Battery Size Calculator

Created by Mehjabin Abdurrazaque
Reviewed by Wojciech Sas, PhD and Rijk de Wet
Last updated: Jan 18, 2024

Omni's battery size calculator (or remaining battery capacity calculator) explains in detail how to check the battery capacity for both lithium-ion and lead-acid batteries.

Our tool has many uses — whether you want to know how much longer your drone will fly after already using it for a few hours, or if you want to compare lead-acid and lithium-ion batteries in terms of their battery capacity, the battery size calculator does it all!

How do I calculate the discharging time of a battery?

You just need to divide the battery's ampere-hour or amp-hour (Ah) rating with the desired load current to estimate the discharging time or run time:

  • discharging time = amp-hour / load current

The amp-hour rating helps us determine the maximum steady current supplied in an hour. Suppose your battery has a 20 Ah rating on it. Using the battery runtime formula, we find out that it can supply 20 A for an hour, 10 A for 2 hours, 5 A for 4 hours, or 1 A for 20 hours.

Do you know that you shouldn't discharge batteries completely? Learn how to estimate how long you can safely use a battery with our battery life calculator!

How do I measure a lead-acid battery capacity?

Now that you know how to calculate the runtime/discharge time of any type of battery, let's learn how to measure the capacity of lead-acid batteries after putting them to use.

We calculate the remaining capacity of a lead-acid battery using the following formula:

BPb=100ILt(100Q)(0.02t+0.6),B_\text{Pb} = \frac{100\cdot I_\text{L}\cdot t}{(100 - Q)(0.02 t + 0.6)},


  • BPbB_\text{Pb} – Remaining capacity of the lead-acid battery (Pb\text{Pb} because it's the chemical symbol for lead);
  • ILI_\text{L} – Load current;
  • tt – Duration for which the power is supplied to the load; and
  • QQ – Percentage of charge that should remain after the battery is used.

How do I check the capacity of a lithium-ion battery after use?

For a lithium-ion battery, the formula for the battery capacity is:

BLi=100ILt(100Q),B_\text{Li} = \frac{100 \cdot I_\text{L} \cdot t}{(100 - Q)},


  • BLiB_\text{Li} – Remaining capacity of the lithium-ion battery (Li\text{Li} because it's the chemical symbol for lithium);
  • ILI_\text{L} – Load current;
  • tt – Duration for which the electrical power is supplied to the load; and
  • QQ – Percentage of charge that should remain after the battery is used.

Do you want to estimate the full capacity of your battery? Check our battery capacity calculator!

🙋 Lithium-ion batteries have a higher energy density than lead-acid batteries. So, we can store more energy in a lithium-ion battery using the same physical space. Therefore, a lithium-ion battery can supply more power than a lead-acid battery of the same size!

How to use battery size calculator?

You can easily find out the remaining battery capacity using our tool by following these simple steps:

  1. Enter the wattage of the appliance in the input box for Application load.
  2. Select the type of your battery from the drop-down list beside Battery type.
  3. Type the voltage rating specified on your battery in the input box for Voltage.
  4. Enter the number of hours (or you can select the duration in other units) you intend to use the battery for.
  5. Input the percentage of charge you want to remain in the battery after use.

Voila! The calculator tells you the Load current and Remaining capacity or the battery size!

⚠️ You shouldn't discharge lead-acid and lithium-ion batteries completely. Discharge lead-acid batteries up to 50% and lithium-ion batteries up to 20% to avoid any irreversible damage and for improved cell life.

Example: To find the remaining charge in your UPS after running a desktop computer of 200 W for 10 minutes:

  1. Enter 200 for the Application load, making sure W is selected for the unit.
  2. Usually, a UPS uses a lead-acid battery. The Battery type is Lead-acid by default. So you don't need to choose the type manually in this case.
  3. Enter 12 for the Voltage as the lead-acid battery provides a total input voltage of 12 V.
  4. Select minutes as the unit from the drop-down list beside the input box for Required duration.
  5. Input 10 in the Required duration input box.
  6. Type 50 in the Remaining charge box.

The calculator displays both the load current 16.7 A and the remaining capacity or the battery size 9.2 Ah!

💡 Learn more about the units of amperes and voltage combined together in our kVA calculator. There, you'll also understand the relationship between the kilovolt-ampere (kVA) and kilowatt (kW) units of power.

Read on to find answers to some of the questions on a battery's capacity and charging. Also, send us your questions on battery capacity in the comment box displayed when you rate the calculator.


What is the difference between ampere and ampere-hour?

Ampere (A) is the SI base unit of the electrical current while ampere-hour or amp-hour (Ah) denotes the electric current produced or consumed in an hour. We use amp-hour to measure the capacity of a battery. We also use watt-hour to measure battery capacity.

What is battery reserve capacity?

The reserve capacity (RC) of a battery tells us how many minutes it can supply a current of 25 A at a potential difference above 10.5 V.

How do I calculate the battery size for inverters?

You can calculate the battery size for inverters using the formula B = P × t / Vdc, where B is the battery capacity in ampere-hour, P is the inverter's power rating, t is the duration of power supply in hours, and Vdc is the DC voltage of the inverter.

How do I calculate the lithium-ion battery size for a varying load?

To calculate the battery size for a varying load which requires I1 in the interval t1 and I2 in the remaining time:

  1. Estimate the average load current — Iav = (I1 × t1 / t) + (I2 × [t - t1 / t]).
  2. Substitute I = Iav in the equation for battery capacity of lithium-ion. B = 100 × I × t / (100 - q) where B is the battery capacity, I is the load current, t is the duration of power supply, and q is the percentage of charge which should remain in the battery after the discharge.

How long will a 100 Ah battery run an appliance that requires 1,000 W?

You can calculate the run-time using the formula, t = (amp-hour × V)/P, where amp-hour is the battery's maximum capacity, V is the voltage of the power supply, and P is the appliance's wattage.

In the US, the household power supply's voltage is 120 V. Therefore, a 100 Ah battery can supply power for 12 hours in the US for a 1000W-appliance.

How do I calculate the charging time of a battery?

If we ignore the losses during charging, divide the battery capacity by the charging current to obtain the battery's charging time in hours.

Mehjabin Abdurrazaque
Application load
Battery type
Required duration
Remaining charge
Load current
Remaining capacity
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