Sizing an off-grid system for a summer cabin - please help

jhauger · February 2015

Hello,

This is my first post but I have been on this site for a while doing my best to learn as much as possible. You guys are amazing and have already made me completely change what I think I need for the system I want. My situation is I have an off-grid family cabin in California that my family uses mostly on random weekends in the summer. During winter months the cabin may sit unused for months in the snow with temps going down to about 20 degree F for lows.

I am trying to size a 24V system that will allow me to go up on the weekends Friday night – Sunday afternoon and be able to consume about 7500 watts for the entire weekend. I also have a Honda EU2000 that I would like to use to bulk charge if needed for an hour or two during the weekend.

In the end I was thinking about going with a 24V system using (6) Maxrate MR-490 (12V 139AH) batteries that I can get through work for nearly free with one year of use on them. I was going to configure the system for 417AH - wiring the batteries in series, with 3 strings total. I was also estimating that a 50A battery charger would work best with my small generator and the 417AH system. I would also use the system with a 1500 to 2000 watt inverter to power 120Vac L1 or L2 of the cabin that is wired for 120/240Volts.

The area I am struggling with is sizing my panels. I would like to use as few as possible because most of the time the cabin just sits un-used and the batteries would be at 100%. I also don’t mind running my generator for a few hours since there is usually something going on that requires me to run my Honda EU2000 or if need be a much larger 12KW diesel generator anyway. If I go with a solar panel setup that has a max amperage output smaller than 5%-13% will it reduce the life of my batteries? I would like to go with about four 100 watt panels but I know that is undersized. Are there other factors to consider if your system sits, un-used for months during the winter? Any advice would be much appreciated. I feel like I have just enough knowledge on this subject to be dangerous

Thank you,

Jake

westbranch · February 2015

Welcome Jake, tell us more about your consumption? What appliances are you using? It sounds like you have some large loads ie 12Kw genny...

BTW that should be 7500Wh shouldn't it?

rpvietzke · February 2015

I ran a 440Ah/12V system in Vermont for several years with 3 125W panels and a small 40A "old style" charge controller. Like you, it was a weekend cabin with limited loads and had plenty of charge time during the week with no loads. The batteries were enough to get me through most weekends without starting the generator. Summer months were completely fine. The winter was a lot harder and I switched to 3 250W panels and a larger MPPPT charge controller... That worked GREAT. Even with cloudy days there was enough to get through the weekend except for the worst of December and January. Most of the time I could even run a small refrig in the summer and (god forbid) a Keurig coffee pot in the mornings too.

Panels are cheap these days. Depending on where you are and what sun you have, I'd argue its worth it to oversize them and worry less.

Rob

jhauger · February 2015

Hi Westbranch,

The cabin is large, about 2000 sq ft and my brother and I share the use of it with our friends and families. We do have a lot of heavy equipment there, including a bull dozer, welders, etc. We also have a primary generator 12KW and a smaller 7KW generator. Both are overkill for the cabin itself but come in handy when needed. A couple years ago I bough the little Honda EU2000 and love it because it is quiet and can pretty much run everything I need in the cabin 80% of the time. The heavy load items in the cabin are the gas clothing dryer, vacuum and microwave. We have a propane refrigerator and stove.

My goal is to reduce the use of noisy generators and make it through a weekend without having to run them much. I estimate that typically on a Friday evening when we arrive we would use around 2000Wh, then Saturday around 3000Wh, followed by 2000Wh on Sunday. Then we would be gone for a week or two before returning for another weekend. I am rarely there for more than 5 days at a time. The family watches more TV than they should on a 32" flat screen w/direct TV. Which also watch movies at night. That along with lighting is the main draw. I would also like to have enough inverter to run a vacuum cleaner before we leave. I would also love to understand a good option for powering both L1 and L2 of the 120/240V system in the cabin. I don't have anything that requires 240V but it would be nice if I could make all the outlets hot with 120Vac.

I think that pretty much sums up what we have. I also am fortunate to have access to more batteries than I probably need. I could probably get between 12-16 of the 12V 139 AH batteries if I wanted them. In fact that was my plan until I started reading this forum and realized I would need a ton of panels to support them. I am trying to keep my cost down to around $1500 for the main components, the batteries are not include in that number.

Thank you Westbranch,

Jake

westbranch · February 2015

Ok, if you can run the Eu 2000 when you need to run that vacuum and the dryer and limit the battery loads to lights, AND change them to LEDs you should be able to run off the batteries. One option is to just power the lights with the battery and all other circuits get gen power. goto go as getting error messages constantly right now

jhauger · February 2015

Hi Rob,

Did you have any issues with your overall battery life? I guess if they sit all winter charged and getting topped off by the solar panels and a good charge controller then they should have pretty decent longevity. Yeah maybe you are correct about the panels, if I bite the bullet and buy enough panels now I won't have to ever worry about having enough again.

Thanks,

Jake

westbranch · February 2015

I should have added that if you decide to stay with a fridge you would want a 1500 or 2000w inverter and a 24V bank... matter of fact if you stay with thise undersized batteries , 24V is almost a must to get good performance from your system.
A fully charged FLA or AGM will stay happy being cold and topped up 'daily' or weekly as it may.
Concur about getting extra panels as they may not be around in a year or 2..

rpvietzke · February 2015

Jake - We're on year 8 with Concorde AGM's. I went with these because they are "inside" the cabin space, I had little kids at the time, and I didn't want to fuss with venting/watering. We also do not heat the cabin in the winter and the AGM's are a little more tolerant to really cold temperatures. They definitely are not what they were in the first year, but I can't complain about resilience or how they have held up. They do get a little cycling for some web security cameras and a DSL modem and they have also suffered from a couple times when the low-voltage cutout didn't work and the were drained hard. We can still show up on a Friday night, watch a movie or two, play the radio, run the lights, run the well & pressure pumps, etc. with no problem, (except late december, early january when there is little sun AND everything is really cold.

I really can't complain overall. The only time we have an issue is when we hit them with the Keurig coffee maker for a 2nd cup the morning after a day of clouds... Oh, and occasionally when we have a really bad run of cloudy days they hit low-voltage shut down and require the sun to come back, or a ride up to run the generator for a while.

We're building a bigger place next to the first cabin and I went with more traditional trojan T105's there... Lots more battery bank for the larger place and very inexpensive. But, there is also a basement, opportunity to super-insulate and keep the batteries "warm" and a lot more load in the larger space... Still learning about maintaining them.

Sizing an off-grid system for a summer cabin - please help

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