery capacity (Ah)} \times \text{Battery system voltage (V)}\)

It then applies usable depth of discharge and system efficiency:

\(\displaystyle \text{Usable factor} = \left(\frac{\text{Usable depth of discharge}}{100}\right) \times \left(\frac{\text{System efficiency}}{100}\right)\)

\(\displaystyle \text{Usable battery energy (Wh)} = \text{Nominal battery energy (Wh)} \times \text{Usable factor}\)

Runtime estimate is:

\(\displaystyle \text{Runtime estimate (h)} = \frac{\text{Usable battery energy (Wh)}}{\text{Load (W)}}\)

The Usable factor is the combined multiplier produced by depth of discharge and efficiency. It is not an independently entered battery characteristic.

Calculation Example

Enter the following values:

Field	Entered value
Battery capacity	200 Ah
Battery system voltage	24 V
Load	500 W
Usable depth of discharge	80%
System efficiency	90%

Nominal battery energy:

\(\displaystyle 200\text{ Ah} \times 24\text{ V} = 4{,}800\text{ Wh}\)

Usable factor:

\(\displaystyle 0.80 \times 0.90 = 0.72\)

Usable battery energy:

\(\displaystyle 4{,}800\text{ Wh} \times 0.72 = 3{,}456\text{ Wh}\)

Runtime estimate:

\(\displaystyle \frac{3{,}456\text{ Wh}}{500\text{ W}} = 6.912\text{ h}\)

Result	Value
Nominal battery energy	4,800 Wh
Usable battery energy	3,456 Wh
Runtime estimate	6.912 h
Usable factor	0.72 x

Under the entered assumptions, the battery system supports a steady 500 W load for approximately 6.912 hours.

Load and DC Current Review

The runtime result is an energy estimate, while conductor and overcurrent decisions are based on current. Battery-side current can be substantially higher than AC-side current because battery systems often operate at lower DC voltage.

A preliminary DC current estimate may be expressed as:

\(\displaystyle \text{DC current (A)} \approx \frac{\text{Load (W)}}{\text{Battery system voltage (V)} \times \text{System efficiency}}\)

For the 500 W, 24 V, 90% efficiency example:

\(\displaystyle \frac{500}{24 \times 0.90} \approx 23.15\text{ A}\)

That current estimate can inform subsequent design work for DC conductors, battery disconnects, overcurrent protection, terminals, lugs, and voltage-drop review. It does not replace an equipment-specific calculation. Inverter surge demand, charging current, fault-current characteristics, conductor length, terminal temperature rating, insulation temperature rating, and manufacturer installation instructions can change the required conductor and protective-device selection.

For a battery inverter installation, evaluate the DC battery conductors separately from the AC output branch circuit or feeder. The two circuits can have different voltages, current levels, conductor sizes, raceway fill conditions, and overcurrent protection requirements.

Field Verification

Battery runtime changes with conditions that are not represented by the entered values. Verify the actual battery and system characteristics before relying on a runtime estimate for operational planning.

Battery chemistry and manufacturer discharge data can cause delivered capacity to vary at different discharge rates.
Battery temperature can reduce available capacity and affect charging behavior.
Battery age, state of health, internal resistance, and BMS limits can reduce usable energy.
Inverter standby consumption and conversion losses may vary with loading.
Motors, compressors, transformers, pumps, and other intermittent loads can create startup or overload demand beyond their running watts.
Long battery conductors can introduce voltage drop, which may cause inverter low-voltage shutdown before calculated energy is fully available.
Parallel battery strings require careful conductor routing, connection design, overcurrent protection, and current-sharing verification.
Battery enclosures, ventilation, working clearances, disconnecting means, equipment listing, fire requirements, and utility-interactive equipment rules must be reviewed for the actual installation and AHJ requirements.

The calculation provides a runtime estimate only. It does not account for battery chemistry, discharge curves, temperature, inverter overload, BMS operation, equipment listing, fire requirements, utility conditions, or code-compliance determinations.

Why is runtime only an estimate?

Actual runtime changes with battery chemistry, age, temperature, discharge rate, inverter efficiency, and cutoff settings.

Can I use Wh instead of Ah?

This page takes Ah and voltage, then converts to Wh. Use the Battery Capacity Calculator when planning from load and desired runtime.