Connecting off grid homestead to the grid--am I crazy?

[halfmoonhomestead]

I'm about to purchase a cabin in the middle of the woods. The house site is completely off grid, with a small solar panel array, some lead-acid batteries, and an ancient pair of 20-amp inverters, providing your typical residential 240/120V single phase power.

Now, we plan on making this cabin our permanent residence, and we know that the 2.2kW solar array just won't be enough for us in the darker months.

There are really two options to increase our power supply: First, we can get a bigger array and more batteries. I'm sure the dated inverter and charge controller will need to be replaced as well, and we would probably want a Generac or something similar to provide power for those long periods when the sun just refuses come out. After all things said and done, even if we were to do most of the labor ourselves, we're probably looking at a $10,000-$20,000 price tag, if we're lucky.

Then there's the second option: we connect to the grid! The road is about 800 feet from the house. Paying the utility company to run service to our house would most definitely be more expensive than vamping up our solar setup. Plus, we don't want to see ugly power lines run through our beautiful land.

But then I did some calculations, and realized that laying some underground cable might be feasible. 120V, 5% drop, 800 feet, aluminum wire at 30 amps requires 4/0. And you can get 500 feet of direct burial 4/0-4/0-4/0 from Home Depot for $3000.

So here's the plan:

1. Build a little shed at the entrance to our driveway
2. Ask the utility company to install service for our "shed". They'll need to pop in a transformer and a meter. Not 100% sure what that would cost, but I'd guess $1-2K?
3. Dig an 800 foot trench with my backhoe from the shed to our house
4. Throw in 800 feet of 4/0-4/0-4/0. $6000 with 200 feet leftover to use elsewhere
5. Connect the wire to a transfer switch at the main panel in the house with a 30-amp breaker. Down the road, it would be nice to upgrade to a grid-tied inverter so we can sell power back, but for now it would just be treated as a backup, completely separate from our current setup. Batteries drained? Flip to the grid.

Some other things to note:

• Where I live, permits and inspections are rarely needed for anything. You can build a whole house by yourself and no one bats an eye.
• I know 30 amps is not much, but it's 50% more than our current inverter output! Besides, our range, dryer, and hot water heater are all propane, and we heat with wood. The biggest load would be when we have a toaster, coffee pot, and microwave on at the same time.

I know it's unorthodox, but I can't see any reason why it would be unsafe, and it would save us a ton of money. I know that digging an 800 foot trench is no small task, but with the right equipment and no hard deadline, I don't see why we couldn't get it done.

Has anyone ever done or heard of something similar to this? Am I insane or does this actually make sense? What am I missing?

 

[SamDeleted]

After all things said and done, even if we were to do most of the labor ourselves, we're probably looking at a $10,000-$20,000 price tag, if we're lucky.

That seems like a lot of money, what do you expect your power use to be per day? Winter/summer

 

[SamDeleted]

But then I did some calculations, and realized that laying some underground cable might be feasible. 120V, 5% drop, 800 feet, aluminum wire at 30 amps requires 4/0. And you can get 500 feet of direct burial 4/0-4/0-4/0 from Home Depot for $3000.

Is it possible to do it at 240v ?

 

[halfmoonhomestead]

That seems like a lot of money, what do you expect your power use to be per day? Winter/summer

Admittedly it's a loose estimate, based off what I've heard others paying in the area, plus the fact that the entirety of the current system is 15-20 years old, and most components would need to be replaced/upgraded.

As for doing only 240V, that would save costs on the wire, but would require a beefy transformer at the house, which could be north of $1500

 

[zanydroid]

Why are you calculating at 120V @ 30A? Vs 120/240 @ 15A

 

[zanydroid]

Also 5% drop is for voltage-sensitive loads. Your resistive kitchen loads won't care about -5% voltage. 5% is pretty extreme though.

120/240V @ 15A is below the minimum allowed service for a dwelling.

 

[pabear89]

ohh where to start, from the being there doing that off grid living to the this is going to be easy to do part.
Difference is the nearest power pole is down by the last mailbox 12 miles away.
Each has its own good and bad parts depends on what you wish to do.
I have seen many that have trouble free set ups but seems that I am not one of them.
Good luck with your plan I hope it all works out for you.

Even with the problems that pop up now and then, Living in the quiet woods has been the best move I have ever made.

 

[Hedges]

Do you have a stream with some flow and drop? Even seasonal?

Photovoltaic has become extremely economical. I think even with batteries you can do it for under $0.10/kWh. What does your utility charge?

A 2kW PV array can be had for $500, maybe $300 these days. In other words, 20kW of PV panels is affordable, biggest cost it mechanical mounts. How would 10x in production help your winter months?

 

[halfmoonhomestead]

Why are you calculating at 120V @ 30A? Vs 120/240 @ 15A

This is definitely something I'm a bit stuck on. My understanding is that if I size the wire to handle 30 amps at 120 volts, then I effectively have 30A*120V*2 = 7200 W of power available before passing that 5% drop. Basically, I could handle two 120V/30A loads at the same time, if they're on different sides of the split voltage. Or, a single 240V/30A load.

The weird part is that having voltage drop as a percentage of supply voltage means that passing more voltage through the wires requires a smaller wire... which is really unintuitive. And you're right... 5% is totally arbitrary and probably overkill for how we're using it. but better to oversize than undersize.

 

[halfmoonhomestead]

Photovoltaic has become extremely economical. I think even with batteries you can do it for under $0.10/kWh. What does your utility charge?

A 2kW PV array can be had for $500, maybe $300 these days. In other words, 20kW of PV panels is affordable, biggest cost it mechanical mounts. How would 10x in production help your winter months?

I honestly haven't done a ton of research, but these estimates seem like an order of magnitude lower than anything I've found. Do you have any suggestions on where to look to find affordable panels and batteries like this?

We could definitely make 20 kW work for us in the winter.

 

[zanydroid]

This is definitely something I'm a bit stuck on. My understanding is that if I size the wire to handle 30 amps at 120 volts, then I effectively have 30A*120V*2 = 7200 W of power available before passing that 5% drop. Basically, I could handle two 120V/30A loads at the same time, if they're on different sides of the split voltage. Or, a single 240V/30A load.

The weird part is that having voltage drop as a percentage of supply voltage means that passing more voltage through the wires requires a smaller wire... which is really unintuitive. And you're right... 5% is totally arbitrary and probably overkill for how we're using it. but better to oversize than undersize.

If you have two 120V 30A loads going at the same time, there will be zero neutral current. So you can calculate things based on 240V voltage drop.

More voltage for smaller metal wire gauge is why we use high voltage for T&D. More voltage -> bigger insulation though.

 

[zanydroid]

I honestly haven't done a ton of research, but these estimates seem like an order of magnitude lower than anything I've found. Do you have any suggestions on where to look to find affordable panels and batteries like this?

Look at the EG4 PowerPro for an example of the most economical battery form factor right now in a packaged product. It might be possible to get cheaper version for something that goes indoors (cheaper enclosure). I don't know of a product off hand for that.

My grid-tie system was like $1.2/W-DC for microinverters and new/modern solar panels (though not the most stylish) for rooftop mount. You can get well below that if you find a deal on say the New Old Stock Hyundai panels Signature Solar has (had?) pallets of. Previous generation panels, still perfectly OK.

I'm not sure what the extra cost for ground mount would be, never looked into it. That might account for why your estimate is in $10-20K range

We could definitely make 20 kW work for us in the winter.

It's not going to be 20kW of output. That is the DC size. The actual output is weather dependent.

 

[halfmoonhomestead]

If you have two 30A loads going at the same time, there will be zero neutral current. So you can calculate things based on 240V voltage drop.

Right, that makes sense. So I guess what I'm designing for here is the unusual case that we have a single 120V/30A load on only half of the circuit, and not wanting to pass under 120V*0.95 = 114V.

 

[Hedges]

It is an order of magnitude lower than an installed system.
I used to say $1/W for grid tie PV hardware, but with PV panel deals I think I can do it for $0.50/W, bringing cost of power down to $0.01/kWh (amortized over 20 years.)

Lithium batteries are around $0.05/kWh (amortized over cycle life.)

We buy PV panels from Santan solar (new and used, good/bad/ugly), also Inxeption (bunch of us just paid $2000 per pallet for 31 pieces, 370W, brand new.)

LiFePO4 Server rack batteries are $1200 to $1700 for 5kWh. EG4 PowerPro is rain proof and $4000 for 14kWh, and a UL listed ESS with that plus an inverter.

If you size battery for 3 days of no sunshine, most of the time should only cycle 15%, and lead-acid forklift battery could last 20 years, or AGM 10 years.

I like SMA inverters. The Sunny Islands can be found for $1200 to $3000, used and new.

What is difficult is making a very small system at low price point, except with the cheap imports.

 

[zanydroid]

Right, that makes sense. So I guess what I'm designing for here is the unusual case that we have a single 120V/30A load on only half of the circuit, and not wanting to pass under 120V*0.95 = 114V.

When would this happen? If it happens a lot you could just balance the circuits.

For a kitchen you would often put the two required 20A breakers on opposite legs. Hitting in kitchen 30A = trip one breaker because you plugged stuff into the wrong side.

 

[halfmoonhomestead]

When would this happen? If it happens a lot you could just balance the circuits.

For a kitchen you would often put the two required 20A breakers on opposite legs. Hitting in kitchen 30A = trip one breaker because you plugged stuff into the wrong side.

Again, you're right that I'm designing for an extreme case that, if designed well, would never matter.

Cutting back the requirement to 5% drop across 240V, then the needed wire size goes down to 3AWG copper or 1AWG aluminum. Basically cutting the cost of the wire in half, it seems

 

[halfmoonhomestead]

It is an order of magnitude lower than an installed system.

Ah yes, that seems to be the status quo with absolutely everything these days.

Thanks for all the info. As I said, I haven't done much research into DIY, and it sounds like I should do more, as it looks like it's more affordable than I thought.

That being said, it looks like if I loosen my voltage drop requirements a bit, running a wire still seems like it would be feasible. For example, this 2 AWG wire is $1200/1000 feet, and should handle 30 Amps with only a 13 volt drop. And with the added bonus of possibly selling power back to the grid at some point, seems like it might be worth the investment. Plus, virtually zero maintenance costs.

 

[schmism]

Talk to your utility. Some are really good about providing free drops of xxxx feet with reasonable rates for various things. (others not so much) Some allow you to buy the conduit and bury it and they pull their wire in it. 800' will be high voltage with a pad mount transformer.

If you can get grid power for less than 10K certainly do that. That doesn't prevent you from spending another 5K-10K on solar and reducing your reliance on the grid.

 

[Ampster]

As for doing only 240V, that would save costs on the wire, but would require a beefy transformer at the house, which could be north of $1500

I am not sure I understand why 120V would be less? Most transformers on the grid near residential provide 24 V split phase. No additional transformers are necessary because L1 to Neutral and L2 to Neutral are bothe 120 volt. I think the higher voltage would reduce voltage drop and possibly reduce the size of wire from the pole transformer to your home.

 

[Ampster]

more voltage through the wires requires a smaller wire... which is really unintuitive

It is ituitive if you think in terms of power. The explanation from a physics standpoint is that wire capacity is based on current not voltage. For the same power, if you increase voltage the current decreases. That is why kilovolt distribution and transmission lines are so thin, because they run and thousands of volts.