Better to plan an intermittent heavy load or a steady, lighter load?

[cracked_ribs]

I have an off-grid cabin that I spend weekends and occasional weeks at. In some cases I'd probably be there for a month, but never longer than that, as far as I can imagine, and that would almost certainly be in the summer. So far I've been doing fine with heavily insulated coolers but if I'm up there for a week or ten days I'll sometimes pick up a bit of extra ice etc.

I have a little Honda 1500w generator but I've only ever run it when I needed a power tool. My place is on an island and it's really quiet so I avoid noisy gear as much as possible. These days I'm spending more and more time at the cabin so I thought I might look at a small PV setup. Here's what I'd like to run:

LED lighting rather than the old kerosene lamps I use now
A laptop and minor electronics charging
Ideally a 25w VHF base station

And some kind of refrigeration.

I'm completely used to coolers so I'm fine with either a chest fridge, OR a chest freezer run intermittently to make ice just to keep coolers cold. In the winter a block of ice can last me for days. In the summer obviously I get more sunshine, so without knowing much about the effects on the batteries I'm leaning more towards the freezer. It would also allow me to freeze fish that I catch locally when over on the island.

I'm in Canada, north of Seattle but in a drier and sunnier climate thanks to mountains on Vancouver Island.


Before I get too far into planning, I thought I would ask about whether there's a major disadvantage to intermittently running something like the Sundanzer DCF165, versus running the DCR165 for longer periods?


As you can see my requirements are pretty minimal but I would rather do this properly, regardless.

[Sunking]

Yes it makes a difference for a given size battery. The difference lays in how batteries discharge and Peukert Law which states:

Change in capacity of rechargeable lead acid batteries at different rates of discharge. As the rate increases, the battery's available capacity decreases.

What that means is the faster you discharge, (more amps). the less capacity a battery has. Example a 6-Volt Golf Cart battery is rated at 225 AH at the 20 hour discharge rate of 11.25 amps. All that means is the battery can deliver 11.25 amps for 20 hours or 11.25 amps x 20 hours = 225 AH.

That same battery discharged at the 1 hour rate of 79 amps is only a 79 AH battery. Same battery discharged at the 100 hours rate of 1 amp is a 300 AH battery.

OK asking fo refrigeration is a big deal with Solar. You whole system has to be designed around it and dominates 90% of your electric use. That would mean to run everything but the fridge could be a simple small 100 watt panel, 10 amp controller, and a 12 volt 80 AH battery. Want a fridge, dig deep now you need a 1000 watt system, an very expensive 80 amp MPPT controller, and monster size 12 volt 1000 AH 700 pound $2000 battery you replace every few years.

My advice is invest in a Yetti Cooler. I am a cooler snob, once I got the Yetti, Threw all other coolers away. I am in Panama and keep the Yetti on the back patio full of drinks. I change ice every 4 to 6 days depending on use. They are expensive but a lot less expensive than the solar to run and operate a fridge.

[cracked_ribs]

I definitely see that to add a fridge is the big challenge - it struck me after a bit of reading here that I could probably do everything else with a car battery and a cheap 12 volt panel, but that to run a fridge would, exactly as you say, require a significant investment.

That's why I got to thinking maybe I'd be able to intermittently run a freezer, just to make enough ice to fuel an industrial cooler. But it sounds like Peukert's Law means that even though it would be an occasional load, it would still mean a large system because the discharge rate would be higher, so the batteries would have to be bigger, so the array would have to be bigger to prevent the batteries from getting wrecked, and so on.

If I said I only wanted the ability to make ice once a week to fuel a Yeti cooler, does that spark any interesting or creative workarounds?

Although I guess making 10 lbs of ice and keeping 6 cubic feet cooled to 4 degrees C for three days is basically the same thing anyway in terms of energy input.

I mean I can always run multiple coolers and bring one up that's just ice and use it to refuel the others, it's just nice to have the ability to limit your resupply when you're on an island because going for supplies is a bit of a production.


Anyway if there's one thing I've learned reading here it's that solar is expensive, inefficient, and a way bigger headache than the layman would expect so I'm not really surprised if there's no work around. I've done well with coolers in the past, even in really hot places. But let me just suggest this scenario: if you had the largest Yeti, say, and you needed to design a solar system for the sole purpose of giving it ice once every week...what would that look like? Still a 1000 watt system?

[DanKegel]

Making ice once a week is what you'd do with the generator, I think.

This forum has a bunch of discussions of fridges/freezers, search for e.g. sundanzer, whynter, edgestar, sun frost.

[Mike90250]

i'd just buy ice once a week.

[Sunking]

Buy a Yetti Cooler and a bag of ice once a week. Much less expensive, way more effective and reliable.