Inverter Peak Power

[cdsolar]

Do I understand the following issue properly? Note: I do not have any of this equipment yet....

1. Let's say we have the Victron 1200/24 Phoenix Inverter for my 24V battery system, (120V AC output)
2. The Peak/Surge rating for the Inverter is 2200W
3. And the efficiency rating is 91%

Let's say we have a device that has a startup W of the full 2200W. That is, the device needs 2200W to start.
Since AC Watts == DC Watts, without including efficiency, the device will need 2200W on the DC side too.
BUT, since the efficiency is .91, then, I need 2200/.91 watts = 2418W on the DC side for the
hopefully brief time the startup watts are required.

On the DC side, this is 2418/24 = 100.75 Amps

Of course, if my system battery bank is rated at 100 Amps, I am perhaps going to cause a breaker to flip (if I have it wired correctly with a 100A breaker).

Do I understand properly?

Thanks

 

[sunshine_eggo]

Do I understand the following issue properly? Note: I do not have any of this equipment yet....

1. Let's say we have the Victron 1200/24 Phoenix Inverter for my 24V battery system, (120V AC output)
2. The Peak/Surge rating for the Inverter is 2200W
3. And the efficiency rating is 91%

Let's say we have a device that has a startup W of the full 2200W. That is, the device needs 2200W to start.
Since AC Watts == DC Watts, without including efficiency, the device will need 2200W on the DC side too.
BUT, since the efficiency is .91, then, I need 2200/.91 watts = 2418W on the DC side for the
hopefully brief time the startup watts are required.

On the DC side, this is 2418/24 = 100.75 Amps

Of course, if my system battery bank is rated at 100 Amps, I am perhaps going to cause a breaker to flip (if I have it wired correctly with a 100A breaker).

Do I understand properly?

Thanks

Mostly.

Peak efficiency is at about 30% rated max continuous, so you get 91% @ about 360W. It tapers off in either direction. Very low power usage is very inefficient. The surge efficiency is not great.

The general convention is to use .85 as an efficiency factor. It's conservative in most cases.

Wires and fuses/breakers are sized for the max continuous current. Fuses and breakers do not trip instantly, and most surges are for a very short duration, so fuses are not blown and breakers do not trip in most surge cases.

You should size your conductors and OCP on the basis of:

1200/24/.85 = 58A
58A fused at 1.25X = 72.5A

You can always go thicker, but your fuse/breaker should be 1.25X wire rating to prevent nuisance OCP.

 

[MichaelK]

There is another component about surge capacity that you need to think about; time. Typically, the biggest surges occur with large AC motors that start under load. Things like compressor motors, air-conditioners, well-pumps, ect. A lot of big motors might need 500-1000 milliseconds from "on" to "running".

The problem with a lot of the lower budget inverters is that they have very short surge times, compared to their surge wattage. An inverter company may claim their inverter can surge to 200%, but what they don't tell you is that the 200% surge can only last for one 60Hz cycle, or ~16 milliseconds. Serious low-frequency inverters designed for motor starting can surge 200% for 5 to up to 60 seconds.

 

[cdsolar]

Thanks for the information, michael. I don't I have to worry about pumps or air conditioners for my barn. I just have to worry about lights, network equipment such as a mesh wifi router and an antenna back to the house, and an occasional garden tool (check watts before use). But it does help to understand the limits.