Calculate Charging Time for Energy Storage

Name: Calculate Charging Time for Energy Storage
Author: Jürgen Kummer

Calculator for the time required to charge a power storage device or battery to a specific level. The device has a maximum storage capacity and is charged at a constant power output. Charging it from a certain level to a higher level takes a specific amount of time, which can be calculated here.
For the percentage values, you can specify the starting and ending values, or the difference, i.e., by how much charge you want. Please also provide two values for storage capacity, power output, and duration to calculate the third value. The result is a theoretical ideal value.

Formula: t = E / P

Example: A battery storage system with a storage capacity of 6.5 kilowatt-hours (kWh) is charged by a photovoltaic system with 2100 watts. To charge it from 10 to 90 percent, it would take just over 2 hours and 47 minutes at a constant power output. Of course, the output of photovoltaic systems is not constant but varies depending on the position of the sun and the weather. Therefore, this 2100 watts should be understood as an assumed average value.

The actual charging time of energy storage systems deviates in practice from theoretical calculations. Even with lithium iron phosphate (LiFePO₄) batteries, which are frequently used in home storage systems for photovoltaic installations, the charging process does not always occur at a precisely constant power output. Towards the end of the charging process, the charging power is usually reduced to prevent exceeding the maximum cell voltage and thus protect the battery. However, this effect is generally less pronounced with lithium iron phosphate batteries than with many other lithium-ion batteries, such as those used in smartphones or electric cars, where the charging power is often reduced more significantly towards the end. Additionally, the battery management system can balance the individual cells within the storage system, which can also add some extra time. Factors such as temperature, charging electronics, and the battery's condition also influence how quickly energy is actually absorbed.

Last updated on 03/11/2026. Author: Jürgen Kummer

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Current charge level:		%
Target charge level:		%
Charge:		%

Storage capacity E:
Charging power P:
Time t:	h m s