This is on
e of the most common questions I get concerning large loads. Others include pool and spa pumps, large shop equipment like air compressors, and even commercial grade freezers. I devoted quite a few pages to this question in my book The Battery Powered Home. The answer?
On two things, in particular:
The startup power of the load
The startup power the battery system can provide
Pumps, compressors, motors, etc. all have a startup (a.k.a. surge) rating that is expressed either in
amperage or their NEMA Code. Startup amperage is labeled as LRA on the device nameplate. Locked rotor amps tells us how much current is needed for the rotor to overcome an at-rest condition. Once the device starts up, it slows to its operating current (power).
Kind of like when you start your car- the tachometer needle shoots over to the left to 6 or 7 thousand RPM and then comes to rest at about 1000 RPM.
On the left side of this pool pump label we see that it states 9.0 or 4.5 amps required for operation. However, this is not the startup requirement but rather its normal operating current. Above the amps rating the label also states two voltage ratings: 115/230. If the pumps is using a single pole breaker (115V) then it will require 9 amps to run. If it is connected to a 2-pole breaker (230V) then it only needs half the current to run. Double the voltage, half the current. Higher current means larger wire sizes and larger overcurrent protection (breakers). This pump label does not state its LRA rating, only the NEMA Code M. To decipher this Code, we must refer to a handy chart.
This chart is all over the Internet, but this one I got from The Engineering Toolbox, a site I refer to a LOT. The chart shows the Code and the kVA (power) required per horsepower to start the load. For ease of explanation I will just refer to kVA as kilowatts (kW). Click HERE to learn about kVA.
From The Engineering Toolbox:
In general, it is accepted that smaller motors require higher starting kVA than larger motors.
Standard 3 phase motors often have these locked rotor codes:
9.0 - 9.99
1 1/2 to 2
L or M
9.0 - 11.19
8.0 - 8.99
7.1 - 7.99
7.5 to 10
6.3 - 7.09
5.6 - 6.29
The pump Code M for our pool pump means that it will require 10 to 11.19 kW to start, average being 10.6 kW, per horsepower. The pump label indicates that this is a 1/2 HP pump. That part of the label is a little torn, but it is readable. So, we now have to take half of 10.6 kW and we get to 5.3 kW of startup. See, that wasn't so hard, was it?
If this was a 2.5 HP pump then the startup requirement would be 10.6 * 2.5 = 26.5 kVA. We need to convert these values to amperage just in case our battery system spreadsheet expresses it this way.
5.3 kVA at 115V = 46 A
5.3 kVA at 230V = 23 A
26.5 kVA at 115V = 220 A
26.5 kVA at 230V = 110 A
Sometimes you will luck out and there will be a locked rotor amp rating on the device. Let's look at this Carrier 2 ton air conditioner label to find its LRA value. Its operating current and power are circled as well as the breaker size, but can you find the LRA?
There it is! It is a beefy 72 amps. It easily puts our dinky 1/2 HP pool pump to shame. That is much easier than tracking down a NEMA Code, right?
Now let's look at an energy storage system to see if it can start our pump. The ESS spec sheet will define their startup capability in a few ways. No, they don't make it easy sometimes. It may be called maximum output or overtly stated as surge. Or both.
Here is a part of a spec sheet from a popular ESS manufacturer. We need to find the section that states output power while in backup mode, i.e. when the grid is down and we are relying solely on ESS power.
This manufacturer states a surge rating of 6.6 kW, standard for most energy storage systems of this type. Great, now let's compare that to the pool pump requirement of 5.3 kVA.
6.6 > 5.3 so we are good. This ESS, while in backup mode with the grid down, would be able to easily start this 1/2 HP pool pump. Let's check that 2.5 HP example.
26.5 kVA will test the mettle of most residential ESS. We would need 26.5 kVA / 6.6 = 4 units of this manufacturer to meet this demand. Oh, but we need to find out if they can stack 4 units to reach this requirement. I already know that they cannot. Very few can. Those who say they start large loads like this may or may not be accurate.
But there is hope! I will always recommend a soft start kit for large loads like this even if the ESS can start it. I have seen to many larger loads trip the ESS off even though the spec said they could start them up. Soft start kits for AC units are very common and they reduce the amount of current drawn by the device. A quick Internet search will provide thousands of pages to choose from, albeit they are for RV's unless you refine your search. Here is an example of one: Briskheat Soft Starter.
That's it! If you found this useful please like and share. I will be happy to answer any questions in the comment section.