ESS Real Talk: Starting large loads - the definitive guide

One of the most common questions ESS designers have with regard to ESS capabilities is the ability for the system to start large loads like air conditioners, pool pumps, well pumps, air compressors, etc. These large loads can put a strain on even the most robust of battery inverters so it is important to understand capability vs. load.

The crucial spec we need to investigate has many names- surge, start-up, starting current, etc. But it is called something very specific on the device nameplate. We are looking for the NEMA start Code or the locked rotor amp (LRA) rating. Let's start with the LRA since it is the easiest to work with.

Here is an air conditioner label with the LRA rating marked. 

The next method also relies on the device nameplate. In this example, let's look at a pool pump label.

Notice how this label does not have a locked rotor amp rating, but the start-up requirement is still there. On this label, the start-u requirement is referred to as Code, which is short for NEMA Start Code. Now we need to find out what this Code means. The Code table translates the letter for us.

The letter corresponds to a kVA rating per horsepower of the device. Using our well pump label Code of L means that it will require 9 to 10 kVA per horsepower to start this pump. The nameplate states that this is a 3/4 HP pump so we just need to multiply 9 and 10 by .75 to get the startup: 6.75 to 7.5 kVA.

Now we need to compare the AC start-up requirement to the battery inverter startup spec. ESS will state their start-up rating as "maximum power", usually followed by a short time frame (seconds or milliseconds). As long as the device startup < battery inverter spec then the device will start. If the startup requirement > battery inverter then some design considerations must be made.

1. Do not put the device on the same AC panel the battery will power when the grid is down.
2. Stack more battery inverters until the inverter rating is greater than device startup.
3. Use a soft start kit to reduce the surge requirement on the battery inverter.

Regardless of which approach the designer takes, it is always a good idea to follow up with a lot of customer expectation management!  And that's it! Please post your comments and questions below.

What is the largest load a customer wanted you to put on their essential load's panel? How did it go?