goteborg avatar image
goteborg asked

Guest Cabin gets all the solar

Hello! I am building a small guest house (195 square feet / 18 sq meters) behind my home, and want to add a system like this:

*Tied to grid via 30 amp shore power plug-in similar to an RV, but don't want to feed the grid

*Guests can stay and enjoy life 'off grid' using solar and battery, and if battery gets low or there is a need to use appliance that requires more than batteries can provide, just plug in shore power

*If my home loses electrical power temporarily this is a type of backup shelter

I'd like to use Victron components. Based on my initial research is this ok?

1/ two solar panels on roof

2/ Victron MPPT 100/30 charge controller

3/ Victron Multi Plus Compact 12/2000

4/ Victron’s Venus GX

5/ Victron's BMV-700

What kind of AC/DC panel (fuse box) is appropriate? Any suggestions regarding the specific type of batteries to use for this?

Thank you for any advice!

gridfusesbattery manufature
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1 Answer
snoobler avatar image
snoobler answered ·

It's very important to conduct a thorough analysis of the anticipated energy consumption of the building and the available solar conditions. Once you have both of those worked out, you can design an appropriate system.

Without these, you're just guessing and the chances of you designing a system that works as desired is very poor.

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goteborg avatar image goteborg commented ·

Thank you Snoobs. The new backyard cabin will be vacant most of the time, with no energy consumption at all other than topping off the battery bank. The roof is flat and there are some trees to the northeast blocking the sky but the entire view south and west is wide open with no obstructions. Lots of clean open sky. Here in the Pacific Northwest USA the annual generation capacity of a 1kW system is 1000kWh.

I want the cabin to be have fully charged battery(s) so that when someone comes to stay, they can turn on lights, make coffee, charge phones, it would be nice to have LCD displays to indicate consumption. Ideally the battery bank would be similar to what we had in our RV; just one house battery could keep our lights on for a week.

There will be a plug-in also, such that when someone is staying there a basic extension cord will provide power also. The solar/battery scheme is novelty is some sense because there is this grid extension cord, but I want to have a shelter in case of power outage and I want to have solar to increase awareness and as a stepping stone to perhaps converting our home in future.

Given this, would you suggest an appropriate setup? Thank you!

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snoobler avatar image snoobler goteborg commented ·

Without knowing your loads, you can't size batteries/panels. If you arbitrarily pick a battery without considering the loads, I think you'll be pretty disappointed if you only get a few hours of power.

Given that you plan to use this as a "stepping stone" to expanding solar to your home, it's all that much more important that you get off to the right start and take an ordered design approach.

I presume you got the 1000kWh/year from a resource like PVWatts. That's good. you should also look at the monthly details:


Summer solar will be fantastic, but fall/winter/early spring is going to be underperformers.

It's important that you use these details for planning purposes.

Your guest house is likely subject to city and/or county building codes. Almost all U.S. locations require compliance with the NEC, and the NEC requires "listed" components appropriate for that portion of the code. This usually means "UL" listing. The MP inverter you've selected is only listed for UL 458, which is suitable for RV/marine; however, for residential installations, they must be UL 1741. Unless your local authority accepts alternate listings, you won't be in compliance with code. Even if you're trying to do this "under the radar" and avoid code compliance, that may have further implications with your homeowner's insurance. If you violate code where it's required, insurance will deny claims if they can even imagine a way it could contribute to a fire.

If you mount panels on the roof, you have a hole new set of regs to comply with. If ground mounting is an option - way better choice.

All that said, again, it's important to design a system for the intended purpose. Your cabin has power requirements based on what's in it and how it will be used. You can calculate a daily energy need.

Try using this:

You will get a Wh/day requirement.

I can tell you based on your choice of MPPT and system voltage, that the maximum solar you can accommodate is 14.4V * 30A = 432W. In December, this would only produce a daily average of 1.38h * 432W = 596Wh, but in July, you'd get 6.20h * 432W = 2,678Wh - over 4X difference. It might make a great summer refuge, but it might be severely disappointing in the colder months.

Selecting all the equipment for a system is the LAST step in the process.

  1. Determine daily power needs:
  2. Determine solar availability.
  3. Design a system (PV size, battery size, inverter size based on #1 and #2) that meets the needs.
  4. Source components.

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