Back-of-the-Envelope PV Setup for RV

AuricTech

This is my first post, though I’ve been lurking for a few days now….

In the next couple of months, I plan to purchase a used motorhome as a primary residence. For the most part, I’ll have it parked at an RV park with shore power available. I plan to install a PV setup, for three reasons:

1. To ensure power availability for boondocking;
2. To reduce my reliance on shore power while at the RV park (for monthly [or longer] residents, all the RV parks I’ve found in the area charge extra for shore power);
3. To ensure power availability in the event of power outages (the US power grid isn’t getting any younger!).

Obviously, since I don’t yet own the motorhome, it’s impossible to calculate loads with any accuracy. I do know that, as of right now, the only AC-powered device I’m likely to use on a routine basis is a laptop computer (and yes, I do plan to get a Kill-A-Watt to see how much power it actually draws). Everything else will be DC (I think it’s a safe assumption that the majority of the hotel load in any motorhome will be DC-powered). Installation will initially have the PV panels mounted on a portable frame, with the charge controller permanently mounted near the batteries. That way, I can deploy the panels at an optimum angle, without having to deal with parking angles or climbing onto the roof to adjust the panels. With all that in mind, here are my initial thoughts on a useful PV system for this application:

EQUIPMENT:
* 2 100-watt Renogy monocrystalline bendable panels, rated at 17.8V Vmp/5.62A Imp (I’m leaning toward the bendable panels for their 3-lb. weight, but the 100-watt Renogy polycrystalline panels have identical STC ratings)
* Bogart TriMetric TM-2030 battery monitor
* Bogart SC-2030 30A PWM charge controller (assuming it’s available in the next couple of months) or a Morningstar ProStar 30A PWM charge controller (I’ll likely end up growing the setup once I gain some experience, so I’m going with a 30A charge controller to give room for expansion)
* 1 Universal Group 31 12-volt AGM battery, with a C/20 rating of 110 AH (inexpensive learner battery) (ETA: changed link to NAWS)
* 1 Goal Zero Yeti 400 solar generator (I’ll be running my laptop from the Yeti’s 300W PSW inverter, and then recharging the Yeti during the day from house power)

Keeping in mind that it won’t be strictly necessary for this setup to provide all my power needs right from the start, what suggestions do y’all have to improve this back-of-the-envelope setup? Again, keep in mind that I won't have any hard numbers on expected loads until I actually buy the motorhome. Also, if it makes a difference, I live in SE Arizona, southeast of Tucson.

Cariboocoot

Re: Back-of-the-Envelope PV Setup for RV

Welcome to the forum.

Good thing you haven't spent any money yet.

Your RV may well come with batteries to begin with, and as far as solar goes it would be a good idea to accommodate those batteries. A 110 Amp hour 12 Volt battery is not going to supply you with much power at all. Almost 100% certainty you will need at least twice that.

Now the detached panel idea is good from a gathering sun point of view, but it does have drawbacks of damage and theft that you'll have to look out for. To make it viable, skip the Renogy 100 Watt panels and especially skip the PWM type charge controller. There will likely be some significant distance between panels and controller and to make up for that you'll want to run higher than 17.5 Vmp array. That means you'll need an MPPT type controller to down-convert the Voltage into current. It also means you'll be able to get cheaper-per-Watt "GT style" panels.

In all likelihood you'll be starting out with a pair of GC2 batteries and about 400 Watts of panel to charge them with. Go visit some RV dealers and see what they've got. Pay particular attention to their electrical systems regarding batteries, AC devices, and those wretched built-in 'converters' (big DC power supplies) they're always foisting off on people.

Photowhit

Re: Back-of-the-Envelope PV Setup for RV

AuricTech wrote: »

(I think it’s a safe assumption that the majority of the hotel load in any motorhome will be DC-powered).

Doubtful, most will likely be AC with an inverter somewhere, your refrigerator alone will want more power than a 200 watt array can supply, and in Southern Arizona, I would suspect you'll want some air conditioning?

AuricTech

Re: Back-of-the-Envelope PV Setup for RV

Cariboocoot wrote: »

Welcome to the forum.

Good thing you haven't spent any money yet.

Thanks for the welcome!

As for not having dropped any coin on this yet, it would be foolish to spend money on a PV system for an RV that I don't yet own, especially since I have no way of accurately judging the power load I'll need to support. The initial guesstimate of my requirements is based on HandyBob's article "The RV Battery Charging Puzzle, in which he points out that, for some of his acquaintances, 125-250 watts of PV array is quite sufficient for their boondocking needs. If I end up needing more, with either the charge controllers I have in mind, I have headroom to grow the system by another 200 watts of roof-mounted PV panels

Your RV may well come with batteries to begin with, and as far as solar goes it would be a good idea to accommodate those batteries. A 110 Amp hour 12 Volt battery is not going to supply you with much power at all. Almost 100% certainty you will need at least twice that.

My plan is to buy an old-but-running motorhome from Craigslist or other classified ads, so I'm not sure how reliable the house battery bank is likely to be. As for battery capacity, I suppose that matching what comes with the motorhome is the best place to start, then building the PV array from there, using the 10% rule-of-thumb I've seen on this forum.

Now the detached panel idea is good from a gathering sun point of view, but it does have drawbacks of damage and theft that you'll have to look out for. To make it viable, skip the Renogy 100 Watt panels and especially skip the PWM type charge controller. There will likely be some significant distance between panels and controller and to make up for that you'll want to run higher than 17.5 Vmp array. That means you'll need an MPPT type controller to down-convert the Voltage into current. It also means you'll be able to get cheaper-per-Watt "GT style" panels.

In all likelihood you'll be starting out with a pair of GC2 batteries and about 400 Watts of panel to charge them with. Go visit some RV dealers and see what they've got. Pay particular attention to their electrical systems regarding batteries, AC devices, and those wretched built-in 'converters' (big DC power supplies) they're always foisting off on people.

I see your point, especially about the possible need for extended wire runs for detached panels, which I'll definitely take into account before building anything. OTOH, according to this calculator, if I can keep the wire run down to under 12 feet total (one-way) of 6AWG, and keep the amp load down to 14 amps or lower, I should see a voltage drop of no more than 0.137 volts (1.14%). Would that be sufficient to eliminate the need for a MPPT charge controller with GT-style panels?

Photowhit wrote: »

Doubtful, most will likely be AC with an inverter somewhere, your refrigerator alone will want more power than a 200 watt array can supply, and in Southern Arizona, I would suspect you'll want some air conditioning?

I expect to use propane for the refrigerator when I'm not on shore power, and I can't imagine having an RV-based PV array powerful enough to run an air conditioner. A 70-watt evaporative air cooler for a couple of hours a day might be feasible (at least until monsoon season hits and raises the humidity), and I'd have to add that into my expected load, but I don't expect to move in to a motorhome until October, when it won't be quite as hot. :cool: That would give me time to add capacity to account for a swamp cooler.

Thanks to both of you for your feedback so far!

Assuming that my initial guesstimate is somewhere in the ballpark, am I at least on the right track with the fundamentals?

BB.

Re: Back-of-the-Envelope PV Setup for RV

"GT" panels have Vmp, typically in the range of 30 VDC... Some are more and some are less.

To "efficiently" charge a 12 volt battery bank, you need a rated Vmp around 17.5 to 18.6 volts as "optimum".

If you go with Vmp below 17.5 volts, the batteries will start to have difficulty charging on hot days. As you go over ~18.6 volts, you are "throwing away power" with PWM (pulse width modulation) controllers...

More or less, the basic efficiency for PWM controllers is:

14.5 volts charging / 17.5 volt panel Vmp ~ 83% efficiency (that is typical and you cannot do much better)

For a "GT" type panel with Vmp~30 volts:

14.5 volts / 30 Vmp ~ 48% efficiency

More or less, the typical solar charge controller efficiency (rated panel power to actual current*voltage into battery bank) is around 77% (taking into account panel+controller+wiring+dirt on panel+less than clear day+etc.) numbers.

You need to:

1) do your power estimates (and work on conservation--usually cheaper to conserve than to generate power)
2) design the battery bank to power your loads
3) design the solar array to properly recharge the battery bank
4) design the array based on Watt*Hours per day used and amount of average sun by season/location)
5) do the paper design (or three)
6) pick the hardware based on needs and costs.

-Bill

AuricTech

Re: Back-of-the-Envelope PV Setup for RV

I see by Bill's reply that I wasn't entirely clear. All else being equal, I would prefer not to use GT panels.

As best I can tell (using the voltage drop calculator to which I linked in my previous post), if I can keep voltage drop down, by proper choice of wire gauge/wire run length, to under .2 volts, the Renogy panels (100W @ 17.8V) I'm considering would fall within the voltage range at the charge controller that Bill mentioned as optimum. Am I tracking so far, or am I off-base?*

Also, given my expected off-grid AC power needs (the only routine AC device I expect to use off-grid is my laptop computer), what do y'all think about my current plan** of using a Goal Zero Yeti 400 as my inverter?

Again, all your feedback is appreciated as I bring this project to fruition over the next few months!

*Mix that metaphor!
**Pun intended, of course!

BB.

Re: Back-of-the-Envelope PV Setup for RV

What you are thinking sounds fine...

I don't know anything about the Goal Zero inverter--I would suggest you also look at the MorningStar 300 Watt TSW 12 VDC inverter too... It has a remote "on/off" input and "search mode" (use relatively little power until it senses a >6 Watt AC load, then turns on 100% vs approximately 1 "pulse" cycle every second looking for a load).

-Bill

AuricTech

Re: Back-of-the-Envelope PV Setup for RV

BB. wrote: »

What you are thinking sounds fine...

I don't know anything about the Goal Zero inverter--I would suggest you also look at the MorningStar 300 Watt TSW 12 VDC inverter too... It has a remote "on/off" input and "search mode" (use relatively little power until it senses a >6 Watt AC load, then turns on 100% vs approximately 1 "pulse" cycle every second looking for a load).

-Bill

The Goal Zero Yeti 400 is one of their largest power packs. It has both a 33Ah SLA battery (with multiple charging options) and also a 300W TSW inverter. While it's considerably more expensive than the Morningstar inverter to which you linked, the Yeti's flexibility and ease of installation (there is no installation) are why I'm leaning toward that solution for AC power. That being said, I was already favorably impressed by the Morningstar SureSine inverter. Ah, well, I still have time to decide these things before I start spending. After all, I haven't even completed Step One (buy the motorhome), so Step Two (optimize the motorhome for efficiency) and Step 3 (calculate the load) are speculative at best for now....

BB.

Re: Back-of-the-Envelope PV Setup for RV

I tend to shy away from "all in one" solutions unless they fit the application/load profile well.

In general, the "solar generator" units tend to have a relatively large inverter and small battery.

If you are planning on our typical rule of thumb (25% discharge per day for two days of storage and 50% maximum discharge).

• 33 AH * 12 volts * 0.85 inverter eff * 25% discharge per day = 84 WH of AC power

99-200 Watt*Hours of "useful" stored energy is not a lot... Run a typical laptop for 3-6 hours or so.

2x 6 volt ~220 AH golf cart batteries in series is a pretty useful bank (12 volts at 220 AH). Put in a second parallel string, and you are looking at 12 volts @ 440 AH (if you need more power).

• 12 volts * 220 AH * 0.85 inverter eff * 0.25 discharge per day = 561 Watt*Hours per day (stored energy)

A lot better than a 33 AH @ 12 volt battery pack. You cannot run a 300 Watt inverter on a 33 AH battery very long (if you go much below 50% discharge, batteries tend to age pretty quickly and fail).

And I would not suggest pulling 300 Watts from a 33 AH battery, current wise:

• 300 Watts * 1/10.5 volts minimum * 1/0.85 inverter eff = 33 Amps at 12 VDC under full load

You would pull the battery empty in 1 hour (or less). UPS systems will pull this kind of current level from AGM/GEL type batteries--But the batteries tend not to last very many deep discharge cycles.

On the other hand, not many people will call a case holding 2-4 "Golf Cart" batteries very portable (at ~67 lbs per battery).

-Bill

Cariboocoot

Re: Back-of-the-Envelope PV Setup for RV

I see no point in buying an all-in-one 33 Amp hour battery & 400 Watt inverter when you will have other, larger batteries. It is redundant. That little battery will provide less than 200 Watt hours before it is dead. Fine for a little radio while camping but it isn't going to do much else.

Get the power requirements nailed down first so you don't buy unsuitable equipment.

AuricTech

Re: Back-of-the-Envelope PV Setup for RV

Cariboocoot wrote: »

I see no point in buying an all-in-one 33 Amp hour battery & 400 Watt inverter when you will have other, larger batteries. It is redundant. That little battery will provide less than 200 Watt hours before it is dead. Fine for a little radio while camping but it isn't going to do much else.

Mostly for ease of installation; if the motorhome I end up buying has any cigarette-lighter-style 12V sockets that are powered by the house battery bank, all I would need to do is plug the Yeti in. Meanwhile, I would have a bit more AC power (300W TSW) available than one can reasonably expect from an inverter that can draw all its power from a 12V cigarette-lighter-style socket. Portability for other use would be another plus.

Get the power requirements nailed down first so you don't buy unsuitable equipment.

I'm planning to buy the RV by early June, which will give me a couple of months to nail down the expected loads (and employ conservation measures to reduce them) before I'll need to build the system and move in. That being the case, I'm going to keep an eye on this thread for additional comments, but it's definitely going to the back burner until I buy the RV.

Thanks to all for your input!

BB.

Re: Back-of-the-Envelope PV Setup for RV

The typical lighter plug is fused at ~10 amps... That is around 120 watts of useful power and enough energy to recharge a ~100-200 AH 12 volt battery (but not really, deep cycle batteries need higher charging voltages than car/vehicle batteries--Plus you need many hours to recharge--Driving the RV is typically not going to give you enough charging time).

Planning on your solar panel(s) to give you enough day to day energy, plus a small/fuel efficient backup geneset (Honda eu family or Yamaha inverter-generator in the 900 to 1,600 watts or so) with a good quality AC to DC battery charger is usually going to work better in the long term (and hopefully give you batteries more life vs running under charged most of the time).

-Bill