Running window AC unit on solar
July 22, 2021 8:27 AM   Subscribe

If I want to run a window AC unit on solar, what equipment do I need and where would I buy it?

I'm okay only running this unit when it's sunny out, so I don't want a battery. If it doesn't work at night or on cloudy days that's fine. I guess I need panels and an inverter? The unit runs on 120 V AC. The owner's manual says it will pull up to 15 amps but in reality it pulls more like 7 amps.

How do I turn this information into an appropriate number of panels, and where do I buy the panels and an inverter? I have a sunny place where I can set the panels. And is there more to it than what I've listed here?

This setup will not interact with the grid at all so I don't need to worry about connecting to the grid.
posted by Tehhund to Technology (12 answers total) 2 users marked this as a favorite
Check out this article - may give you some ideas.
posted by The_Vegetables at 9:01 AM on July 22, 2021

I don't think you can have an off-grid solar setup without a battery.

You need a battery to supply a stable source of voltage to power the A/C unit. Your solar panel is going to deliver a varying DC current—it's not going to be stable enough to power devices directly.
posted by vitout at 9:04 AM on July 22, 2021 [17 favorites]

A lot of people have done these calculations for RV AC units - How Many Solar Panels Do I Need to Run My RV AC?.

I think you are going to need a battery to smooth out the process - sometimes you will have less/more sun, and also the amount of power the AC draws is not consistent - it uses a lot of extra power when the compressor kicks on, so your inverter needs to be able to handle that (or I guess there are ways to make the AC start more gently).
posted by mskyle at 9:15 AM on July 22, 2021 [4 favorites]

Without a battery (or some other energy-storage device), I think you'd have to over-build the panels significantly in order to supply the "inrush" current used by the A/C at the first moment when you turn it on. It may only draw 120V @ 7A when running continuously, but I strongly suspect that in the instant after you turn it on, current draw is significantly higher.

This page, which is aimed at people running A/C off of small generators (but is also applicable to small solar installations), says: "The maximum start up current is referred to as "Locked Rotor Amps" (LRA) because at the first moment when the rotor is at standstill it appears as if it's locked. This current will drop significantly when motor accelerates to about 75% full speed. The LRA is typically 3 to 8 times continuous operating current (called full load amps, or FLA)."

You basically have two options for the battery/inverter/controller: you can buy an all-in-one unitzed solution (like the one in The_Vegetables' link above, only you need a significantly bigger one), or you can build one from parts, which is more complex but cheaper.

Supposedly you can get a 2000W continuous / 4000W surge, unitized solar "generator" from China for $2400.

You could probably do better than that with locally-sourced lead acid batteries (surplus forklift batteries used to be fairly popular, although more recently I've read that people are getting ex-Tesla battery modules somehow), a charge controller, and an inverter. Depends on how much time vs. money you want to spend.

Alternately, DC-powered air conditioners do exist, for exactly this sort of application. There's no prices listed, though, which makes me think they're probably quite expensive.

One other option to consider, is to drop the requirement of being entirely off-grid, if you have access to 120VAC. Something like this lets you power fairly large electric appliances using solar panels, but will pull from the AC supply when demand exceeds what the panels can supply. So when you flip the A/C on, the inrush/LRA current will be pulled from the AC supply, but then once the motor is running, and assuming you have the appropriate number of solar panels, the steady-state operation will be via solar. This is what I would do, TBH, if you are not actually in an off-grid location.
posted by Kadin2048 at 9:41 AM on July 22, 2021 [7 favorites]

"Soft start" is the name of the step-up technology you can use to overcome the initial power draw of AC AC units.

I generally try to keep up with available tech via DIY Solar Power with Will Prowse, who is currently in the process of standing up a full home solar array with central AC and has a shed that runs a typical residential window unit on solar, more in the RV style.
posted by Lyn Never at 9:45 AM on July 22, 2021 [2 favorites]

Response by poster: I hadn't thought about the draw when the AC starts up (I already have the unit so I'd rather not buy another one), and I also had not thought about how having an appropriately-sized battery would let me run the AC with fewer solar panels since they will charge the battery even when the AC isn't running. The AC runs a maximum of 3 hours per day so I guess I should get panels and a battery big enough to run it for that time.
posted by Tehhund at 10:08 AM on July 22, 2021

12 VDC mini split system, $1695, 6000 BTU. Meant for an RV but I don't see why it couldn't be installed on a wall (or a piece of wood mounted in the window-frame for the season and passing the lines through). Pictures on Amazon seem to support that concept.
posted by snuffleupagus at 11:16 AM on July 22, 2021 [1 favorite]

Save yourself the headache of getting separate inverter, controller, battery and doodads (+ housing all that) by looking into combo units, often called solar generators or portable power stations. Goal Zero is a popular (good, perhaps over-priced) brand but there are dozens of others. Look for ability to attach (or chain) more than one panel and keep in mind the unit will probably take lower wattage panels than the typical rooftop ones.
posted by Press Butt.on to Check at 11:21 AM on July 22, 2021 [1 favorite]

It's not going to be cheap by the way, if that was part of the goal. Much more than the AC itself.
posted by Press Butt.on to Check at 11:23 AM on July 22, 2021 [1 favorite]

Hi, I'm baking my second load of sourdough of the day in a toaster oven with my offgrid solar setup as I write this.

Get a kill-a-watt meter and find out the actual draw of your AC unit. Base the system size on that. It will probably be in the neighborhood of 800-1000 watts of solar, so 3 or 4 panels. The Epever Tracer solar charge controller is a common and inexpensive controller for this power range. Any 1500 watt inverter should do although you may want to consider the inverter's surge capacity for that compressor start load.

As to the battery, most charge controllers need a battery to work at all -- they can't power up to look at the panel voltage without already having a source of power. Deep cycle or lithium, but the latter is overkill for this, unless you need to run the AC at night. It does make sense to overpanel rather than buying a larger battery, because batteries are the part of the system that is going to be killed by overuse, misuse, time, etc.
posted by joeyh at 11:54 AM on July 22, 2021 [5 favorites]

Inverter air conditioners generally have low to zero inrush current (they start at a very low speed and ramp up) so you could conceivably run a 500W inverter air conditioner using a 500W inverter without a battery, but I agree with others: to do this right, you should get Solar Panels + Solar Charge Controller + Battery + Inverter.

Mini Splits usually are inverter based, but now you can even get window-style inverter air conditioners.
posted by soylent00FF00 at 4:41 PM on July 22, 2021

I work in the solar industry in Australia. I've been installing off-grid power systems for nearly 30 years. I get asked about this kind of thing regularly and it is rarely, if ever, worthwhile from a cost point of view.

You need some sort of storage because the inverter will be running the AC, not the solar, and they have minimum requirements for delivering their rated power. The battery has to be able to provide the required current without the voltage dropping too much.

The cost of off-grid systems here is about 50% batteries and 50% everything else. By not using the grid that you seem to have available you will be doubling the cost at a minimum. It's probably more like increasing it by 3-4 times given the different and extra equipment required for off-grid versus grid connect.

I would use all that money to put in a grid connected system. It will run your AC, but it will also reduce your power bill by allowing self consumption - you using the power generated by the solar - and maybe also by providing a feed in tariff from your electricity supplier, which will offset the power you use when the sun isn't shining.

In Aus, these systems are mass market, high volume products that are very cheap compared to anything off-grid.
posted by mewsic at 9:53 PM on July 22, 2021 [3 favorites]

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