Battery & Power
Size a battery bank for your inverter load
Enter the running load and target runtime to estimate the bank capacity required.
Calculation breakdown
Planning estimate only. Confirm battery chemistry, discharge-rate performance, inverter DC limits, surge demand, BMS, cable and fuse sizing with the equipment manufacturers.
How to Use the Inverter Battery Capacity Calculator
Enter the combined running load and the amount of backup time required. Then select the battery-bank voltage, usable-capacity percentage, inverter efficiency, and planning reserve.
The calculator works backward from the energy required by the load. Instead of estimating runtime from an existing battery bank, it calculates the nominal amp-hour capacity needed to support the selected load for the target time under the assumptions you enter.
Use the result to compare possible battery-bank sizes before selecting individual batteries. The calculated Ah requirement does not by itself confirm that a particular battery model, inverter, BMS, charger, or wiring arrangement is compatible.
Information You Will Need
Connected Running Load
Enter the combined continuous wattage of all equipment expected to operate from the inverter during the backup period.
Include only devices that need to remain powered.
Use measured consumption when available because appliance nameplates and power-adapter labels may describe maximum input rather than typical running demand.
If the load changes substantially during normal use, calculate more than one scenario instead of relying on one optimistic wattage.
Desired Backup Time
Enter the required operating time in hours and minutes.
Choose a target that reflects the outage or off-grid period you actually need to cover.
If requirements vary, calculate separate cases such as:
- minimum acceptable backup time;
- normal target;
- longer contingency period.
Battery-Bank Voltage
Select the nominal DC voltage required by the inverter, such as 12V, 24V, or 48V.
This is the voltage of the complete battery bank, not necessarily the voltage of one battery.
Confirm the inverter’s supported DC input voltage before using the calculated Ah result to plan a bank.
Usable Battery Capacity
Enter the percentage of nominal battery capacity you intend to use before reaching the planned discharge limit.
This value should reflect the actual battery model, operating strategy, BMS limits, and manufacturer guidance.
It should not be treated as one universal percentage for every battery sharing the same chemistry.
Inverter Efficiency
Enter the estimated percentage of battery-side energy that becomes usable AC output.
Where possible, use efficiency information that reflects the expected operating load. A published peak-efficiency figure may not describe performance at every load level.
Planning Reserve
The reserve adds extra nominal battery capacity after usable-capacity and inverter-efficiency adjustments.
It can provide headroom for factors such as:
- moderate variation in running load;
- uncertainty in efficiency;
- future capacity reduction;
- occasional operation beyond the normal target;
- rounding to a bank that can actually be assembled.
The reserve should have a purpose. It is not a substitute for obtaining better load data or following manufacturer-specific derating requirements.
Inverter Continuous Rating
This optional value compares the entered running load with the inverter’s continuous watt rating.
Enter watts, not VA.
Startup and surge requirements still need to be checked separately.
How the Required Battery Capacity Is Calculated
The calculator uses a reverse-sizing process.
1. Calculate the Energy Required by the Load
Load energy (Wh) = Running load (W) × Backup time (hours)
A 500W load operating for four hours requires:
500W × 4 hours = 2,000Wh
of AC load energy.
2. Account for Usable Capacity and Inverter Efficiency
The battery bank needs more nominal energy than the appliances consume because only the selected fraction of battery capacity is being used and some energy is lost during DC-to-AC conversion.
Base nominal bank energy = Load energy ÷ (Usable capacity × Inverter efficiency)
Percentages are converted to decimals.
For example:
50% = 0.50
and:
90% = 0.90
3. Add the Planning Reserve
Required nominal energy = Base nominal energy × (1 + Reserve percentage)
A 20% reserve therefore uses:
1.20
as the multiplier.
4. Convert Watt-Hours Into Amp-Hours
Once the required nominal energy is known:
Required capacity (Ah) = Required nominal energy (Wh) ÷ Bank voltage (V)
The resulting Ah requirement applies at the selected complete battery-bank voltage.
Worked Example: 500W for Four Hours
Suppose the system must support a 500W continuous load for four hours.
Use:
- Bank voltage: 24V
- Usable battery capacity: 50%
- Inverter efficiency: 90%
- Planning reserve: 20%
Load Energy
500W × 4 hours = 2,000Wh
Base Nominal Bank Energy
2,000Wh ÷ (0.50 × 0.90) = 4,444.44Wh
Before reserve, the bank therefore needs approximately:
4.44kWh nominal
Required Energy After Reserve
4,444.44Wh × 1.20 = 5,333.33Wh
Required Amp-Hour Capacity
5,333.33Wh ÷ 24V = 222.22Ah
The calculated requirement is therefore approximately:
222.2Ah at 24V
If the calculator rounds this upward to 250Ah, that value is a practical planning size rather than a recommendation for one particular battery product.
A 24V 250Ah bank contains:
24V × 250Ah = 6,000Wh nominal
Under the same 50% usable-capacity and 90% efficiency assumptions:
6,000Wh × 0.50 × 0.90 = 2,700Wh
of estimated AC energy is available.
At a constant 500W load:
2,700Wh ÷ 500W = 5.4 hours
That is approximately:
5 hours and 24 minutes
The extra runtime compared with the four-hour target comes from the planning reserve and the additional capacity created by rounding the bank above the calculated minimum.
Why Required Ah Changes With Battery-Bank Voltage
Amp-hours cannot be compared meaningfully without voltage.
Using the example’s required nominal energy of approximately 5,333Wh:
| Bank voltage | Required capacity |
|---|---|
| 12V | 444.4Ah |
| 24V | 222.2Ah |
| 48V | 111.1Ah |
All three represent approximately the same nominal energy:
12V × 444.4Ah ≈ 24V × 222.2Ah ≈ 48V × 111.1Ah ≈ 5,333Wh
The smaller Ah number at 48V does not mean the 48V bank stores less energy.
Voltage and amp-hours must be considered together.
Do not choose a bank voltage simply because it produces a smaller Ah requirement. The inverter, charger, batteries, BMS, and any connected DC equipment must support the selected voltage.
Minimum Capacity, Reserve, and Rounded Bank Size
The calculator separates three related values.
Base Capacity
This is the nominal capacity required after accounting for usable battery percentage and inverter efficiency but before adding the planning reserve.
It represents the mathematical requirement under the entered assumptions.
Capacity With Planning Reserve
The selected reserve adds headroom to the base requirement.
For example:
200Ah × 1.20 = 240Ah
for a 20% reserve.
Practical Rounded-Up Capacity
A real battery bank may not be buildable at the exact calculated Ah value.
If the requirement is:
222Ah
rounding down to 200Ah would remove part of the calculated requirement.
Rounding upward to:
250Ah
preserves the target and adds some additional capacity.
The rounded figure still does not prove that one 250Ah battery exists at the required bank voltage or that the target can be assembled from the battery model you intend to use.
When you need to convert the required voltage and Ah into complete series and parallel strings, use the Battery Series Parallel Calculator.
Use a Reserve for a Specific Reason
A larger reserve increases more than the displayed Ah result.
It can also increase:
- battery cost;
- physical size;
- weight;
- charging time;
- required number of batteries.
Use reserve to cover defined planning uncertainty rather than as a replacement for accurate inputs.
For example, if the connected load is uncertain, measuring the load is better than adding a very large reserve solely to hide that uncertainty.
Reserve also increases energy capacity, not inverter output power.
A larger battery bank cannot make an undersized inverter support a load beyond its continuous or surge capability.
Check the Inverter Load Separately
The optional inverter-rating calculation uses:
Load percentage = Running load ÷ Inverter continuous rating × 100
For a 500W load on a 1,000W inverter:
500W ÷ 1,000W × 100 = 50%
This checks how much of the entered continuous watt rating the running load uses.
It does not confirm that the inverter can start every connected appliance.
Refrigerators, pumps, compressors, air conditioners, power tools, motors, and some power supplies may briefly require substantially more power during startup.
Check both:
- continuous output rating;
- surge or peak capability and permitted duration.
A larger battery bank cannot compensate for an inverter that cannot supply the required AC power.
For manufacturer context on choosing inverter output around expected continuous demand and the supporting battery system, see Victron Energy’s inverter/charger selection guidance.
Understand the Estimated DC Current
The calculator also estimates battery-side current:
Estimated DC current = AC load ÷ (Bank voltage × Inverter efficiency)
Using the 500W load at 24V and 90% efficiency:
500W ÷ (24V × 0.90) = 23.15A
At 12V:
500W ÷ (12V × 0.90) = 46.30A
At 48V:
500W ÷ (48V × 0.90) = 11.57A
For the same AC load, increasing bank voltage reduces the approximate DC current.
That current can matter when checking:
- battery discharge capability;
- BMS limits;
- cables;
- fuses;
- disconnects;
- connectors;
- busbars.
The calculator does not size those components.
If you want to compare the estimated battery current with the bank’s Ah capacity, use the Battery C-Rate Calculator.
The displayed current represents the entered running load. Startup surges can create much higher temporary current.
Turn the Ah Requirement Into a Complete Battery Bank
Suppose the calculation requires:
24V 250Ah
and the selected batteries are:
12V 125Ah each
Reaching the required 24V bank voltage takes two batteries in series:
12V × 2 = 24V
One two-battery series string remains:
125Ah
To reach 250Ah, two identical 125Ah strings are connected in parallel:
125Ah × 2 = 250Ah
The result is:
2S2P using four batteries
This is a mathematical configuration example.
The actual battery model must support the proposed series and parallel arrangement.
When increasing a bank built from fixed battery sizes, work with complete compatible strings. A 24V bank built from pairs of 12V batteries cannot normally be expanded by inserting one unmatched extra battery into only one series string.
Check Charging Requirements as the Bank Gets Larger
Increasing battery capacity can extend backup time, but the larger bank must also be recharged.
For example, moving from a 200Ah planning bank to a 400Ah bank without changing the charger can materially increase recharge time.
Once you have chosen a realistic bank arrangement and charger current, the Battery Charging Time Calculator can estimate how long the selected state-of-charge window may take to restore.
This is a separate question from inverter capacity sizing:
- this calculator asks how much battery capacity is required;
- the charging calculator asks how long that bank may take to recharge.
Use Scenarios Instead of One Perfect-Looking Result
A battery-capacity estimate becomes more useful when uncertain inputs are varied deliberately.
Try combinations such as:
- normal vs high running load;
- minimum vs preferred backup duration;
- typical vs conservative inverter efficiency;
- current battery assumptions vs longer-term planning assumptions.
This shows which input has the largest effect on required bank size.
For example, if a modest increase in connected load pushes the requirement from one buildable battery configuration to another much larger one, reducing non-essential load may be more practical than adding another complete battery string.
If the battery bank already exists and the question is instead how long it may run the load, use the Inverter Battery Backup Time Calculator.
The two calculators solve opposite planning problems:
Capacity sizing: load + target runtime → required battery bank
Runtime estimation: known battery bank + load → estimated backup time
How to Interpret the Results
Minimum Calculated Capacity
The calculated Ah requirement after applying usable capacity, inverter efficiency, and planning reserve at the selected bank voltage.
Practical Rounded-Up Bank Size
A planning capacity above the calculated minimum.
It still needs to be converted into a compatible bank of complete batteries or strings.
Load Energy Required
The AC watt-hours consumed by the entered running load over the selected backup period.
Required Nominal Bank Energy
The calculated battery-side nominal watt-hours required after accounting for usable capacity, inverter efficiency, and reserve.
Estimated DC Current
The approximate battery current required to support the entered continuous AC load at the selected bank voltage and inverter efficiency.
Inverter Load Check
The percentage of the optional continuous inverter watt rating used by the connected running load.
Common Sizing Errors
Calculating Ah Without Specifying Voltage
A requirement of 200Ah is incomplete without the associated bank voltage.
A 24V 200Ah bank contains twice the nominal energy of a 12V 200Ah bank.
Using the Inverter Rating as the Appliance Load
The inverter’s watt rating describes its output capability.
It does not mean the connected equipment continuously consumes that amount.
Entering Surge Watts as Continuous Load
Startup surge must be checked against inverter and battery current capability, but a brief peak should not automatically be multiplied by the entire target backup duration.
Applying Inverter Efficiency Twice
If the entered load is already the AC power consumed by the appliances, apply the inverter-efficiency adjustment once.
Do not manually inflate the AC load for the same inverter loss and then apply the efficiency field again.
Treating Reserve as Usable Capacity
These inputs perform different jobs.
Usable capacity limits the fraction of nominal battery energy included in the base sizing calculation.
Planning reserve adds extra nominal capacity after that base requirement is determined.
Rounding to an Impossible Battery Bank
A numerical Ah target does not prove the bank can be assembled from the selected batteries.
The final configuration must use complete compatible series strings and remain within the manufacturer’s connection limits.
Assuming a Larger Battery Always Solves a Power Problem
Increasing nominal battery capacity can increase energy and potential runtime.
It does not automatically increase:
- inverter continuous output;
- surge capability;
- BMS current limit;
- cable capacity;
- protection-device rating.
Energy capacity and power capability must be checked separately.
Compatibility and Design Limits
Before selecting batteries, confirm:
- inverter DC input voltage;
- inverter continuous output rating;
- inverter surge capability;
- battery manufacturer discharge limits;
- permitted series and parallel arrangements;
- BMS continuous and peak current ratings;
- charger compatibility and charging capacity;
- cable and busbar requirements;
- overcurrent protection;
- disconnect requirements.
Battery capacity ratings are measured under specified test conditions. Actual delivered capacity can differ with discharge rate, temperature, battery condition, chemistry, and system behavior.
The calculated bank is therefore a planning requirement under the inputs you selected, not a guaranteed measured runtime or approval of a particular installation.
High-energy, safety-critical, or business-critical systems require model-specific performance data, appropriate protection and redundancy, and suitable professional design review.