Off-Grid, Newb, general guidance

[Crudely0097]

Hey Folks - Nublet #9001 here looking to offload all my thinking and learning. Looking for general guidance or direction so I can focus my research appropriately.

Short: Father lives off in the woods, and is old enough that no food storage is getting to be an issue. Has a generator & found a small chest freezer we can use. Looking to enable chest freezer usage, 90% + solar, with battery bank to be charged via generator when >5 days of no-solar generation.

Needs:
1. Run small 120v freezer (300W max draw, average .7kwH/day )
2. Cheap
3. Battery, with Generator as charge source?
4. Temperature Resistant

System Basics Concept:
- Panels
- Used, 3 x 400W? target 600W total, PVWatts puts at 3kWh/day for a floor.
- Inverter
- [12/500/20 Victron](https://www.victronenergy.com/upload/documents/Datasheet-MultiPlus-500-1200VA-120V-EN.pdf)
- Charge Controller
- 12/500/20 Victron
- Batteries
- 12V, 150AH? Lithium expensive, do lead for cheaper & double? Exterior, no climate control.
- Communication
- ????

 

[Crudely0097]

Cost is a prime driver, used is perfectly fine.

 

[Mattb4]

Welcome to the Forum.

Cost may be a big factor but the prime driver for solar is loads and supply. Watts= Volts X Amps. Loads are the things you wish to power and is rated for watts running, starting and in watt hours for amount used over a day. Example is your freezer. You list 300 watts running, probably 8-900w starting and a energy use of 700wh per day. If you have tested the freezer with a Kill-A-Watt type meter to confirm these numbers that would be great.

Knowing the load helps with calculating the supply. Let's say with conversion losses that your 700wh is closer to 1000wh for the day. To supply 1000wh with PV you would need to know your average production hour number for your location. Example Summer is 5 hours. You can than take 1000wh/5h=200w worth of solar panels.

Obviously the sun does not shine all the time so you need to store sufficient energy to carry you through the day. To store electrical power you will need a battery. Taking the 1000wh load and using 1 day as storage with a 12vDC battery that gives you 1000wh/12v=83.33ah (amp-hour). If this is a lead acid type battery that it is best to only use 1/2 of available amp-hour you would need ~175ah worth.

Picking an inverter that can run your loads requires you to have sufficient capacity to start and run the load. So in the case of the freezer you would want a 1000w inverter.

I hope this helps get you going.

 

[MichaelK]

I see several mistakes, misconceptions. First off, 3 X 400W does NOT equal 600W. I'd suggest though that 600W is definately not enough solar, and even 1200W is a bit low. Secondly, you definately do not want to run this on a 12V system. That is just too limiting. Go with either a 24V or 48V.

I've got a freezer running in my 24V workshop, and I'm finding that I need ~2.5kWh of power day in, day out. That's with the freezer on 24/7, the inverter on 24/7, and whatever lights and power tools I happen to be running each day. I'm running my 24V workshop system with 2000W of solar, and has performed well 12 months of the year.

I'll assume that with power available, Dad will not want to be sitting there in the dark listening to the freezer run? So, include some extra to power lights, TV, ect. With the freezer, and other stuff, I'd suggest you should plan for maybe 3.5-4.0kWh of power consumption per day. That should give Dad a reasonably comfortable 21 century lifestyle.

Since panels are the cheapest component in a solar system these days, I'd suggest maxing out on those. Getting four of those 400W panels would go well, wiring them 2S2P into just any MPPT controller. You might add even more if you orient additional panels East or Westward instead of just South. You'll be putting out more power per day without exceeding the amp limit of your controller. You might also add more panels buying a higher voltage-limit controller, say and Epever Tracer 8420AN?

Assuming you consume maybe 3.5kWh of power per day, and maybe at least 1/3 of that is nighttime after sunset, let's say you need to draw at least 1.0-1.2kWh of power from the batteries. CostCo has a cheap 220Ah golf-cart at 6V right now for 110$. Four of those to make a 24V battery bank. Assuming you never want to drain them less than 50%, at works out to be 220Ah X 24V X 50% = 2.6kWh of power. That should be able to get you through a night, and maybe one extra day if you're stuck in a storm system. With only a 24V golf-cart, you'll likely need to start up a generator to charge the bank by day three to charge the batteries back up. Of course, bumping up to 48V doubles that margin.

 

[Crudely0097]

Absolutely appreciate the thorough responses. I think the one I'm going to be looking at most closely is the choice of what Voltage to run at, I didn't do much homework there.

The max draw is actually measured at the wall, and I was *shocked* to see peak pull of 267VA on compressor startup after a couple hours disconnected. Small freezers seem to pull significantly less amperage than I assumed.

Re: Total Usage - it's his thing. Growing up the sawmill had power, but that's it. At 79 he ain't gonna change, which is why that power set is so low. Phone charger would be the most likely other use. Probably going ground mount to avoid him going up a ladder.

4 panels makes more sense anyways from a wiring flexibility perspective, I put 600w as my pvwatts array size, and just assuming used panels will have at least 10-15% lower production than rated. I'm a bit concerned about temperature sensitivity - seems like batteries & inverters have a pretty narrow operational band?


Edit: I'm very different, we have a 18kW grid tied system on my place and it's still not enough.

 

[Mattb4]

Well if you like to start from a supply perspective (4-300w panels) and work out from that what you can do the concepts are the same. 4-300w panels over a 5 hour average would produce 1200w X 5h=6000wh.

1200w/12vDC=100a max charge rate. 1200w at 24vDC= 50a max charge rate.

Batteries to store 6000wh would be 6000wh/12v=500ah 6000wh/24v=250ah.

Depending on SCC and inverter idle and conversion losses your 6000wh is likely 4000wh usable for loads. Once batteries are charged you can not store anymore and panels do not push power, loads draw it.

 

[Rednecktek]

2. Cheap

- [12/500/20 Victron]

MultiPlus-500-1200VA

Cost is a prime driver

These statements contradict each other. You can have cheap, OR you can have Victron.

I agree that you're going to want to step up the power game a bit and provide for things like lights and a coffee pot, maybe another freezer in the future. A small garden shed and some Styrofoam insulation panels can save you a lot of trouble housing everything. At least overpanel your system a bit to account for winter. Something in the 3Kw 24v range would be a good goal and leave you with room to grow the loads when your dad realizes he needs the new Playstation6 and Starlink so he can p0wn n00bs online or the like.

Usable watt per dollar wise, LFP's are a better long term investment as it'll cost you about the same as AGM's, take up half the space, be 1/4 the weight, and last twice as long. Doing anything to insulate, like a big Coleman cooler or a dead chest freezer, can go a long way towards keeping your batteries happy. RV heating pads inside and a cheap thermostat can be rigged up pretty easily to take care of the particularly cold months.