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davestreck
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# Posted: 5 Jul 2015 05:08pm - Edited by: davestreck
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We've been weighing refrigeration options for our small off-grid cabin. We currently have 130w of solar feeding a 215Ah 12v battery bank. Its just enough for basic electrical needs (12v LED lights, cell phone/laptop charging etc).
Last year we bought a small DC chest fridge (this one), and tried it out for a 2 week trip. We loved it, and it made the stay so much more pleasant. However: its small, it can act as a freezer or a fridge but not both simultaneously, and it drew a bit more power than our system could comfortably supply. Not much more, but I could definitely tell that I was draining the battery bank faster than the panels could handle.
So, this year we decided to upgrade our solar to approximately 400w. With that much more power we are able to consider a larger fridge/freezer combo.
My question is this: considering we can get a cheap 120v conventional fridge/freezer that only uses 270kwh per year (like this one), which works out to about 740wh per day, should we even consider buying a DC fridge? We already have an inverter that should easily handle a small AC fridge. For the price of a dedicated DC fridge we could buy an AC fridge plus two new 150w panels and still have money left over.
What am I missing? What is the advantage of a DC fridge that justifies the extra cost?
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bldginsp
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# Posted: 5 Jul 2015 07:01pm
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Do you know how much more efficient a DC fridge is versus powering an AC through an inverter? That may be less relevant now that panels are $1/watt.
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LastOutlaw
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# Posted: 5 Jul 2015 09:41pm - Edited by: LastOutlaw
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Have you considered a chest freezer conversion?
I use one at my cabin and love it.
The draw is around 500 watts in a 24 hour period running on max cold.
The bottom can be used like a freezer while the top area is used like a fridge on max cold setting.
I put the ice from my cooler in ziplocks in it when I get there and then use them on the way home when I leave.
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creeky
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# Posted: 6 Jul 2015 09:25am
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Imho I say No. It's not worth going dc. AC is ever so slightly less efficient but the cost savings pay for themselves.
I would make sure my inverter was pure sine. I'll bet dollars to donuts that the morningstar inverter would run your fridge easily and it has minimal overhead.
Your 200 amp storage is pretty slim for the fridge at 20% discharge. maybe 400 solar / 400 storage? How old are those batteries? I'd be tempted to beat them to pieces for a couple of years and then replace. with lithium of course. 
my .02.
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davestreck
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# Posted: 6 Jul 2015 04:47pm
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Quoting: LastOutlaw Have you considered a chest freezer conversion? I use one at my cabin and love it. The draw is around 500 watts in a 24 hour period running on max cold. The bottom can be used like a freezer while the top area is used like a fridge on max cold setting.
What model chest freezer do you use? 500wh per day sounds good to me.
Quoting: creeky I would make sure my inverter was pure sine
That might tip the scale in favor of a DC fridge. My inverter is a cheapo Chinese 1000w. Do reefer compressors really need pure sine? 1000w pure sine inverter will probably run $200 or so.
Quoting: creeky Your 200 amp storage is pretty slim for the fridge at 20% discharge
Oops, 215 was a typo. Three 105Ah AGMs for a total of 315Ah. The batteries are 2 years old.
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creeky
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# Posted: 6 Jul 2015 07:00pm
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well. the money you save on the fridge pays for the inverter.
the fridge/freezer combo pays for itself with food savings and ice and ... I mean look at what (gary o, stop reading now) Gary O is paying just for ice. I mean.
and 300 ah AGM. you're golden I think.
as to the inverter: look carefully at the morningstar. the folks here who have 'em like 'em.
so now for my experience/life lesson
five years ago: modified sine wave killed expensive tool batteries. hey. it's your guess whether your tools batteries will die. I'm just sayin' this is what happened to me and other owners of those tools. and it's proven that msw is harder on motors. so fridges etc that have to pump it up. maybe you're better off runnin' cool.
then two years ago: when my 3000 watt pure sine with 35 watts overhead on idle dies through no fault of it's own, sort of. I replaced it with a 1200 watt with 10 watt overhead on idle. And an "eco" mode that I've never used but uses milliwatts on idle. Okay it cost 2.12765 times the cost of the 3kw. But a good pure sine inverter will run stuff you won't believe. My "1200" watt. Runs my fridge. My microwave. My dishwasher. My vacuum cleaner. Oh. and it's 34C (94F) outside, my AC. And even any two at the same time.
Tho. If i"m vacuuming and the fridge turns on, the "bump" in startup draw will reset the microwave clock. eh. I can live with that.
and to be fair. the inverter has a max load of 2500 watts. just like most inverters, double surge over rating. but this inverter, the time on max load. measured in? not milliseconds. like some you see. not seconds. like others. 25 minutes is the run time on max.
so. that's why I like the morningstar for small systems. sort of a general rule. look at the max run time on the max load. kinda tells you a story.
and a good ac system will outdo a good dc system just because you can add so many devices without the dc price. or because they're actually available.
tho. dc appliances, esp. in the 12v range. it's not such a bad world.
whew. that got long.
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davestreck
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# Posted: 7 Jul 2015 05:12pm
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Quoting: creeky whew. that got long.
Informative, though!
OK, $180 for the 3.1cf AC fridge, $330 for 2 150w panels and another $180 for a Sunforce 1000w pure sine inverter. Add $30 for wiring and fusing and I'm at $720. More than I wanted to spend but still reasonable to have a "real" fridge/freezer at the cabin (even if it is on the small side). And as an added bonus I'll be less likely to fry my cordless tool batteries.
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creeky
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# Posted: 8 Jul 2015 09:26am
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and divide by the 20 year life span and that's, um, wait, er, somebody needs a remedial math class, ah call it 36 bucks a year. seems pretty fair.
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