Formula Used
Real watts from VA: watts = VA × power factor.
Adjusted load: adjusted watts = real watts × load use factor.
AC energy: watt hours = adjusted watts × backup hours.
Battery energy: DC Wh = AC Wh ÷ inverter efficiency.
Design energy: design DC Wh = DC Wh × reserve factor × aging factor ÷ temperature factor.
Required capacity: Ah = design DC Wh ÷ battery bank voltage ÷ depth of discharge.
Series batteries: series count = target voltage ÷ single battery voltage, rounded upward.
Parallel strings: parallel count = required Ah ÷ single battery Ah, rounded upward.
How to Use This Calculator
- Enter your total appliance load in watts or volt amps.
- Use power factor when the load is entered as volt amps.
- Enter the required backup time in hours.
- Add inverter voltage, efficiency, and safe discharge limit.
- Add reserve, aging, and temperature allowances.
- Enter the voltage and Ah rating of one battery.
- Press the calculate button to view the battery bank result.
- Download the result as a CSV or PDF file.
Example Data Table
| Load |
Backup Time |
Bank Voltage |
Efficiency |
DoD |
Battery Size |
| 800 W |
3 hours |
24 V |
90% |
80% |
12 V 150 Ah |
| 1500 W |
5 hours |
48 V |
92% |
70% |
12 V 200 Ah |
| 2200 VA |
4 hours |
48 V |
90% |
80% |
12 V 220 Ah |
Inverter Battery Capacity Planning
Battery sizing looks simple at first. Yet small losses can change the final answer. An inverter does not pass all battery energy to the load. Cables, temperature, battery age, and depth of discharge also matter. This calculator includes those details. It helps you build a bank that matches the actual backup target.
Why Capacity Matters
Battery capacity is the stored energy available for your devices. For inverter systems, capacity is often shown in amp hours. The same amp hour value gives different energy at different voltages. A 100 Ah battery at 12 V stores less energy than a 100 Ah bank at 48 V. That is why the calculator converts loads into watt hours first.
Important Design Factors
The connected load sets the power demand. Runtime sets the energy demand. Inverter efficiency increases battery demand because some energy becomes heat. Depth of discharge protects the battery from being drained too far. Reserve margin supports extra load, startup surge, or future expansion. Temperature derating accounts for weak performance in cold or hot conditions. Aging allowance helps the bank stay useful after months of use.
Practical Battery Bank Design
The calculator also checks series and parallel layout. Series batteries raise voltage. Parallel strings raise amp hour capacity. A higher voltage bank can reduce current and cable stress. Too many parallel strings can be harder to balance. Matching battery age, model, and capacity is important. Use proper fuses, breakers, and cables. Follow the inverter manual and battery maker limits before installation.
Reading the Result
The recommended amp hour value is the minimum design target. The installed bank result uses the battery size you enter. It rounds series and parallel counts upward because partial batteries cannot be installed. The estimated runtime shows how long the rounded bank may run the selected load. Real runtime can vary. Battery chemistry, load shape, cable length, ventilation, and maintenance affect performance.
Use this calculator for planning, comparison, and estimates. For critical backup systems, confirm the design with a qualified electrician. Record each design case before buying hardware. Compare several voltages and battery sizes. Check charging current too. A battery bank is only useful when the charger can refill it within the required time safely.
FAQs
What is inverter battery capacity?
It is the battery storage needed to run your load for a selected backup time. It is usually shown in amp hours, but watt hours give a better energy comparison.
Why does inverter efficiency matter?
No inverter is perfectly efficient. Some battery energy is lost as heat. Lower efficiency means you need a larger battery bank for the same load and backup time.
What depth of discharge should I use?
Use the safe value recommended by the battery maker. Lead acid batteries often need lower discharge limits. Lithium batteries may allow deeper discharge, depending on model and BMS limits.
Why is reserve margin included?
Reserve margin adds extra capacity for startup surge, future loads, wiring losses, and uncertain runtime needs. It also helps avoid running the bank at its exact limit.
What does series battery count mean?
Series count shows how many batteries are connected end to end to reach the inverter voltage. For example, two 12 V batteries create a 24 V bank.
What does parallel string count mean?
Parallel strings increase amp hour capacity. Each string has the same series voltage. More strings provide more capacity, but they need balanced wiring and matching batteries.
Can I use this for solar inverters?
Yes, it can estimate battery storage for solar inverters. Charging source size is not calculated here, so check solar array and charger limits separately.
Is the result exact for every battery?
No. Real runtime depends on battery chemistry, age, temperature, discharge rate, cable losses, and inverter behavior. Use the result as a planning estimate.