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Small Cabin Forum / Off-Grid Living / How to calculate Air Conditioner usage for solar panels & battery storage
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mg93108
Member
# Posted: 12 May 2020 19:20
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Hi everyone...I'm new here
There are lots of variables here but I'll try to give some info that will help a bit
It gets very, very hot out here. I'm installing a solar system since AC is not an option.
The cabin is 432 sqft (40 sqm).
The hottest temps get to about 117 F (47 C).
The cabin is not well insulated and receives no shade.
I'm wanting to install a mini-split AC heat pump system of 12,000 btu's (1 ton).
Running it during the day is no problem. There is abundant sun here.
My real question is about battery storage.
For other items that will discharge the batteries, it's really very, very little. We use propane for cooking, no microwave, propane frig, etc. We will have a few LED lights and we will laptops/phones during the day.
Does anyone have a similar sized setup?
I've tried running calcs but keep running up against how many hours a day/night the AC will be running.
Thank you all for your help!!

mg93108
Member
# Posted: 12 May 2020 19:21
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Here's a pic of the cabin
IMG_3209.JPG
IMG_3209.JPG


mg93108
Member
# Posted: 16 May 2020 23:43
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Nobody?

FishHog
Member
# Posted: 17 May 2020 08:23
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Only you can tell us how many hours your Ac will be running. The specs on the unit will tell you the usage. Then it’s just math to see how many kwh you need to run it through the night then size your batteries and solar panels accordingly

Ac is tough to run off batteries but it can be done if you get enough sun through the day to charge your bank back up

mg93108
Member
# Posted: 17 May 2020 10:20
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If I knew that, I wouldn't have to as the original question. "How to calculate Air Conditioner usage for solar panels & battery storage". If you have done this, I'd love to hear your thoughts. There are a number of people who do this. It seems that the whole issue is predominantly related to proper battery size (for me given that I have abundant sunshine.

Nobadays
Member
# Posted: 17 May 2020 11:46
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I think what is being said is.....

1) You need to estimate how many hours per night/ non-charging hours you expect the A/C to run.

2) You need to know the wattage of the unit you intend to buy. This varies between units.

I looked at a Seer 19, 12,000btu unit. The specs said 1020 watts. To round this off... a VERY ROUGH estimate, 1kw/ hour of operation. With a 48v system that would equal about 21ah/ hour.

As FishHog says... it's tough to run A/C off solar. It's going to take a big battery bank to run the A/C during non-charging hours as to NJ ot deplete them so low you damage the batteries.... and a large solar array to ensure you can bring them back up to full charge every day.

As an aside... think lithium for this demanding task.

Nate R
Member
# Posted: 17 May 2020 16:51
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Lots of variables here. THermal mass of the building, how well insualted it is, etc... These variables will contribute to what the needed overnight cooling load is. REALLY tough to guess/figure that out. If I were you, I'd be inclined to run a 10K BTU window unit on a generator over a hot weekend with a kill-a-watt and see if it ever shuts off overnight or what the cycling looks like. Datalogging the load would be even better, as you could really find compressor runtime then, and find BTU usage that way.

And then, yeah, AC on solar CAN be done, there is a DC heat pump out there for this task, too.... But not an easy application.

Notes
Member
# Posted: 17 May 2020 16:51
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A properly sized air conditioner based on your climate, load, insulation will cycle every 2-3 times per hour, with every cycle of run time anywhere between 10- 15 minutes. Look for a inverter type unit. (Low start surge). LG has a new DC inverter driven window unit that’s very efficient.

clintondiya
Member
# Posted: 19 May 2020 04:01
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My central A/C unit uses 2.4 kW when it's cooling. When it's about 90 ºF, it runs probably 70% of the time. At 100 ºF it'd probably be running nearly 100% of the time. In order to sustain this, I would need, thus, at least 2.4 kW of solar generating capacity.

I have 12 kW grid-tied. So that's fine for me! But for you, getting 2.4 kW for under $2,000 would likely be quite a challenge.

Hopefully a cloud doesn't go over your house on a hot day like they do at my house.

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