Transfer Switch

[Nakoa Woodruff]

Hi, I'm designing a 18kW off grid power station and would like to know how big does my battery storage need to be? I will be using solar and pma turbines. I want to power my home and two barns.

[Rob Beckers]

Hi Nakoa,

The most-used rule for sizing off-grids is to provide 3 days of storage with nothing coming in. For lead-acid batteries they're sized to be 50% down at the end of the 3 days. In practice even on a cloudy day something is coming in (with solar PV), and lead-acid can be discharged to 80% DOD no problem, so in actuality there's more than 3 days in the tank this way.

That means if you are using 8 kWh per average day it works out to 24 kWh in available storage. The 50% discharge part means a battery bank that stores 48 kWh. For a 48 Volt system that works out to 1000 Ah in storage.

Some size their batteries smaller. That is perfectly legit, it just means the generator will kick in more often, so more money in fuel, but less in batteries. The 3 days is just what's used most often (and what I've done many times in designing off-grids).

-RoB-

[Nakoa Woodruff]

Thanks. I will look at bigger batteries.