How Long Will a Battery Last With an Inverter? (Calculator)

One of the most common concerns that irritate solar power system owners is the battery running duration. This is very important since it tells you how much time your inverter will power your house. This question could be easily answered by determining the following:

  • Battery bank voltage and current
  • Battey type and DOD %
  • Inverter maximum power capacity
  • Inverter efficiency

Let’s get into each of them, and at the end, you will have a simple, easy calculator for calculating your battery running time.

Battery Bank Voltage and Current

Determining the total voltage and current of your battery bank is very easy. If you were engaged in the installation of your solar power system, you already know the answer. However, if you don’t, then you need to take a look at them and watch how are they connected.

Parallel Connection

Suppose you see batteries connected to each other by connecting the positive (+) (red) terminal to the positive terminal, and the negative (-) (black) terminal to the negative terminal, as in the above image. In that case, they are connected in parallel, and we need to add their capacity (Ah), but the voltage will stay the same.

For example, if the two batteries are 200Ah and 12V, then the total capacity will be 400Ah at 12V.

Series Connection

Suppose you see batteries connected to each other by connecting the positive (+) (red) terminal to the negative (-) (black) terminal, as in the above image. In that case, they are connected in series, and we need to add their Voltage (V), but the current will stay the same.

For example, if the two batteries are 200Ah and 12V, then the total capacity will be 200Ah at 24V.

Combined Connection

If you have 3 or more batteries, you might notice that they are connected in series and parallel at the same time, as in the above picture. We have 4 batteries divided into two groups. Each group of two batteries are connected in parallel; however, the two groups are then connected in series with the green line.

In this case, you will use the combined knowledge you just learned for parallel and series connections. For example, if each battery is 200Ah 12V, then each group will have 400Ah 12V since they are connected in parallel. We will then treat each group as a single BIG battery.

So now we have two BIG batteries, each is 400Ah and 12V. These two BIG batteries are connected in series, so we will keep the current same and will add the voltage to have 400Ah and 24V.

Battery Type and DOD %

It is very important to determine the battery type you have and the recommended depth of discharge (DOD%) by the manufacturer. The following table shows the most common battery types with the DOD% for each type:

Battery TypeDepth of Discharge (DOD) %
Lithium-ion Batteries80% to 95%
Lead Acid Batteries50%
Nickel-cadmium Batteries90%
Saltwater (Sodium-ion) Batteriesup to 100%
Sodium Nickel Chloride Batteries80%

However, getting the exact specs through your battery manufacturer is always recommended. But if you don’t have access to this data, then this table is a good starting point.

The DOD% is a crucial parameter in our calculations since it tells us how much percentage of the battery’s capacity is allowed to discharge. For example, if your 200Ah battery is lead acid, then you must not exceed the 100Ah limit (50%).

Inverter Specs

You should also determine two important parameters from your inverter. It is the maximum power for your inverter and the inverter efficiency. The power is fundamental, and you probably know how much power your inverter is (1kW, 3kW, 5kW…). If you don’t know how much power is your inverter (which is strange!), then you could look at its sides or back for the specs sheet.

While looking at the specs sheet to determine the inverter power, you should also look for the inverter efficiency %. This number describes how much power will the inverter convert and how much is lost.

Calculation Process

Now that we have all the needed values, we will start the calculation process. Don’t worry; all these calculations will be done by a calculator that we will provide you at the end.

First, we will calculate the total power capacity that the battery could supply using the following equation:

Battery Power Capacity (Wh) = Battery Capacity (Ah) x Battery Voltage (V) x DOD%

Let’s say my battery is lead acid 200Ah 12V, with 50% DOD:

Battery Power Capacity = 200Ah x 12V x 50%

Battery Power Capacity = 1200 Wh

After that, we will use this number to find the duration the battery could run the inverter. Let’s say my inverter is 1kW = 1000 W with an efficiency of 95%. The equation is:

Battery Running Time = ( Battery Power Capacity (Wh) / Inverter Power (W) ) x Inverter Efficiency %

Battery Running Time = ( 1200 Wh / 1000 W ) x 95%

Battery Running Time = 1.14 Hours or 1 Hour and 8 Minutes

So, a 200Ah 12V lead acid battery with 50% DOD could power a 1kW inverter with 95% efficiency at maximum load for 1 Hour and 8 Minutes. Now using the knowledge that you learned in this article, you will be able to use the following calculator easily.

You will need to input the total battery bank capacity in Ah and the total voltage V. You will also have to choose the battery type to determine the DOD%. Of course, you could also enter this value manually. Finally, you have to provide your inverter power in kW or in W and its efficiency. The result will be automatically calculated in hours and minutes.

FAQs

How long will a 12-volt battery run a 500-watt inverter?

The following table shows how long can a battery run a 500-watt inverter at full load with 95% efficiency:

Battery Capacity (Ah)Lead Acid battery with 50% DODLithium battery with 90% DOD
100 Ah1 hour 8 minutes2 hour 3 minutes
150 Ah1 hour 43 minutes3 hour 5 minutes
200 Ah2 hour 17 minutes4 hour 6 minutes
250 Ah2 hour 51 minutes5 hour 8 minutes
300 Ah3 hour 25 minutes6 hour 10 minutes

How long will a 12-volt battery run a 1000-watt inverter?

The following table shows how long can a battery run a 1000-watt inverter at full load with 95% efficiency:

Battery Capacity (Ah)Lead Acid battery with 50% DODLithium battery with 90% DOD
100 Ah34 minutes1 hour 2 minutes
150 Ah52 minutes1 hour 32 minutes
200 Ah1 hour 8 minutes2 hour 3 minutes
250 Ah1 hour 26 minutes2 hour 34 minutes
300 Ah1 hour 43 minutes3 hour 5 minutes

How long will a 12-volt battery run a 2000-watt inverter?

The following table shows how long can a battery run a 2000-watt inverter at full load with 95% efficiency:

Battery Capacity (Ah)Lead Acid battery with 50% DODLithium battery with 90% DOD
100 Ah17 minutes31 minutes
150 Ah26 minutes46 minutes
200 Ah34 minutes1 hour 2 minutes
250 Ah43 minutes1 hour 17 minutes
300 Ah52 minutes1 hour 32 minutes

Ahmad Ghayad

Ahmad Ghayad is the Creator of SolarsHouse.com, a website created to help people build and understand how solar power systems work. He is a passionate Mechanical Engineer that worked in the building and maintaining several solar power systems for houses and organizations during a severe domestic power outage in his country.

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Andy
Andy
9 months ago

Hi Ahmad,
I was wondering about another aspect that affect battery life – how fast you withdraw the power. How does this affect the battery capacity? As I understand it, a small load will run for longer than a simple multiple of a higher load. If you have a 2KW inverter, it will not run for 4x the duration of a 500W inverter or maybe it will damage the battery? What is the maximum power you can withdraw from a battery?

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