What size batteries do I need for my inverter?

There are several factors that will determine the size of battery bank you need to run an electrical system with an inverter. Listed below is a series questions you should consider when determining which batteries you should use with your inverter.

Which devices do I want to run?

If you are planning on running devices that consume large amounts of power (e.g. air conditioners, induction cook tops) or if you plan on running multiple devices simultaneously, then you will need a large battery. This is because power hungry AC devices will deplete small batteries quickly and may exceed their maximum discharge current limits. Generally speaking, larger batteries can be discharged at a faster rate than smaller batteries. A high maximum discharge rating is important in order to supply the large current draw of an inverter.

For example, a 1500W kettle consumes roughly 120A of DC power at its peak. This exceeds the maximum discharge current limits of the 125Ah B-TEC, 100Ah Slim, and 100Ah eLITE batteries (which have 100A max discharge limit). If a 1500W kettle were used with any of these batteries, it would cause them to shut down and could potentially damage to their internal BMS. To avoid this, the smallest B-TEC you would want to use in this scenario is 200Ah.

When choosing the batteries you plan to use with your inverter, it is important to remember you should always follow the discharge limits of a single battery. This applies even if you have multiple batteries connected in parallel. More information on maximum discharge currents and parallel connections can be found here.

How long will I be running my appliances?

Another consideration when determining the size of your battery bank is how long you plan to run your devices for. Even small devices can consume a lot of power if they are left running for long periods of time. To work out approximately how much power your device will consume, convert its power consumption into amps, factor in the inverter efficiency, and multiply that number by the amount of time you plan on running it.

For example, a 200W CPAP machine should consume around 14.8A of current (200W/13.5V=14.8A). If you assume that your inverter is around 90% efficient and consumes an additional 1.5A to operate, then the actual amount of current it will consume is 18A (14.8A/0.9+1.5=18A). Since the CPAP machine will be running the entire time you are sleeping, multiply its power consumption by 8 hours for a total of 144Ah of power per day. This means that even though your CPAP machine only consumes around 200W of power, you will still need a minimum of a 200Ah B-TEC in order to run it for a single night

What DC loads are you running

The inverter will not be the only device drawing power from your batteries. Most off grid systems have DC electrical devices (e.g. camping fridge, lights) that run from your batteries directly without the need for an inverter. The power consumption of these devices will also need to be factored in when determining the size of your battery bank.

How long will you be off grid and how will you recharge?

The final factors in determining the size of your battery bank will be your ability to recharge and how long you plan to stay off grid. As you can see from the above examples, running power hungry devices from your inverter, or operating small devices for long periods of time, can quickly drain your batteries. If you plan to stay off grid for an extended period of time, then you should consider adding a DC2DC charger or solar controller to your system to help keep your batteries charged.

For more information on inverters, check out our inverter video series here.