Why your Battery Size might be wrong! A look at C-rates.

Are you certain you have the right battery size in relation to your inverter? Follow the steps below to check your battery size (We will only focus on Lithium LiFePO4 batteries in this tutorial).

First of all, find all the technical specifications or data-sheets of your equipment, and then follow these steps:

Step 1: Determine the capacity of your inverter in kW (kiloWatt).
Step 2: Determine the capacity of your battery in kWh and Ah.
Step 3: Determine the C-Rate of your battery.

Step 1: Determine the capacity of your inverter in kW (kiloWatt).
The capacity of your inverter is the maximum AC power (in kW) your inverter can supply at any given moment. If the capacity of your inverter is in kVA (Apparent Power), you can determine the Real Power as follows:

Real Power = 1 kVA × 0.8 = 0.8 kW

As an example, let's look at the Victron MultiPlus II 48/5000/70-50. This inverter has a continuous output power of 5 kVA, or 4 kW (at 25C).

Step 2: Determine the capacity of your battery (in kWh and Ah)
The capacity of your battery is the total amount of electricity that it can store, measured in kilowatt-hours (kWh) or Ampere-hour (Ah). If the battery capacity is measured in Ah, simply multiply the Voltage of the battery with the Ah rating to get to Wh:

Wh = Ah x V

Let's have a closer look at two different batteries, the I-G3N e-stack single module and the Pylontech US2000C.

The capacity of a single I-G3N e-Stack is 5.4 kWh and 105 Ah (Ampere-hours). This means that the battery can supply 5.4 kW in one hour before it is completely empty.

The capacity of 2 Pylontech US2000C batteries is 2 x 2.4 kWh = 4.8 kWh. The capacity in Ah is 50 Ah.

Step 3: Determine the C-Rate of your battery
The C-rate represents the rate at which level the battery is providing energy. While capacity tells you how big your battery is, the C rate tells you how much power a battery can provide at a given moment, or how quickly the battery can be charged or discharged.

Sometimes the battery manufacturers give the C-rate outright. If not, you will have to look at the maximum continuous discharge current (in A) and the capacity in Ah that we determined in step 2. You can then determine the C-rate as follows:

C-rate = discharge current  (A) / capacity (Ah)

• 1 C means that a 4 kWh battery can deliver 4 kW in 1 hour.
• 0.5 C means that a 4 kWh battery can deliver 2 kW in 1 hour.

Now let's look at the C-rates of the two batteries from step 2.

For the I-G3N e-stack, the maximum continuous discharge current is 100 A and the capacity is 105 Ah. Therefore, the C-rate is 100/105 = 0.95 (close to 1). 

For the Pylontech battery, the maximum continuous discharge current is 25 A and the capacity is 50 Ah. Therefore, the C-rate is 25/50 = 0.5. 

What does this all mean?
The C-rate is an important measure to see how much power your system can provide at any given moment and how quickly the battery can be charged.

Let's look at the system with a MultiPlus-II 5kVA and an I-G3N e-Stack 5.4 kWh. The e-Stack can deliver maximum 5.4 kWh x 0.95 (C-rate) = 5.1 kW. This battery will be more than able to deliver the 4 kW of the MultiPlus, at maximum power.

For the system with a MultiPlus-II 5kVA and 2 x Pylontech US2000C. The 2 Pylontechs can deliver maximum 4.8 kWh x 0.5 (C-rate) = 2.4 kW at any give moment. This 2.4 kW is a bit more than half the power the MultiPlus can deliver (4 kW). To maximise the inverter and make sure your system can produce 4 kW when connected to the battery, you might need either 3 (3.6 kW) or 4 (4.8 kW) Pylontech US2000C modules!

This means roughly that for similar outputs, you need 4 Pylontech US2000C batteries (4.8 kW output) for a similar output as 1 I-G3N e-stack battery (5.1 kW).

So, we would recommend you go through these steps to determine whether your battery size is correct. Your system might be under-performing and not reaching its full potential, and you might have to add additional battery capacity. Please contact us for any questions.