Inverter Battery Backup Time Calculator – Estimate Hours

How the Inverter Battery Backup Time Calculator Works

The Inverter Battery Backup Time Calculator estimates how long a configured battery bank can support an AC load using bank configuration, charge range, battery condition, running load, inverter efficiency, and inverter self-consumption.

Describe the Battery Bank Correctly

Enter the bank as batteries in each series string and equivalent parallel strings, not only as a total battery count.

Series connections increase bank voltage while Ah remains unchanged within the string. Parallel strings increase total Ah while bank voltage remains unchanged.

For four matching 12 V, 150 Ah batteries arranged as two batteries in series and two equivalent strings in parallel:

• batteries per series string: 2
• parallel strings: 2
• total batteries: 4
• bank voltage: 24 V
• bank capacity: 300 Ah
• nominal bank energy: 7,200 Wh

Two 12 V, 150 Ah batteries connected in series form a:

24 V, 150 Ah

string, not a 24 V, 300 Ah string.

If you need to determine combined voltage, Ah, or Wh from a battery arrangement first, use the Battery Series Parallel Calculator.

The estimate assumes reasonably matched batteries and equivalent strings. Significant imbalance or a weak battery can reduce practical backup time.

If an inverter battery-system voltage is entered, it is used to compare the configured bank voltage with the inverter’s nominal DC system voltage.

Worked Example: One 12 V, 150 Ah Battery With a 300 W Load

This example uses a flooded lead-acid battery.

The 50% planned minimum is an illustrative calculation setting rather than a universal battery limit.

1. Calculate Battery-Bank Energy

Bank voltage:

12 V × 1 = 12 V

Bank capacity:

150 Ah × 1 = 150 Ah

Nominal bank energy:

12 V × 150 Ah = 1,800 Wh

2. Calculate Energy in the Selected Charge Range

The selected range is:

100% − 50% = 50%

At a 100% battery-condition factor:

1,800 Wh × 0.50 × 1.00 = 900 Wh

Selected battery energy:

900 Wh

3. Calculate Battery-Side Demand

At 90% inverter efficiency:

300 W ÷ 0.90 = 333.33 W DC

Add the separately entered 10 W inverter self-consumption:

333.33 W + 10 W = 343.33 W DC

4. Calculate Backup Time

900 Wh ÷ 343.33 W = 2.621 hours

Estimated backup time:

About 2 hours 37 minutes

Example Results

Watts, VA, and Inverter Capacity Are Different

Backup time is based on real power in watts, while inverter compatibility can also depend on apparent power in volt-amperes.

When apparent power and power factor are known:

Real power = VA × Power factor

For a 500 VA load with a power factor of 0.80:

500 VA × 0.80 = 400 W

The runtime calculation therefore uses:

400 W

while the inverter may still need to support the full:

500 VA

Schneider Electric’s official real, reactive, and apparent power guidance explains the distinction between real power in watts and apparent power in volt-amperes.

For example:

• connected load: 400 W, 500 VA
• inverter rating: 600 W, 450 VA

The watt requirement is within the inverter rating, but the 500 VA requirement exceeds its 450 VA capacity.

When only watts are known, an accurate VA requirement cannot be inferred without additional power-factor or apparent-power information.

Startup Demand

Motors, compressors, refrigerators, pumps, transformers, and some power supplies can require substantially more power during startup than during normal operation.

Suppose:

• other loads already running: 300 W
• additional startup demand: 1,200 W

Combined startup demand:

300 W + 1,200 W = 1,500 W

With a 2,000 W inverter surge rating:

Surge margin = 2,000 W − 1,500 W

= 500 W

A positive margin does not guarantee successful startup. Surge duration, battery-side current, voltage sag, and appliance requirements can still affect operation.

Account for Inverter Efficiency and Self-Consumption

Inverter efficiency and self-consumption represent different parts of battery demand.

Inverter efficiency accounts for conversion losses while supplying the AC load.

Self-consumption is additional DC power used by the inverter itself.

Use a separate self-consumption value only when it is not already included in a measured DC-input or efficiency figure.

For example, if efficiency was calculated from total measured DC input at the same load, inverter overhead may already be included. Adding it again would count the same demand twice.

Battery Condition and Planned Minimum

The planned minimum charge controls how much of the selected charge range is included in the backup-time estimate.

The battery condition factor adjusts modeled battery capacity.

For the worked example:

Do not reduce the entered Ah value for degradation and then apply the same reduction again through the condition factor.

If entered Ah already represents reliable present-day measured capacity, a 100% condition factor may be appropriate.

The planned minimum is a calculation boundary, not an exact inverter or battery-management cutoff.

If starting charge equals planned minimum charge, selected energy and estimated backup time are zero.

Lead-Acid Discharge-Rate Effects

A simple Wh-based estimate assumes the selected battery energy remains usable at the calculated load.

Lead-acid batteries can provide less usable capacity at higher discharge rates, so high-load estimates may be optimistic.

This calculator does not apply a Peukert adjustment. When discharge-rate effects are important, use manufacturer discharge curves or battery-specific rate data.

Nominal Current and C-Rate

The calculator reports nominal battery current and discharge C-rate as planning outputs.

For the worked example:

Nominal current = 28.61 A

Nominal discharge rate ≈ 0.19C

For dedicated current-to-capacity calculations, use the Battery C-Rate Calculator.

These values do not replace maximum-current, voltage-drop, wiring, protection, or surge calculations.

Accepted Inputs

These are calculator validation limits, not recommended battery or inverter ratings.

Planned minimum charge cannot exceed starting charge. Startup demand and inverter surge rating are used together when the surge comparison is enabled.

Optional inverter-voltage, continuous-watt, continuous-VA, and surge comparisons apply only when the corresponding values are supplied.

Calculation Method

Battery-bank voltage:

Bank voltage = Voltage per battery × Batteries in series

Battery-bank capacity:

Bank Ah = Ah per battery × Parallel strings

Nominal bank energy:

Nominal bank Wh = Bank voltage × Bank Ah

Selected charge fraction:

Selected fraction = (Starting % − Planned minimum %) ÷ 100

Condition-adjusted selected energy:

Selected Wh = Nominal bank Wh × Condition factor × Selected fraction

When apparent power is entered:

AC load W = Load VA × Power factor

Battery-side demand:

Battery-side W = AC load W ÷ Inverter efficiency + Self-consumption

Estimated backup time:

Backup time = Selected Wh ÷ Battery-side W

Nominal battery current:

Bank current = Battery-side W ÷ Bank voltage

Nominal discharge rate:

C-rate = Bank current ÷ Bank Ah

When continuous-watt capacity is entered:

W utilization = Running load W ÷ Inverter continuous W × 100

When apparent load and continuous-VA capacity are entered:

VA utilization = Load VA ÷ Inverter continuous VA × 100

When startup demand and inverter surge capacity are entered:

Surge margin = Inverter surge W − Startup demand W

The calculator uses underlying unrounded values for calculations. Display precision changes only how results are shown.

Calculation Boundaries

The Inverter Battery Backup Time Calculator estimates energy-based AC backup time and optional inverter-capacity comparisons.

It does not determine:

• detailed battery discharge curves or exact cutoff behavior;
• battery BMS or inverter protection behavior;
• inverter waveform suitability;
• startup and surge behavior such as VA, required duration, or battery-side surge current;
• voltage sag or cable voltage drop;
• wire, fuse, breaker, or connector sizing;
• battery charging time;
• required battery-capacity sizing.