Short answer:
You need one 12V, 100Ah lithium battery or four 12V, 100Ah lead-acid batteries.
Let’ dive deeper to why that is.
This article will have two sections:
- Determining the amount of batteries
- Determining the running time of a 1,000W inverter
Continue reading to find out.
Determining the amount of batteries for a 1,000W inverter
Step 1. Determine Current draw
The current draw depends on the battery voltage. Most readers of my website will have a 12V battery, so we will use 12V as an example.
1,000W/12V= 83A
The inverter will draw a current of 83A from the battery.
If we repeat the same calculations for a 24V and 48V battery system:
1,000W/24V= 41A
1,000W/48V= 20A
We can see that the current will decrease if we increase the battery voltage.
We will use the current draw in step 3.
Step 2. Determine C-rate
Next, we need to consider the battery C-rate. These are the C-rates for the most used battery chemistries:
- Lead-acid: 0.2C
- Lithium-ion or LiFePO4: 1C
The C-rate is how much current we can draw from a battery. We have to multiply the C rate by the capacity of the battery.
- Lead acid: 100Ah battery * 0.2C = 20A
- Lithium: 100Ah battery * 1C = 100A
Step 3. Determine the Amount of Batteries
To maximize the lifespan of our batteries, we need to consider the C-rate of the battery.
Remember from step 1 that a 1,000W inverter on a 12V battery will draw 83A?
Lead-acid
According to the C-rate (step 2) of a single 12V 100Ah lead-acid battery, we can only draw 20A.
To maximize the lead-acid battery life, we need four 12V 100Ah batteries. This is how:
12V 100Ah battery * 4 in parallel = 12V 400Ah battery
400Ah * 0.2C = 80A of current draw
The current draw of 83 Amps matches the current draw of the C-rate.
We will have to connect four 12V 100Ah lead-acid batteries in parallel.
Lithium
We can have a higher current with lithium because lithium batteries have low internal resistance.
To maximize the lithium battery life, we need one 12V 100Ah battery. This is how:
100Ah * 1C = 100A of current draw
We can see that lithium is preferred. Check out my expert review article: Best lithium battery
Fuses and Wire used
F1: 83A * 1.25 = 103A -> 100A fuse
W1: 4AWG or 25mm²
How long will a 1,000W inverter run?
The runtime of a 1,000W inverter depends on:
- The battery capacity
- The load you’re running
This can get technical, so if you don’t want to read all these steps you can fast-forward to the example.
To calculate the runtime, you need to follow these steps:
- Determine the load (in watts) you want to run on the inverter. For example, if you have a 400-watt appliance, the load will be 400W.
- Determine the battery capacity (in amp-hours or Ah) and voltage (V) connected to the inverter. The battery capacity and voltage will determine how much energy can be provided to the inverter.
- Calculate the available energy in watt-hours (Wh) from the battery. Multiply the battery capacity (Ah) by the battery voltage (V).
- Factor in the depth of discharge (DoD) and efficiency. Most batteries shouldn’t be fully discharged to maximize their lifespan. For example, if you’re using a lithium-ion battery with a recommended DoD of 80%, multiply the available Wh by 0.8. For lead-acid, this is 0.5. Also, consider the inverter efficiency (typically around 85-90%). Multiply the adjusted Wh by the efficiency factor (0.85 or 0.9).
- Divide the battery available energy (Wh) by the energy consumption of the load (W) to find the runtime in hours.
Let’s calculate this with an example. The Most-used example my readers ask for is a fridge.
A fridge typically draws an average of 50 watts. This includes the off-times.
We have a 12V lithium battery with a capacity of 100Ah, as shown in the diagram before. We need to calculate the total amount of usable energy in the battery:
12V * 100Ah = 1,200Wh
Now we can only use 80% of that capacity for the battery to have a long lifespan. We also need to factor in the efficiency of the inverter at 90%:
1,200Wh * 0.8 usable capacity * 0.9 inverter efficiency = 864Wh of usable stored energy
Now we need to divide the available energy with the used energy:
864Wh/50W = 17 hours or run time.
If you increase the battery capacity you can run the fridge for longer.
Conclusion
You need one 12V 100Ah battery or four 12V 100Ah lead-acid batteries in parallel to run a 1,000W inverter.
We have also calculated the runtime of the inverter with a fridge which was 17 hours.
If you have questions, feel free to leave a comment or send me an email.
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I’m an off-grid enthusiast. I created this website to give clear and straight-to-the-point advice about solar power. I’m also the author of the book ‘Off-grid solar power simplified‘. Read more about me on my about page, check out my Youtube channel, or send me a message.