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:
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% 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 |
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?
Hi Andy,
When it comes to the rate at which power is withdrawn from a battery, it does indeed have an impact on the battery’s capacity and overall performance. The relationship between load and battery duration is not linear, as you correctly mentioned. Let me explain in more detail.
Batteries have a specified capacity, usually measured in ampere-hours (Ah) or watt-hours (Wh), which indicates the amount of energy they can store. The capacity rating assumes a certain discharge rate over a specific period of time. For example, a battery with a capacity of 100Ah might be rated to provide 5 amps of current for 20 hours (100Ah / 20h = 5A).
If you withdraw power from a battery at a higher rate than its recommended discharge rate, the effective capacity will decrease. This phenomenon is known as “Peukert’s law” and is more prominent in certain types of batteries, like lead-acid batteries.
Let’s consider a lead-acid battery with a capacity rating of 100Ah. According to Peukert’s Law, the effective capacity of the battery decreases as the discharge rate increases. Suppose the battery is discharged at a rate of 5 amps, which is within its recommended discharge rate. In this case, the battery would theoretically last for 20 hours (100Ah / 5A = 20h). However, if the discharge rate increases to 10 amps, which is double the previous rate, the effective capacity of the battery decreases. Due to internal resistance and other factors, the battery might only last for, let’s say, 15 hours instead of the expected 20 hours.
This example illustrates how the rate of power withdrawal affects the battery’s capacity and the duration for which it can provide power.
To answer your specific question about inverters, if you have a 2KW (2000W) inverter and a 500W inverter, and both are connected to the same battery, the 2KW inverter will not run for four times the duration of the 500W inverter. The higher load will draw more current from the battery, causing the battery voltage to drop faster, which leads to a reduced effective capacity.
Suppose you have a battery connected to two inverters: a 2KW (2000W) inverter and a 500W inverter. Let’s assume the battery has a capacity of 100Ah.
If you use the 500W inverter, which draws a lower load, it will consume approximately 500W / 12V = 41.7A of current (assuming a 12V battery system). If we consider a typical lead-acid battery’s discharge rate of 0.5C (half the battery capacity), the battery could theoretically provide power for around 2 hours (100Ah / 41.7A = 2.4h).
However, if you use the 2KW inverter, which draws a higher load, it will consume approximately 2000W / 12V = 166.7A of current. As the current draw increases, the effective capacity of the battery decreases due to Peukert’s Law and other factors. Let’s assume the effective capacity is reduced by 20%. In this case, the battery would last for approximately 1.6 hours (2h – 20% = 1.6h) under the higher load.
This example demonstrates that a higher load (2KW inverter) will result in a reduced duration compared to a lower load (500W inverter) when connected to the same battery.
Moreover, continuously discharging a battery at a very high rate can indeed damage the battery and reduce its overall lifespan. Each battery has a maximum power or current rating specified by the manufacturer, and exceeding this limit can result in overheating, internal damage, or even a safety hazard.
The maximum power or current withdrawal limit of a battery depends on its design, construction, and specifications. Let’s consider a lithium-ion battery with a maximum continuous discharge rate of 50A, as specified by the manufacturer.
If you attempt to withdraw power from the battery at a rate higher than 50A, you would exceed the battery’s maximum power withdrawal limit. This can lead to several issues, such as excessive heat generation, voltage drop, potential damage to the battery’s cells, reduced lifespan, or even a safety hazard like battery swelling or leakage.
It is crucial to adhere to the manufacturer’s specifications and guidelines regarding the maximum power or current withdrawal to ensure the battery’s optimal performance and longevity.
To determine the maximum power you can safely withdraw from a battery, you need to refer to its datasheet or consult the manufacturer’s specifications. It’s important to use an inverter or load that is within the battery’s recommended discharge limits to ensure optimal performance and avoid potential damage.
I hope this explanation helps! If you have any further questions, feel free to ask.
Best regards,
Ahmad