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 Type | Depth of Discharge (DOD) % |
---|---|

Lithium-ion Batteries | 80% to 95% |

Lead Acid Batteries | 50% |

Nickel-cadmium Batteries | 90% |

Saltwater (Sodium-ion) Batteries | up to 100% |

Sodium Nickel Chloride Batteries | 80% |

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:

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 = ( 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% DOD | Lithium battery with 90% DOD |
---|---|---|

100 Ah | 1 hour 8 minutes | 2 hour 3 minutes |

150 Ah | 1 hour 43 minutes | 3 hour 5 minutes |

200 Ah | 2 hour 17 minutes | 4 hour 6 minutes |

250 Ah | 2 hour 51 minutes | 5 hour 8 minutes |

300 Ah | 3 hour 25 minutes | 6 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% DOD | Lithium battery with 90% DOD |
---|---|---|

100 Ah | 34 minutes | 1 hour 2 minutes |

150 Ah | 52 minutes | 1 hour 32 minutes |

200 Ah | 1 hour 8 minutes | 2 hour 3 minutes |

250 Ah | 1 hour 26 minutes | 2 hour 34 minutes |

300 Ah | 1 hour 43 minutes | 3 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% DOD | Lithium battery with 90% DOD |
---|---|---|

100 Ah | 17 minutes | 31 minutes |

150 Ah | 26 minutes | 46 minutes |

200 Ah | 34 minutes | 1 hour 2 minutes |

250 Ah | 43 minutes | 1 hour 17 minutes |

300 Ah | 52 minutes | 1 hour 32 minutes |