One of the most common questions ESS designers have regarding ESS capabilities is whether the system can 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 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-up requirement is labeled "Code," which is short for NEMA Start Code. Now, we need to find out what this Code means- fun, right? 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, we know 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.
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). The device will start as long as the device startup < battery inverter spec. If the startup requirement > battery inverter, then some design considerations must be made:
- Do not put the device on the same AC panel the battery will power when the grid is down.
- Stack more battery inverters until the inverter rating is greater than the device startup.
- 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!
If you have any questions about energy storage system design, please contact our Customer Success team.