Choosing the correct inverter size?


New Member
Feb 24, 2021
I am running off grid. Need to power a 110V 13.5 BTU split A/C unit, (3000 starting watts and 2000 run watts is my guess) maybe 1 small portable room AC unit and a ceiling fan. I think 5-6KW inverter is adequate, but would a larger size inverter, say 10KW be more efficient or run cooler? Is there a downsize to having an oversized inverter?
There's lots of things to consider when picking an inverter. You need to add up all your individual power consumers, including lights, computers, appliances, etc. Assume they were all one at one time, and that adds up to your max continuous power need. Anything with an electric motor will have an inrush current / power. That usually is for a fraction of a second. Most of the better inverters have two power ratings: one for continuous and one for surge. As long as the power including the inrush is below the surge rating of your inverter, you should be fine.

One of the things that will be driven by this total power is the voltage of your DC system. There's lots of different opinions on where the boundaries are, but a 12VDC system is appropriate if you are planning on less than 2000W. A 24VDC system is fine up to around 4000W. Above that, you should probably do a 24VDC system. This is partially because of the larger wire sizes needed for high watts at low voltage, and partially because it is hard to find a quality inverter that really provides power above these thresholds. You may find some no-name 10kW 12VDC inverters. These are fake, rip-offs, and will not work. Remember, 10kW at 12VDC is over 800 Amps, without even accounting for the inefficiency of the inverter.

You didn't say what DC voltage you were planning on, but the power consumption should be a primary driver.

As for a negative impact of having an oversized inverter: First, if you stick with a quality inverter, buying oversized can be very expensive, especially if you are sticking with true sine wave output. Second, the tare loss (power consumed by the inverter itself when there is no loads) tends to be higher as the inverter watt rating goes up.
Right now I have 6x275 watt panels, a midnite 250 charge controller and 4x100AH batteries=48 volt.
your 3000 watt inverter is right, but you will need to get that battery bank beefed up if you wanna run it for very long.
if we are running our AC we use a good majority of 1800 AH@48v capacity that we have (not lithium so ours would be around 900ah in lithium at 48V.
I am running off grid. Need to power a 110V 13.5 BTU split A/C unit, (3000 starting watts and 2000 run watts is my guess)

If that split A/C is inverter drive, then no surge and 3000W as you suggest or even 2000W starting.
Otherwise ...

If 2000 watts by the nameplate, I would guess 10,000 starting watts.

I've measured smaller units, with a scope and current probe so I could see individual AC cycles. A meter isn't fast enough.
It was a small 400W window unit. Under mild conditions (not cooling a room on a hot day), running wattage was 1/2 of rating on label. Starting wattage for 0.16 seconds was 5x rating.

Some inverters quote surge rating, but for about 20 milliseconds which is not enough. Some quote surge rating for multiple seconds which is good.
Many offer a surge 2x continuous rating, some 3x.

If your A/C has VA (volts x amps) of 2000W on the label, expect 10,000W surge and a quality 5000W inverter is probably need to start it.

So as you say, "5-6KW inverter is adequate". Consider specs for SI-6048 (my picture)

11,000W surge for 3 seconds.

At 2000W or 33% of rated power, 95% efficient.

Larger inverter is better of course, efficiency and no-load power is the only concern.

Different brands vary about 4:1 in no-load power. But, the more efficient one generally cost more. This can be offset with more PV and more battery, also a cost, so include those in the dollar comparison.

You may be able to find a nice 120/240V unit that meets your needs so you have more flexibility. But you'll have to be sure of how much surge is available at 120V.