UPS runtime formula

In this article, you will learn the method of calculating or estimating Power Inverter or  Uninterrupted Power Supply (UPS) using this UPS runtime formula. Why do you want to? Because you want to know the time which the system will take before it runs off. That is; run out of power supply from power backup battery (low power from the battery).

Power Inverters as you probably know, they convert power stored in form of Direct Current (DC) to a fluctuating or Alternating Current (AC) form. So, mind you that they themselves don’t create electrical power or energy but, they convert the power stored from one form (DC) to the other form (AC). This is in obedience to the law of conservation of energy which states that; energy can neither be created nor destroyed but can only be changed from one form to another.

The Uninterrupted Power Supply (UPS) is the developed version of the Power Inverter. Its  design and program is to immediately switch to its Power Inverting system in a very short period of time (around 10 to 25 milliseconds) when there is an outage or interruption from the mains power supply. Whenever the UPS is on mains, the system switches its output to mains and the battery is being on charging system at this period.

OK!!! You can study more in advance UPS system and circuits.


Before we start the calculations or estimations, first put these factors in mind.

  1. The capacity of your battery: That is; the power your batter can store or deliver in an hour which is charge capacity. This is measured mostly in Volts-Ampere Hour (VAhr) or Watts-Hour (Whr). As I stated earlier, the Inverter can only change this power which is in DC form to AC. Don’t expect more output Watts hour (Whr) more than that of your battery, in fact, expect less.
  2. The ability of your inverter or UPS: This is the maximum output power your inverter or UPS can drive. This is mostly measured in Volt-Ampere (VA) or Kilo Volt-Ampere (KVA) or even the Watts (W) or Kilo-Watts (KW). Make sure that your inverter can give higher output power more than the load’s power to be driven.
  3. The load power to drive: That is; the total power of the appliance(s) that the inverter or UPS will power. This is measured in Watts (W) or Kilo-Watts (KW)

The power of your battery in hour or charge capacity (VAHr) is directly proportional to the runtime t, (Hr) provided that the load (W) is constant.

UPS runtime Formula

Where t is the runtime in an hour (Hr), VAHr is the power of the battery in hour or Charge capacity, and W is the load power which is the constant

This means that larger battery power or charge capacity will cause the system to have a longer runtime.

The formula above is true and gives almost 98% accurate estimates if your battery is healthy. That is, battery fully charged and hasn’t exceeded its recharging cycles or damaged. Also, the efficiency of the inverter matters a lot. With this formula you can calculate or estimate the runtime, battery charge capacity, and also the load.

Example 1

If an Inverter or UPS’ battery is healthy and it rates 12V7.2AHr and you want to power a load of 30W. What will be the runtime?


From the formula above, we can estimate the runtime to be 2.88Hr which approximately is 2Hr, 53minutes.

Example 2

If you have a healthy battery rated 12V45AHr and you want to run a load of 200W. How long can the system take powering the load?


The time it will take is thus;

Power Inverter or UPS runtime Calculation

After this time you will have to recharge the battery or you will shorten the life span of the battery if frequently discharged lower to 10V. If it is an UPS, it’ll switch off automatically to preserve the battery. But if it is a merely Inverter, you will otherwise have to do this manually or design a low voltage cut off circuit for the inverter.

Fully charged 12V lead –acid battery mostly reads up to 13V when tested with voltmeter.

This method of calculating or estimating Power Inverter or  Uninterrupted Power Supply (UPS) runtime simple and effective! Isn’t it?

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