Grid Interactive RV?

CraziFuzzy · December 2014

So, I have a few different wishes that are converging.

- I have a 25 ft travel trailer that is used once a month or so about 9-10 months of the year. The rest of the time it is parked along side my house.

- Now that my wife is getting more accustomed to RV live, we have started doing more dry camping. This has led me to increase the battery storage on the RV (currently 2 golf cart batteries)

- To aid in the off-grid stay time, I'd like to add a few solar panels to keep the battery charged, and minimize messing with a portable generator.

- I also would like to install an inverter in the RV to allow some infrequent use of 110 appliances (coffee maker, microwave, etc) off the now solar maintained batteries.

So, this means I'm looking at buying:
Solar Panels
Charge Controller
Inverter
Associated mounts and cables

This looks an awful like a compete off-grid solar system, which makes sense.

So, here's the rub. MOST the year, the RV is sitting on the side of the house, connected to a 120V/30A outlet. It seems to me it might make even more sense, with all this going in at once, to instead make the system grid-interactive, which would provide the ability to sell excess solar back to the grid as the RV is parked (technically, it would just be offsetting my house loads). The first 12V grid interactive inverter I've come across is the Outback GFX1312, which looks like it will do the job quite nicely (which it should for its ample price tag), but I'm wondering what other offerings are out there.

Also, any opinions on this plan?

solar_dave · December 2014

Re: Grid Interactive RV?

Your first problem is that grid interactive usually requires an interconnect agreement with the utility. One of the requirement for most of those is it has to meet NEC and be permitted with the AHJ. The AHJ generally dictates how it must be attached to the main panel (breaker size, AC disconnect switch for the fire department, ...) and I doubt they will buy off on using your RV as a power source.

Would it work? Probably, but if your get caught by the utility (i.e. a smart meter seeing the back feed is one way, the other is usually seeing the panels) they can disconnect you and then you have to re-qualify your whole electrical system

Cariboocoot · December 2014

Re: Grid Interactive RV?

Welcome to the forum.

What solar_dave said is true: it could not be done legally.

You could always tap the output of the RV system to run a few things though. It would in essence become a battery back-up sub panel. This would require both the AC IN and AC OUT having lines on them: grid feeds to AC IN and loads connected to AC OUT. The trick in this case is to arrange it so the AC IN only connects as needed. For that you need a healthy relay controlled by automatic generator start circuitry: the grid power becomes a generator that will connect when the AGS says the batteries are low because the solar can't maintain them.

The down side is that you would be shortening the life of the batteries by cycling them more as opposed to keeping them in Float while not in use. It is debatable which is more economical, but I'd hazard a guess that storing them is as most places do not have electric rates so high as to make off-grid power economic at all.

And of course you would not be selling and surplus to the utility so there would be no financial offset there.

Some may advocate the use of a hybrid GT inverter in this sort of application with the 'SELL' turned off. That would also work, but again may not be of any financial benefit long-term.

CraziFuzzy · December 2014

Re: Grid Interactive RV?

solar_dave wrote: »

Your first problem is that grid interactive usually requires an interconnect agreement with the utility. One of the requirement for most of those is it has to meet NEC and be permitted with the AHJ. The AHJ generally dictates how it must be attached to the main panel (breaker size, AC disconnect switch for the fire department, ...) and I doubt they will buy off on using your RV as a power source.

Would it work? Probably, but if your get caught by the utility (i.e. a smart meter seeing the back feed is one way, the other is usually seeing the panels) they can disconnect you and then you have to re-qualify your whole electrical system

First of all, the panel sizes I'm talking about are far less than my base load of the structure, so reverse power is highly unlikely to ever happen.
Secondly, 'seeing' solar panels on the roof of an RV is hardly a reason to suspect anything odd going on.
Third, the rules on grid-tie panels connected via a power plug (as the RV is) are somewhat unclear. As far as the residential electrical system is concerned, the entire RV is a single appliance.

solar_dave · December 2014

Re: Grid Interactive RV?

CraziFuzzy wrote: »

First of all, the panel sizes I'm talking about are far less than my base load of the structure, so reverse power is highly unlikely to ever happen.
Secondly, 'seeing' solar panels on the roof of an RV is hardly a reason to suspect anything odd going on.
Third, the rules on grid-tie panels connected via a power plug (as the RV is) are somewhat unclear. As far as the residential electrical system is concerned, the entire RV is a single appliance.

Just saying you are on your own if you do it, Plug in appliances are certainly not covered by solar back feed om the NEC. Also if you have a fire, even if the cause is not from your install, the insurance company can then not payoff.

Blackcherry04 · December 2014

Re: Grid Interactive RV?

Depending on your service meter ( Smart Meter ) you may be in fact be charged for the power you inject into the house system, so you may end up with a net loss and be paying for what you generate.

CraziFuzzy · December 2014

Re: Grid Interactive RV?

Blackcherry04 wrote: »

Depending on your service meter ( Smart Meter ) you may be in fact be charged for the power you inject into the house system, so you may end up with a net loss and be paying for what you generate.

Like I said, nothing will be backfeeding through the meter, so this isn't an issue. I have near 1kW of continuous load in my house, even when unoccupied, due to electronics loads (servers). I'm looking at at most 700W of solar here from the RV with perfect sun.

Don't get me wrong, I would certainly go through the permit process to do this, but does anyone genuinely believe an inspector would even know what they were looking at with a setup like this? It just seems a waste to have those panels pointing at the sun all the time and not be able to use them.

Cariboocoot · December 2014

Re: Grid Interactive RV?

CraziFuzzy wrote: »

Also, any opinions on this plan?

This is what you asked for. This is what you've got.

Evidently you've already made up your mind to do this so why ask about it?

You've been told what the situation is. Beyond that it's up to you.

CraziFuzzy · December 2014

Re: Grid Interactive RV?

Cariboocoot wrote: »

This is what you asked for. This is what you've got.

Evidently you've already made up your mind to do this so why ask about it?

You've been told what the situation is. Beyond that it's up to you.

I'm not ungrateful for the responses. I asked the question with the thought that this is something that has been done before. I'm looking for opinions on not just the regulatory aspect of it, but the technical/equipment side as well.

Cariboocoot · December 2014

Re: Grid Interactive RV?

In that case a GFX Outback would be a good choice because not only can the AC IN be used bidirectionally on the shore power input, but also could be used in conjunction with a generator for gen support.

Figure out the peak load needs for the RV.
Figure out how much battery the daily Watt hours will require or how much can be fitted.
Figure out how much PV would be needed to recharge battery or what the max you can fix on the roof is (assuming you don't want mobile panels to supplement).
Balance the batteries/PV/gen use to what you find is most suitable (this will require multiple recalculations).

From the peak loads and the ultimate PV array and battery bank size you can see what capacity inverter best suits your needs. Note that there is a potential problem with running a large inverter from a small battery bank when in grid-tie mode. At 12 Volts you would need a lot of Amp hour capacity to smooth out the AC ripple: about 400 Amp hours per kW of inverter capacity. This probably means you should indeed look at the small inverter (GFX1312) and a double set of golf cart batteries (440 Amp hours @ 12 Volts) if they will fit plus an array of about 685 Watts minimum (probably larger as PV orientation on an RV is unlikely to be "ideal") to handle charging. At least for a starting point.

CraziFuzzy · December 2014

Re: Grid Interactive RV?

Cariboocoot wrote: »

In that case a GFX Outback would be a good choice because not only can the AC IN be used bidirectionally on the shore power input, but also could be used in conjunction with a generator for gen support.

Figure out the peak load needs for the RV.
Figure out how much battery the daily Watt hours will require or how much can be fitted.
Figure out how much PV would be needed to recharge battery or what the max you can fix on the roof is (assuming you don't want mobile panels to supplement).
Balance the batteries/PV/gen use to what you find is most suitable (this will require multiple recalculations).

From the peak loads and the ultimate PV array and battery bank size you can see what capacity inverter best suits your needs. Note that there is a potential problem with running a large inverter from a small battery bank when in grid-tie mode. At 12 Volts you would need a lot of Amp hour capacity to smooth out the AC ripple: about 400 Amp hours per kW of inverter capacity. This probably means you should indeed look at the small inverter (GFX1312) and a double set of golf cart batteries (440 Amp hours @ 12 Volts) if they will fit plus an array of about 685 Watts minimum (probably larger as PV orientation on an RV is unlikely to be "ideal") to handle charging. At least for a starting point.

Regarding the battery size/ripple. I don't see the inverter being pushed very hard all that often. Microwave is probably the biggest thing that would be run on it, and that's probably a very infrequent activity, and honestly, one that can likely handle 'dirty' AC just fine. On a small power grid like an inverter powered RV, a microwave likely dishes out more THD than the inverter could produce. The only other large load is the air conditioner, which is not going to be run on battery alone (though the generator support mode on the outback may definitely help with the starting surge when on generator power, letting me keep the generator in ECON mode, or when on limited (15A) shore power.

It looks like to use the advanced features, like the adjustable AC IN limit, and switch between Grid and Gen modes, I'd need the MATE as well, correct?

Regarding array sizing, part of the reason I even started looking at exporting power off the rig, was that it allowed me to maximize the solar array size, as the extra panels would not end up costing more, after about a payoff cycle. I'm gonna spend about $1,100 minimum just getting an inverter, charger and minimal panels, so if after 5 years, I've still only spent that much, but pushed a bit of power into the house, and will continue to be able to do so for years down the road, all the better. And I've got an even larger array for even less worry in the boondocks for free (bottom line).

My bigger concern with the larger than needed array is overcharging when islanded, and what to use as a dump load on the RV. I've got a 1400W 110V electric element in the water heater that could be used, but that's a little big, and only usable if the water heater wasn't already at temp. I could likely force on the furnace's blower as a DC drain any time. I guess it comes down to how much over-availability I've got.

Cariboocoot · December 2014

Re: Grid Interactive RV?

Couple of things:

Yes, you absolutely need a MATE with any Outback inverter. The default values set are almost never right for the system!

No, you can't "overcharge" batteries because of too much array: the charge controller will prevent that. Although you can have too much current available which will heat the batteries up and shorten their lives. A good MPPT controller can limit that, but you would not be able to make use of full array power when you want to. An opportunity load can make use of power available when the batteries are near/at full, but using one to reduce current during Bulk becomes more complicated. On the whole it is best to size things so that the situation never arises (you could go up to 20% peak current for most flooded cells but 15% is better).

The AC ripple is not an off-grid concern it is a grid-tie concern; the exported power needing to be 'clean' enough to match utility power. Off-grid you can get away with murder, so to speak. I have 232 Amp hours on a 3.5 kW inverter, the large size being to accommodate start-up demands rather than continuous use.

A microwave will not create harmonic distortion on the line. Voltage sag, yes. Especially on a 12 Volt system where the 1.2 kW load will be about 115 Amps on the DC side. Try to supply that with a minimalist battery bank and the Voltage will sag drastically. On 220 Amp hours it represent >50% of capacity all at once, and when current goes up real capacity goes down instantly (in that moment of draw) so it is worse than it appears. Another reason to try and fit a larger battery bank.

I have no idea where you can get inverter, charge controller, and PV for $1,100. The FX is going to be closer to $2,000 with a MATE, good controller will be $300 for a small one (Kid 30 Amp) or double that for a larger one (OB FM60), and as cheap as PV is it's still $1 a Watt so there's up to another $1,000 right there. Or maybe you've sourced some used bargains?

CraziFuzzy · December 2014

Re: Grid Interactive RV?

Cariboocoot wrote: »

Couple of things:

Yes, you absolutely need a MATE with any Outback inverter. The default values set are almost never right for the system!

No, you can't "overcharge" batteries because of too much array: the charge controller will prevent that. Although you can have too much current available which will heat the batteries up and shorten their lives. A good MPPT controller can limit that, but you would not be able to make use of full array power when you want to. An opportunity load can make use of power available when the batteries are near/at full, but using one to reduce current during Bulk becomes more complicated. On the whole it is best to size things so that the situation never arises (you could go up to 20% peak current for most flooded cells but 15% is better).

The AC ripple is not an off-grid concern it is a grid-tie concern; the exported power needing to be 'clean' enough to match utility power. Off-grid you can get away with murder, so to speak. I have 232 Amp hours on a 3.5 kW inverter, the large size being to accommodate start-up demands rather than continuous use.

A microwave will not create harmonic distortion on the line. Voltage sag, yes. Especially on a 12 Volt system where the 1.2 kW load will be about 115 Amps on the DC side. Try to supply that with a minimalist battery bank and the Voltage will sag drastically. On 220 Amp hours it represent >50% of capacity all at once, and when current goes up real capacity goes down instantly (in that moment of draw) so it is worse than it appears. Another reason to try and fit a larger battery bank.

I have no idea where you can get inverter, charge controller, and PV for $1,100. The FX is going to be closer to $2,000 with a MATE, good controller will be $300 for a small one (Kid 30 Amp) or double that for a larger one (OB FM60), and as cheap as PV is it's still $1 a Watt so there's up to another $1,000 right there. Or maybe you've sourced some used bargains?

Sorry about not being clear on the cost comment. The $1100 was using 'conventional' RV style systems. A simple charge controller, and a 'dumb' inverter that's turned on when I need it and powers the load independently, plus panels and mounting. My comment was that if i went with the larger system, with better components, and was able to scavenge power from it when not in use, that I'd get the better components, and after a few years of the reduced power bills, my bottom line might still be near the $1,100 mark, and I'd have much more capability and quality.

I got a little side-tracked with the 'overcharge' thought, after seeing the configuration settings to have AUX on the inverter turn on a parasitic load to battle overcharge, but was forgetting that that is overcharge from either a poor/simply solar charger, or something like a wind turbine. With a proper MPPT controller, this, of course, would not be a concern.

Regarding batteries, I certainly have 'space' to double the bank size - the concern is weight distribution. Ideally, the additional 2 batteries would be installed in the rear of the rig, to avoid throwing an extra couple hundred pounds of tongue weight (assuming I could even fit them on the tongue). I could likely squeeze a custom bank of single cell batteries on there and 'split the difference' between 2 GC2's and 4 GC2's, but the current bank is only a year old, so would hate to remove them now.

jimindenver · December 2014

Re: Grid Interactive RV?

When you start using multiple banks, you have to make sure they are balanced so that they charge and discharge properly together. That means equal distances, wire gauge, etc or separate the banks. That's easier in a big motorhome or fifth wheel with a basement or lots of storage but in a trailer, especially our trailer, it's not easy. Two batteries can go on the tongue, more could go under the couch or bed but would still affect the tongue weight but to a lessor degree due to being closer to the axles, The dinette is right over the axles and would least affect the tongue but would add a lot to just one side of the rig. That may not matter as it's opposite the kitchen. The last place is in the rear storage. here the weight affects the tongue weight the most and possible in the worse way. By lightening the tongue you may change the handling of the trailer and even induce sway.

Now if money were no object, nor was loosing the storage space, I put a pair of AGM 8-D deep cycles under the dinette as the inverters bank and a pair of 6v on the tongue for the trailers bank. I could go a one type of battery and make one big system but the balancing would be hard. 6v under the dinette would also need ventilated boxes, just one more thing to do. The other reason for separate banks is that one can always back up the other AND if we turn something on with the inverter and forget it, we still have a running furnace and it gets cold up there, even in the summer.

CraziFuzzy · December 2014

Re: Grid Interactive RV?

I don't worry too much about needing redundant battery banks in a trailer. I've got a redundant battery, with its own engine, in the truck parked in front of it and a portable generator waiting in one of the compartments if something really goes downhill. Now my boat I love having separate house and engine batteries, with an auto charging relay between them that charges the engine first, and won't close to charge the house battery until the engine battery is almost done with bulk charging. Getting stranded on a beach up river because you've been blasting the stereo all day is no bueno.

Doubling battery size would of course help in any situation, whether I go with an inverter and solar setup or not. Just a matter of convincing the boss that she doesn't need that storage under the dinette...

Something I may do before anything is build a coulomb counter for the battery bank, to have a better image of battery condition. That is something that will be of use no matter what way I go, and its a relatively simple and fun project.

Grid Interactive RV?

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