Cost effectiveness of solar panels

[audreyh1]

The trick for boondocking is to have a generator. And a large freshwater tank plus large waste tanks - unless you have a bathroom nearby.

Our motorhome draws too much power even on the chassis batteries for solar to keep up. We have a 100W solar panel, but I don't think it would even keep the house batteries topped off.

Audrey

 

[NW-Bound]

There are folks who use the RV built-in generator so infrequently that they remove it, and only carry a little 1KW portable Honda generator for cloudy days. They do nearly cover the roof with panels though. It still takes extreme frugality in energy usage.

 

[brewer12345]

There are folks who use the RV built-in generator so infrequently that they remove it, and only carry a little 1KW portable Honda generator for cloudy days. They do nearly cover the roof with panels though. It still takes extreme frugality in energy usage.

Yep. Guys like Tioga George and Andy Baird seem to manage this just fine. The wrinkle is that AC usage is pretty much impossible. I would not go that far. At the moment we have a small (16') travel trailer that we mostly use for weekends to a week (and as a guest bedroom in the driveway, and an adult playhouse), but eventually we will end up graduating to something bigger (likely a class C or larger trailer) and at that point I will want to be able to live without shore power or running the genset all the time. A PV system appears to be just the ticket, hence my curiosity. But I am coming from a position of near total ignorance.

 

[NW-Bound]

No RV owner here yet, but I have looked and spent time inside a few. I don't know about big class As, but in a fairly large classs C (30ft), the generator drone would drive me nuts if I have to run it constantly for the AC. I would rather drive somewhere that is cooler. Genset running for a short time (1 or 2 hrs?) to top off the battery or to microwave would be OK.

 

[brewer12345]

I do not disagree, NW, but if you are parked somewhere that is 100F for a while and do not have the option of turning on the AC, you will be sorry. Learned that my very first time out in a rental in NM in the summer.

 

[NW-Bound]

Eh? Why do you have to convince me, a guy who likes to run around in Bermuda shorts in 50-60F?

Full-timer boondockers plan ahead and head for the high hills, and don't let themselves get caught in the heat. If that failed, I would just head for the nearest 30A plug.

 

[Brat]

Sailboats use a combo of solar and windmills to generate power BUT they don't consume a lot. Pay a visit to your local boat yard or check out a boat show with yachts, not a power boat retailer.

We had solar hot water on our roof in Portland, in a shaded area. That puppy melted dip sticks and blew off steam regularly. This was a while back and I see the current home owner has removed the set-up. Based on my experience, it doesn't take much of a system to get water boiling. Doubtless there are better control systems today.

Daughter did a major remodel of a home in the area that qualified for the solar program in San Jose County. Ever the number pusher she said there wasn't sufficient return on investment because the previous owner had very low usage and the program was based on the home's history of electric consumption. She passed. Now that the house is full of kids her IRR may be different.

 

[jimnjana]

I think the move by Lowes to offer DIY solar panels will help reduce the cost of panes even more. Residential Solar will take off in the South as 1) utilities in the South transition away from Coal. For the most part, Duke and the Southern Company Utilities depend heavily on Coal which is one of the reasons why generally electric rates are lower than FL and other parts of US. 2) more contractors offering solar installation 3) States offering more financial incentives

 

[Nords]

But how would I go about figuring out a system for an RV? Assume I would need a mega battery bank, heavy duty inverter/charge controller and maybe 200w of panels?

I guess the first question you have to answer is "why". We enjoyed designing/installing our project and it's going to save us money in a home where we're living long past the payback. I don't know how well those motives transfer to an RV.

Audrey and REW have said a number of times that they don't do their scale of RV to save money, although I guess some full-time RVers manage to reduce their housing costs. In an RV I'd think that the PV system wouldn't get much use unless you went full time, and gas is cheap by comparison.

Quiet boondocking would be the best use I could think of, but again it might cost $6-$8/watt to install a PV system that could power lights and a fridge. The PV would have a terrible time trying to keep up with A/C. I don't know if the reduction in gasoline/generator use would ever recover your costs, and if RV solar is like home solar then no one will ever pay you for the feature on resale.

Sailboats make some use out of PV, but that's the ultimate boondocking and there's a reason that they install windmills. (And I'd hate to drop a piece of rigging on top of a panel.) However the sailing industry has the deep-cycle marine batteries that you'd want for an RV, and maybe "marine" is just a traditional word for a typical RV battery. Anyway you'd want batteries that you could beat all the way down close to cell reversal between sunset and sunrise. And I think most sailboats run 12v or 24v systems throughout.

Unless you had a class "A" you'd probably struggle to get more than 200-300 watts on top. What's an RV electrical system run on-- 12v, 24v, or AC? There are a number of companies selling "microinverters" that mount under each PV panel and produce 120v 60Hz AC. But an RV system would run more efficiently if all the lights & appliances were 12v DC with maybe another small plug-in inverter for non-RV portable AC appliances. The advantage of the microinverters is that each panel has its own inverter and can do its own thing instead of forcing a bigger inverter to deal with a bunch of panels getting different solar exposure. (Of course they drive the cost up a bit.) Add a charge controller for the battery bank, do a lot of wiring work, waterproof the heck out of each roof penetration, and you'd be off the grid.

I don't know how much objective info you'd get from RV manufacturers or aftermarket companies. "Home Power" magazine sometimes comes across more as an industry cheerleader than a guide to affordable PV, but they seem to have several articles on RV solar. (RV Comfort: Anywhere the Sun Shines) They charge a few bucks to read their archive articles but it's well worth it.

I was in our local solar supply store the other day and saw 1000-1750 watt inverters designed to run off 12v batteries. They've been around for years but now they're getting cheap. However instead of using one with a PV system I'd use them for power outages. And instead of attaching it to PV panels I'd attach it to the battery terminals of our Prius and run the refrigerator off the Prius' gas tank. But power outages around here aren't very frequent and if the hurricane smacked Oahu then we'd just revert to 19th-century life for a few weeks.

 

[audreyh1]

There are folks who use the RV built-in generator so infrequently that they remove it, and only carry a little 1KW portable Honda generator for cloudy days. They do nearly cover the roof with panels though. It still takes extreme frugality in energy usage.

Yeah, if you're in a tiny trailer with few appliances/systems and willing to change the generator oil every 16 engine-hours. And getting the fuel into those things is a pain.

We use the built-in generator quite often. If it's really hot out, we run it while driving to help with the cooling. We run it if we need AC while stopped at a rest area.

Yes, the gen is noisy, but if the AC is running, you barely hear it! LOL!

If we are boondocking and not needing AC, we run it about twice a day for about an hour each time.

Personally, I can't imagine not having the built-in gen.

Audrey

 

[audreyh1]

I was in our local solar supply store the other day and saw 1000-1750 watt inverters designed to run off 12v batteries. They've been around for years but now they're getting cheap. However instead of using one with a PV system I'd use them for power outages. And instead of attaching it to PV panels I'd attach it to the battery terminals of our Prius and run the refrigerator off the Prius' gas tank. But power outages around here aren't very frequent and if the hurricane smacked Oahu then we'd just revert to 19th-century life for a few weeks.

FYI - most motorhomes come with inverters. We have a 2000W inverter that can power everything but the A/Cs. So when stopping at a rest area, etc., we can run microwave and everything and don't run the gen unless it's pretty hot.

We have house battery bank 8 golf-cart batteries that powers the inverter. Yes, RV house batteries are deep cycle (aka "marine") batteries. Still, you don't want them to drop below half-charged or else you end up ruining them.

The power switching in our motorhome is so sophisticated and seamless that you don't realize shore power outage unless the A/Cs cut off. As an EE who has designed power supplies for electronics systems I am still amazed how totally "glitchless" our power system is. The motorhome is like one giant uninterruptible power supply!

The fridge - most RVs have fridges that use the older gas refrigeration technology and run on propane when no shore power. So powering the fridge is not an issue when boondocking.

A lot of the lights are 12V and some of the devices (detectors, water pump, etc), but not any of the appliances or audio-visual components (except for the in-dash audio system).

Audrey

 

[samclem]

Using PV to make electricity to power a motor that turns a mechanical compressor which produces a liquid from a gas that can then be expanded back into a gas to produce cooling--that's a lot of steps and losses at every one. I would think adsorprive cooling (of the type used by propane-powered refrigerators) run directly by the sun would be a lot more efficient. Or, I've seen some diagrams of setups that use heat to drive the water out of a solid dessicant (e.g. silica gel), which then dries the incoming air, water is sprayed through the dry air to cool it, and the cool air goes into the living area (like a "swamp cooler" but it works in any climate). The dessicant is "recharged" (by solar energy) and reused continuously. While all of these schemes would work only when the sun was shining (or when you could burn propane as a temporary replacement), they would seem to be more efficient and probably less costly than a PV system. And a lot of fun for basement experimenters!

 

[FUEGO]

I was in our local solar supply store the other day and saw 1000-1750 watt inverters designed to run off 12v batteries. They've been around for years but now they're getting cheap. However instead of using one with a PV system I'd use them for power outages. And instead of attaching it to PV panels I'd attach it to the battery terminals of our Prius and run the refrigerator off the Prius' gas tank. But power outages around here aren't very frequent and if the hurricane smacked Oahu then we'd just revert to 19th-century life for a few weeks.

So this would work? Get a 1000 W inverter and hook it up to any old car's battery terminals? Then run the car as normal and let the car act as a generator? I had thought about getting a set up like this for the occasional extended power outage. But wasn't sure if the 12v DC from the car's battery terminals (or "cigarette lighter" adapter) could provide enough juice long term to power a 1000 W inverter. Or is that Prius-specific due to batteries?

 

[travelover]

So this would work? Get a 1000 W inverter and hook it up to any old car's battery terminals? Then run the car as normal and let the car act as a generator? I had thought about getting a set up like this for the occasional extended power outage. But wasn't sure if the 12v DC from the car's battery terminals (or "cigarette lighter" adapter) could provide enough juice long term to power a 1000 W inverter. Or is that Prius-specific due to batteries?

You can run a 1000 watt inverter from a regular car's battery, but you need to make sure the alternator puts out enough amperage at idle to keep up. The beauty of using this setup with a hybrid is that you can tap the juice in the drive battery via the DC-DC inverter and the car will actually start and stop its engine as needed to keep the battery charged, thus minimizing fuel usage. A few months ago Costco had 1000 watt Xantrex inverters for about $40.

This is a good link on the subject:

Inverter FAQ - DonRowe.com - Frequently Asked Questions about Power Inverters

 

[samclem]

You can run a 1000 watt inverter from a regular car's battery, but you need to make sure the alternator puts out enough amperage at idle to keep up.

Right. As an example, the typical alternator in a family sedan (e.g. Toyota Camry) puts out 80 amps max. At 12VDC, that would equal 960 watts. And, it is not designed to do that for very long--it will get hot and not last very long. And it probably won't produce that much juice at idle, so you'd have to sit in the car with your foot on the gas. Reving a 100HP+ engine continuously in order to produce less than 1000 watts is not an efficient use of fuel.

I also thing you'd want a direct connection to the battery or main bus, that [-]cigarette lighter[/-] accessory plug in most cars probably isn't built for this much juice.

A smaller inverter might work okay for emergency use in a typical car.

 

[rescueme]

This is certainly an interesting subject, and more so since our local utility will be raising their rates in the future.

However, I've found (upon initial investigation) that the current technology, along with the price will not give us a payback until many years in the future (we're currently in our early 60's).

Since our home will be sold when we pass (proceeds to charity), we don't see any advantage to "converting" at this time.

IMHO, if you are young enough, can afford the conversion, and plan to stay in your home till the anticipated payback (plus) years, then go for it.

When we built our current (terminal/retirement) home in 1994, we added an additional $30k+ in items that then were considered “state of the art” (e.g. insulation, off-peak heating recharge, passive heat retention, etc.) Of course, as time goes on technology changes and our current home would be considered outdated (as compared to today). However, it does provide the environment that we want in retirement and we can’t see pursuing what is currently in style (we know that it will become passé in time)

 

[FUEGO]

Right. As an example, the typical alternator in a family sedan (e.g. Toyota Camry) puts out 80 amps max. At 12VDC, that would equal 960 watts. And, it is not designed to do that for very long--it will get hot and not last very long. And it probably won't produce that much juice at idle, so you'd have to sit in the car with your foot on the gas. Reving a 100HP+ engine continuously in order to produce less than 1000 watts is not an efficient use of fuel.

I also thing you'd want a direct connection to the battery or main bus, that [-]cigarette lighter[/-] accessory plug in most cars probably isn't built for this much juice.

A smaller inverter might work okay for emergency use in a typical car.

That was what I was afraid of. I doubt my honda civic's alternator could power a beefed up inverter for very long without burning out and without revving the engine. I guess I'll stick with the 120 watt inverter plugged into the cigarette lighter plug. At least I can power my cable modem, router, and netbook so I can log on here and post while the power is out.

 

[scrinch]

I've had my home's 3 kW rooftop PV system in use for 4 full years now, and so am getting an idea of the real economics of the project. The system gross cost was $20583 (material, labor, permits, new TOU meter), and the net cost after rebates and tax credits was $10678. Operating/maintenance costs over 4 years have been about $20...to buy a pole-mounted PV panel washer/squeegier. I've found that I lose 10-15% efficiency with the accumulation of 6 months of airborne grime. There has been no significant permanent loss of panel efficiency in 4 years, though. My pre-PV power bill averaged about $1000/yr, and now my average annual bill is almost exactly $0 (I owed $6 this past year). So, assuming no big changes in the price of power, the zero-return payout on the investment will be at about 10 years, and the lifetime IRR will likely be about 6-8% (depending on inflation, system degradation, maintenance, etc).

 

[NW-Bound]

In an RV I'd think that the PV system wouldn't get much use unless you went full time, and gas is cheap by comparison.

Quiet boondocking would be the best use I could think of, but again it might cost $6-$8/watt to install a PV system that could power lights and a fridge. The PV would have a terrible time trying to keep up with A/C. I don't know if the reduction in gasoline/generator use would ever recover your costs, and if RV solar is like home solar then no one will ever pay you for the feature on resale.

Audrey has answered your questions, but I'd like to add the 2 cents I have learned by reading.

Full-time boondockers want to enjoy nature in solitude, and do not want to stay on RV campgrounds. They also save money by not having to pay campground fees, though they have to visit once every few days for tank dumping, fresh water refill, and a luxurious shower. A PV would not be economical for weekenders.

I have not read of any RV'er being able to run a fridge from the PV+battery. It just takes too much power, so has to be run off propane. The electricity is sparingly used for microwaving, lighting, laptop PC, small TVs, etc... and must be carefully rationed.

The ammonia-absorption RV fridge is pain in the rump, though. It is expensive, slow to cool, and does not last long. I have read of class-A owners getting rid of it and plugging in a normal residential fridge. Now, they cannot stray far from a 30-A receptacle, but these owners seem to not care about not being able to boondock.

About the AC, it is simply not possible to have enough solar cells for an RV to have a self-contained AC.

There is also the problem with the surge load at starting. The induction motor inside the AC compressor typically needs 2.5X its normal current to start. Just by reading the wattage consumption of a typical 15,000-BTU/hr RV AC, same as a mid-size window home AC which is run off a 15A home outlet and hence by definition needs less than 15A X 115V = 1725W, one would think he can run it with a 2KW portable Honda generator. No, not so. One would typically needs a 4KW generator for the job, even though the AC motor is an inductive load and actually consumes less than 1725W. Additionally, generator ratings are stated for resistive loads (because the numbers look better), but the only resistive loads we use are incandescent light bulbs.

So, lugging around a 4-5 KW generator is no fun. When I buy an RV, I would not want to do without a built-in genset either. It is just too tough to do without, though I would rather use it just for emergencies. I have done a bit of reading and do not think that full-time boondocking is for me either. Too much of a spartan lifestyle for us. Our 2nd home in the boonies is close enough to nature. When traveling, we would want to stay close to the cities and their restaurants. Cajun food anyone?

As an EE who has designed power supplies for electronics systems...

Alright! Do you still have a Tek scope tucked in the basement of your Alpine? I was wondering if there would be room inside an RV to have an electronic bench with an antistatic top, and shelves for the instruments

 

[audreyh1]

You are right about the extreme surge needs for the A/C starting up - in fact our generator has trouble switching both on simultaneously. We have to leave one off until the other starts running. If only I could program in a 15 sec delay between the two - darn! Just a tiny bit of additional intelligence needed in the controller to stage them....

We have no complaints about the performance of our absorption RV fridge (knock on wood that we don't jinx it, it is expense and we've had one serious repair). I'm really glad we don't have a residential fridge. When we are camping with 30A hookup, we actually switch it to gas because sometimes it makes a difference in being able to run both A/Cs.

No - no Tek scope hooked up to monitor our system, that was definitely a past life. You can just tell by absolutely no flickers and all electronics completely oblivious to any switching.

Our inverter is a Xantrex, and we also have a Surge Guard system monitoring shore power and switching over to the inverter if something goes wrong (here you might see momentary light-dimming if it's a brown-out type situation). Anyway - these modern RV power systems are very impressive.

We've done most of our boondocking either overnighting at Walmart on cross-country travel or because we are waiting somewhere for service and they don't have hookups or we got stuck somewhere over a holiday weekend and we are using the overflow camping area. At many of the most scenic locations if they have a decent road to get there, they also tend to have at least some minimal RV hookups, but even if not, we're set for quite a while.

The thing is, some of the serious boondocking areas can be pretty tough off-road situations or just roads not suitable for a heavier, larger RV or have campsites to small to accommodate a larger RV. We do a great deal of state/national park type camping because we really like to be out in the boonies and many of these places have some level of facility for RVs.

If you have a more expensive RV you absolutely do not take any risks with driving it on off road or on inadequate roads or in very heavily treed areas. This is where the camper vans and Class B type vehicles shine.

The only places I hear about serious boondocking is the BLM out in Quartzite and it always sounded like a madhouse so we never wanted to go there.

Audrey

 

[NW-Bound]

That explains why I have seen some RVs boast of having a genset capable to start both ACs simultaneously. Did not know it was a big deal.

No, Tek scope is for when one has the itch to troubleshoot that !@#$ piece of consumer electronics that fails Perhaps time to sit down and design and add in that delay timer yourself?

 

[REWahoo]

You are right about the extreme surge needs for the A/C starting up - in fact our generator has trouble switching both on simultaneously. We have to leave one off until the other starts running. If only I could program in a 15 sec delay between the two - darn! Just a tiny bit of additional intelligence needed in the controller to stage them....

I'm surprised to see this. Our RV has a built-in controller that staggers the start up of our two A/C units whether we're on shore power or running the generator. I thought that was SOP on all dual A/C RV's, but I suppose not. (This is our first).

 

[audreyh1]

That explains why I have seen some RVs boast of having a genset capable to start both ACs simultaneously. Did not know it was a big deal.

Some RVs come with a 10KW gen - wow! Ours is 7.5KW. But seriously, all you need is a short delay. Nope, I'm not touching the controllers in our RV - recipe for disaster! Who knows - there might be an upgrade to our H&AC controller box. Now we could replace that no problem - it's been a while since we looked at our options.

Some RVs have a "power management system" that can be set up to only run one A/C at a time - this would be a useful feature for camping with a 30A connection.

Of course there are some RVs out there with 3 and 4 A/Cs !!!!

It's too tough to mess with consumer electronics! Most of them are so cheap too. At most we've debugged some of the switches/solenoids in our RV and this was easily accomplished with a voltmeter/ohmmeter/ammeter. We more likely to have trouble with plumbing, wall leaks, mechanical components, hydraulics and our hydro-hot in our RV than with the regular electronics systems.

Audrey

 

[NW-Bound]

Kidding again. Yes, these things are now way too small and too complicated to fix. No schematic, no replacement parts. One would also need a SMD facility. It won't fit in an RV.

 

[Brat]

To hear you guys talk of a Tec oscilloscope brings tears to my eyes. What a great company it used to be.