Tie RV solar to home?

Cole

Expedition Leader
I've tried searching but can't seem to find the right combination of terms. So if it's been discussed please send me a link.

....hang with me here for a moment..........:coffeedrink:.....A while back we spent the night in an "Earthship" off the grid home. 90% of the stuff in the house ran off 12v. This was fascinating to me when I realized that 90% of the stuff I need power for is also 12DC.

This has simplified my idea for my "Expo" rig builds since I really have zero use for anything that needs to plug into AC.


I love the idea of adding solar to both my home and to my Sprinter. The first priority is the Sprinter to support our travels. But when we aren't travelling the Sprinter is sitting outside in 300+ days of Colorado sunshine.

Seem like it would be fairly simply to add a "grid tie in inverter" to allow the excess solar from the Sprinter mounted panels to flow into the grid :sombrero:

Maybe even adding a few 12v outlets in the house to charge the 12v appliances in the process.


Eventually expanding the home system as money allows.

Anyone done this? Thoughts? Experiences? Links?
 

workerdrone

Part time fulltimer
Simple yes, legally not so simple. To protect line workers and your property an automatic safety disconnect would need to be installed and inspected.

Easier to keep everything separated, our (RV sized) home solar back up just powers the stereo and the Internet and keeps the batteries charged, basically just enough load to keep the batteries above 80% at all times and reduce our bill a little. So the stereo and Internet is basically plugged into a power strip off an inverter off of the batteries, no grid connection whatsoever, no permits.
 

Cole

Expedition Leader
Have a link to this legal requirement?


It would be no different than plugging in additional solar panels to your system.
 

dwh

Tail-End Charlie
The requirement for a disconnect for a grid-tie is in the National Electrical Code. There are in fact quite few requirements for a solar system that is to be tied to a grid.

The real problem with this idea, is the grid-tie inverter. Good ones, properly rated and approved (and properly connected) cost thousands. There are some El Cheap Chinese "grid-tie" inverters that you can buy and it just plugs into a standard 15a wall receptacle, and you hook a solar panel to that. I very much doubt that those are legally approved for use anywhere in the U.S., though it has been some years since I last looked in to it.

Wiring houses or cabins with 12vdc can be done, and has been done. The biggest issue is the line loss and voltage drop over long wiring runs. For a small cabin, the size of an RV, no big deal, but for a bigger house - say 2 or 3 bedrooms, you are better off just wiring the house normally and using an inverter.

And you'd have to have an inverter anyway to grid-tie.

If you feed from the solar in the RV to a grid-tie inverter, then you would be supplying 120vac to the house - not 12vdc. So exactly how would you go about "adding a few 12v outlets in the house"?
 

dwh

Tail-End Charlie
To get an idea of a practical way to use small solar to "help out" around the house - without having to grid-tie - check out the 2manytoyz web site. He's been doing it for years and has done a pretty good job of documenting what he's done. Scroll down to find the section on solar. It's worth reading the whole thing.:

http://www.2manytoyz.com/
 

workerdrone

Part time fulltimer
If you already have a properly installed grid tied solar system on your home, then yes, it's not a big deal to tie in a few additional panels that happen to be sitting on your RV.

But so many people don't realize the dangers and liabilities of tying into the grid without an auto disconnect - if a section of the grid loses power, the electric company will dispatch repair crews to see what's up.

They'll generally isolate the damaged area and restore as many customers as possible outside of it immediately, then commence repair on the de-energized / damaged part.

If they are not feeding into that section, but your solar panels still are since you don't have the proper disconnect, their transformers still work just fine in 'reverse'. So your 120v grid tie inverter might now be energizing wires at 13,000 volts or something. Not safe.

Even if it is a minuscule amount of current at that 13kv, and nobody gets hurt, they'll quickly become aware that something is energized and not right and will then need to track down the source. This will cause a great deal of confusion, tracing, record checking, etc.

If the source is your illegal solar installation, I'd imagine you'd be in some legal trouble where at the very least you'd be required to pay the crews for all the time they wasted figuring out what's not right. They don't come cheap.
 
Last edited:

dwh

Tail-End Charlie
Well....there are a couple of problems here...


If you already have a properly installed grid tied solar system on your home, then yes, it's not a big deal to tie in a few additional panels that happen to be sitting on your RV.

Nearly all grid-tie inverters are MPPT units, and almost all fixed location grid-tie solar arrays use higher voltage panels or panels rigged in series to create higher-voltage strings. They do it that way because it can save a lot on the cost of copper wire and higher voltage has less line loss.

It would be highly unlikely that some homeowner could easily add a couple of lower-voltage panels to a higher-voltage array.


But so many people don't realize the dangers and liabilities of tying into the grid without an auto disconnect - if a section of the grid loses power, the electric company will dispatch repair crews to see what's up.

They'll generally isolate the damaged area and restore as many customers as possible outside of it immediately, then commence repair on the de-energized / damaged part.

If they are not feeding into that section, but your solar panels still are since you don't have the proper disconnect, their transformers still work just fine in 'reverse'. So your 120v grid tie inverter might now be energizing wires at 13,000 volts or something. Not safe.

Even if it is a minuscule amount of current at that 13kv, and nobody gets hurt, they'll quickly become aware that something is energized and not right and will then need to track down the source. This will cause a great deal of confusion, tracing, record checking, etc.

If the source is your illegal solar installation, I'd imagine you'd be in some legal trouble where at the very least you'd be required to pay the crews for all the time they wasted figuring out what's not right. They don't come cheap.

This is a fairly common theoretical scenario that I've seen on the net for years. But it's completely wrong. In the real world, it doesn't happen.

Grid-tie inverters *require* a frequency from the grid to sync to. If the grid is down, there is nothing for the inverter to sync to, and it won't feed power to the grid.
 
Last edited:

workerdrone

Part time fulltimer
In a proper installation, sure - but I know a few utility people / linemen and they're pretty concerned about home solar screwing with their safety protocols. If it could never happen, why all the concern?

You could also end up with someone like myself, who has a solar / battery setup isolated from the grid, who in the case of an outage ties their system into their home panel to light up their house without turning off their main disconnects.

Same thing with all the people who make illegal 240v extension cords to tie their generator into a dryer outlet or something instead of setting up a proper transfer switch.

Utilities HATE solar and wind installs, and it's not for the reasons you might think - they're not scared at this point of competition, it's just that the inputs are so unpredictable, and their whole business is historically built around very predictable inputs from power plants and the very predictable usage patterns of the customers
 

1stDeuce

Explorer
Digested: Grid tie inverters are expensive. Pushing 12V through wiring without significant voltage drop requires very large guage wire.

Your best bet is to do what dwh and workerdrone are suggesting and just power a few things from your RV to "help out" with the bill. Or invest into a stand alone system for the house and tie the RV panels in when it's parked there, but now you're talking more expense than you likely want to lay out. And again, you're limited by voltage drop.

For a better understanding of voltage drop issues, and how voltage relates to battery charging, read handybob's solar blog. :)
 

dwh

Tail-End Charlie
If it could never happen, why all the concern?

Belt-and-suspenders SOP.

There are possible scenarios which could send power to the grid. For instance a lighting strike burns out something in the inverter and it does back feed power to the grid even though there's nothing for it to sync to.

But a busted inverter is a dangerous problem even if the grid is not down and being worked on.

Another scenario would be physical damage to the wiring system, which shorts power from the house side of the inverter and sends it to the grid side.

But in either of those scenarios, if the linemen detected a backfeed to a section of the grid that they are working on, they probably aren't going to need to track it down to the individual house - they'd just unhook the transformer. A lot of transformers have fuses and/or disconnects (look for the handle at ground level and the long rod running up the side of the pole where the transformer is) which make that easy.

Also, lineman are trained to work on high-voltage without shutting it down. I've done almost a hundred service upgrades, and only once had to get the utility to disconnect the feed at their end - working in a confined underground concrete space where there just was no way to make it safe unless I wore a full rubber suit.

For an overhead drop from the pole, or a normal underground feed, trained electricians just work on it while it's still hot.

I prefer to have the drop hot - that way after I cut it, I can use it to feed a temporary power box so I have power while I work. :)



The *main* reason for a disconnect on a grid-tie is so that firemen can shut down ALL power sources while they do their job. That's also the reason why the code requires clear and obvious labeling, and easy accessibility.

The guys who write the code - The National Fire Prevention Association - try to cover all possible failure modes and achieve a 0% chance of fire, death or danger.



But unless there's damage to the inverter or the wiring - a grid-tie inverter will not supply power to a dead grid.



You could also end up with someone like myself, who has a solar / battery setup isolated from the grid, who in the case of an outage ties their system into their home panel to light up their house without turning off their main disconnects.

Yup, there's another possible scenario - user error.
 

calicamper

Expedition Leader
Home power system unless you have everything unplugged and only plug in and use what you need a few panels on your RV won't hardly light up a light bulb let alone make it worth the effort. Yes you need a proper switch in place which isolates the two power sources from each other which case if your going through the trouble and cost just invest a little more and have some panels put on the house.
 

wirenut

Adventurer
I install grid tied and off grid solar systems for a living.
Your first problem is that there are not battery based grid tie inverters that operate with a 12v battery bank; there are some 24v but most are 48v. So, you couldn't have a home solar system that you could just "tie in" a few extra panels on the DC side. If your Srpinter system was set up for 24v or 48v then it could work. If you have 60 cell modules on the sprinter and use an MPPT charge controller to get it down to 12v then you could disconnect the panels when at home and connect them to Enphase micro-inverters. This would be a rather troublesome process I think.
As for the system in your home, you would need an AC disconnect located by your meter to meet the power companies requirement. (It's their requirement, not the NEC.) You would also have to get the whole system permitted and inspected. This would likely be difficult with the unusual RV tie in. You would then need to file paperwork with the utility to become a net-metered customer. They would install a bi-directional electric meter on your home so that it can keep track of power you buy and power you generate and send to the grid. This paperwork typically requires the signature of a licensed electrician. It also requires the signature of the vendor who sold you the equipment certifying it meet the appropriate UL standards.
This is all to protect the lineman who work on the grid and the power quality of the grid. Someone with non-listed, improperly installed equipment could cause power quality issues. A small system like you describe likely wouldn't but that doesn't matter, same rules for everybody.
A lot of people get all indignant about the power company and all the red tape for "their solar system". They completely forget that it is the power company who owns the grid that they want to hook too with their home brew solar creation.
While you have a decent idea in reality it just doesn't work out.
 

dwh

Tail-End Charlie
NEC 2014:

"705.21 Disconnecting Means, Equipment.

Means shall be provided to disconnect power production equipment, such as utility interactive inverters or transformers associated with a power production source, from all ungrounded conductors of all sources of supply. Equipment intended to be operated and maintained as an integral part of a power production source exceeding 1000 volts shall not be required to have a disconnecting means."


Which particular type of disconnect you use may be up to the utility - but the requirement to have the disconnect is in the code.
 

wirenut

Adventurer
Okay, yes a disconnect is in the code. What I meant was the one located right by the meter that may or may not be redundant to the code required one. You can have all the disconnects you want but if there's not one outside by the meter accessible to the power company then they aren't happy.
 

dwh

Tail-End Charlie
Okay, yes a disconnect is in the code. What I meant was the one located right by the meter that may or may not be redundant to the code required one. You can have all the disconnects you want but if there's not one outside by the meter accessible to the power company then they aren't happy.

Ahh. I see. Yea, true dat - where you put the disconnect (and the meter) is generally up to the utility's spotter.
 

Forum statistics

Threads
185,529
Messages
2,875,555
Members
224,922
Latest member
Randy Towles
Top