Inverter Battery Calculator
An inverter battery calculator helps you estimate how large your battery bank needs to be to run an inverter for a chosen number of hours. It is especially useful for home backup systems, solar storage, RV setups, cabin power, and small off-grid systems where battery voltage, inverter efficiency, and usable depth of discharge all affect the final battery size.
This page uses a practical formula that accounts for inverter power, runtime, system voltage, efficiency, and battery depth of discharge. That matters because the battery bank must supply more DC energy than the AC load alone suggests, especially when inverter losses and battery reserve limits are included.
Short answer: Required battery size in amp-hours can be estimated with Required Ah = (Inverter Watts × Hours) ÷ (Battery Voltage × Efficiency × DoD). For larger systems, 48V is usually the most practical option because it lowers current and reduces cable losses.
Inverter Battery Size Calculator
Estimate required battery capacity, usable energy, and current draw for inverter-powered systems
Inverter Battery Formula
The core sizing formula is:
Required Ah = (Inverter W × Hours) ÷ (Voltage × Efficiency × DoD)
This formula works because the AC energy demand must be converted into the DC energy that the battery bank must provide. In other words, the battery has to cover the load, the inverter losses, and the fact that you may not want to use 100% of the battery's rated capacity.
For example, a 1000W inverter load running for 4 hours on a 48V system with 90% efficiency and 80% DoD needs: 1000 × 4 ÷ (48 × 0.90 × 0.80) = 115.7Ah. In practice, that rounds to about 120Ah at 48V.
| Setup | Formula Result | Practical Rounded Size |
|---|---|---|
| 1kW inverter, 4 hours, 48V | 115.7Ah | About 120Ah |
| 2kW inverter, 4 hours, 48V | 231.5Ah | About 240Ah |
| 3kW inverter, 8 hours, 48V | 694.4Ah | About 700Ah |
| 5kW inverter, 4 hours, 48V | 578.7Ah | About 600Ah |
Common Inverter Setups
1kW Inverter
A 1kW inverter for about 4 hours of backup typically needs around 120Ah at 48V when using 90% efficiency and 80% depth of discharge.
2kW Inverter
A 2kW inverter for about 4 hours needs roughly 240Ah at 48V under the same assumptions, which is already a fairly serious battery bank.
3kW Home Backup
A 3kW inverter running for 8 hours for longer home backup needs roughly 700Ah at 48V, which is a large bank and often easier to think of in kWh.
5kW Inverter
For a 5kW inverter, a 48V system is the practical minimum in most cases because lower-voltage systems create much higher current and heavier cable requirements.
Why System Voltage Matters
Battery voltage has a major effect on current draw. For the same power, a lower-voltage battery system must deliver more amps. That makes cables thicker, increases heating, and can create larger voltage drop issues. This is why inverter systems often move from 12V to 24V and then to 48V as power levels rise.
For example, a 2000W load is about 166.7A at 12V before losses, about 83.3A at 24V, and about 41.7A at 48V. That simple comparison shows why 48V is often preferred once you move into bigger inverter systems. Lower current usually means easier wiring, better efficiency, and more stable inverter operation.
Practical warning: Above about 2kW inverter size, 48V systems are usually the safer and more practical choice. Lower-voltage banks can become bulky and may require very heavy cables to handle the current properly.
Battery Chemistry and DoD
Depth of discharge, or DoD, is one of the most important parts of battery sizing. If you plan to use only 80% of a lithium battery's rated capacity, then the total bank must be larger than the bare energy requirement. That reserve helps protect battery life and reflects the real usable energy available.
LiFePO4 batteries are often sized around 80% usable depth of discharge in practical backup calculations, while lead-acid systems are often limited to around 50% for longer life. This difference is one reason lithium banks can be much smaller than lead-acid banks for the same usable backup energy.
The calculator on this page lets you choose DoD values commonly associated with LiFePO4, broader lithium use, and lead-acid systems. That way, the same inverter load can be sized more realistically depending on the battery type you plan to use.
How to Use This Calculator
Start with the load in watts that the inverter will actually run. If your inverter is rated for 3000W but your backup load is only 1200W, enter 1200W rather than 3000W. The calculator is for battery sizing based on your real demand, not only the inverter nameplate.
Then enter how many hours of backup you want. This is usually the biggest factor in battery size. Doubling the hours roughly doubles the battery bank requirement if everything else stays the same. After that, choose your battery voltage, inverter efficiency, and DoD assumptions.
The single battery size field helps translate the final required capacity into a rough battery count. That is useful when planning around common modules such as 48V 100Ah lithium batteries or 12V 100Ah batteries that may need to be combined in series and parallel.
Practical rule: Think in both Ah and kWh. Ah is useful for battery labeling, but kWh is often the easier way to understand how much real backup energy your inverter system needs.