Going solar? Be realistic about your power needs

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By Greg Illes

So you’ve decided to “go solar.” Congratulations! No doubt you’ve read a lot about panel types and inverter technologies, and maybe received a few quotes. You have probably become familiar with the basic arithmetic of energy management by now, too.

The advice you’ll typically see is to size your panels according to how much power you use each day and, of course, this makes eminent sense. If you’re using 40 amp-hours a day, then you need panel output adequate to recharge those forty amp-hours. If you figure five hours of good sunshine in the middle of the day, then your panels will have to produce eight amps, or about 100 watts.

8 amps x 5 hrs = 40 amp-hours

So that’s it, right? A 100W panel and you’re good to go? Not really.

There are several reasons why the calculation, while entirely valid, is not so simple. Let’s review:

Sunshine Availability — If you’re in the shade or it’s overcast, you’ll get less output from your panels. Even my shade-tolerant amorphous panels drop by 50 percent or more with heavy shade.

Panel Angle — All panels have their power ratings at 90 degrees to the sun. This is never achieved in real practice, even with panels that can be tilted. Even if you get them aimed perfectly at 10 a.m., the sun keeps moving. At a 45-degree angle, you’ll get about 30 percent less power output.

Charge Acceptance — Batteries will not necessarily accept all the available power. As they become more fully charged, acceptance declines. So not all the panels’ power will be absorbed. This physical limitation can only be compensated by more aggressive charging (more power) when the batteries are in a discharged state.

The bottom line is that a typical solar application might need two or three times as much power rating (and sunshine) as is actually used by the rig. This is because all the inefficiencies add up to only getting 1/2 or 1/3 of the rated panel power actually into the batteries.

To be safe, be conservative. Figure an efficiency factor of no more than 50 percent for how much panel power you’ll need. If you’re still uncertain, make sure your system is designed for expansion so that you can add a panel or two if needed. The photo shows my motorhome, using flex panels of 68W each. After going through the learning curve, I ended up going from two, to four, to eventually the six panels shown.

Don’t despair: Despite the uncertainties, you’ll love your solar system and won’t ever want to be without one again.

photo: Greg Illes
Greg Illes is a retired systems engineer who loves thinking up RV upgrades and modifications. When he’s not working on his motorhome, he’s traveling in it. 
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Carson Axtell
1 year ago

I am designing my solar energy system to meet my needs for a minimum of three days of poor sunshine, which basically means putting as many solar panels on the roof as I can fit, and maybe carrying a portable unit, as well. I also intend to carry a small inverter generator of about 800 watts just for recharging the battery bank, and to store all the energy in lithium cells since they seem to be the most efficient and economical long term electric storage setup. (See Will Prowse’s argument for the economics of lithium batteries over lead-acid batteries: https://www.youtube.com/watch?v=Rp8Hspi4BC4)

Gene Bjerke
1 year ago

Rather than getting all bound up in calculations and what-ifs, I just filled the remaining space on my roof (which in a Class B with other stuff up there amounted to two units) and called it done. So far everything has worked out okay. Admittedly, I don’t boondock a lot and I can always turn on the generator if needed.

Wolfe
1 year ago
Reply to  Gene Bjerke

You’ve discovered the difference between a system designer and an engineer. I’ve seen people develop elaborate spreadsheets of their expected power usage, whereas I advise people to just unplug their RV for a day, use it like you would, and MEASURE how much power they need to replace, plus an efficiency margin. General experience seems to say 150W solar will replace efficient boondocking (LED lights, LP fridge electronics, pump, etc), while 600W is more typical usage (modest inverter usage). Running AC from solar is generally never realistic for most people.

Eddie
1 year ago

One thing I have asked about in many places, and the best answer I received was “It depends”, is this. Can you have TOO MANY panels? I mean, if a full battery is a 2 cup measuring cup, you can trickle water into it or hit it with a fire hose and it will never hold more than 2 cups. I will be building a system for my small utility trailer I bought (and am awaiting notice of completion so I can go pick it up), but I am an occasional traveler and will never actually live in the thing like full time RV people do. My power needs will be geared toward 5-7 day trips maybe every 6th week or so, and the consumption will be a CPAP machine, a 12v fridge, a laptop and a 10 inch O2 Cool fan. I will not cook in it because it is so small the smell would linger forever. Plus, part of going to see cities is enjoying their local cuisine.

I had planned on 2 flexible 100w panels (the roof is curved, as are the roof racks where they will be mounted) an MPPT charge controller and a 100ah lithium battery (to keep the weight down). I have a feeling that the math is not as simple as to say “If 1 panel rated at 100w charges my battery in X hours, then 2 panels the same size should charge my battery in half of X.”

I am reading a lot of books and watching a lot of video, but as with all youtube videos that are posted by people who received product for free in exchange for their positive reviews, I take those videos with whole shakers of salt.

Anybody here have any guidance they can offer me?

Drew
1 year ago
Reply to  Eddie

Hello Eddie,

In addition to Greg’s advice I’d recommend reading and following Ray Burr’s site “Love Your RV”. He’s very knowledgeable and has an electrical back round. He is also friends with an expert who builds these systems and can probably give you his info as well. Probably the next most important items in a capable solar system are the batteries. Remember- the batteries actually supply the power you use- not the panels. If I were designing a system, I’d probably get high quality AGM’s. I’d really like to get lithium’s but cost is a factor….so is the charging system for those. -Just my 2 cents.

Drew

19KC69
1 year ago
Reply to  Drew

Just a quick note on wether you should buy AGM or lithium. Yes, lithium is more expensive up front, but look at the benefits. Lithium lasts longer. If maintained properly, you could end up buying 3 AGM batteries by the time your lithium battery wears out. You’ve then spent about the same amount for 3 quality AGMs as you would have on one lithium. Another benefit is you can discharge lithium to 10%, don’t do that with AGM. Also, lithium will accept a huge amount of power input compared to AGM, which means you can use a shorter time period of sunshine. We have had great luck with using the Magnum brand for our solar power needs. They have a “plug-n-play” system that works great. Even on cloudy days with rain we produce power with our system. As far as having too many panels, we are full timers and have four 320w panels which provides us with a sufficient amount of power to cook, run a residential refrigerator, charge our computers and phones, and still have plenty to use if need be. Good luck with your system.

Wolfe
1 year ago
Reply to  19KC69

LiFe batteries are great, and as you say allow using more than 50% of your battery like Pb (FLA/AGM). That said, that double-your-fun costs more than double-as-much, so I still expect to use $80 FLA batteries unless prices tumble soon. Budget rears it’s ugly head!

chris p hemstead
1 year ago
Reply to  19KC69

Lithium don’t slow down acceptance as they reach 99%- no absorb phase.

Wolfe
1 year ago

Technically, Pb vs. LiXX have different charge profiles, but do both “slow” absorption as they near full. Ideally, Pb should go through many different stages of voltages and amperages over 3-7 convoluted “stages.” LiXX only needs two “stages” — Constant Current (at a few C max) up to it’s max voltage and then Constant Voltage maintaining that max voltage while amps taper off — so CV definitely does taper off wattage (power acceptance).

That said, LiXX definitely accepts/delivers power faster, and while a different charge profile, it’s much simpler to avoid damage. LiXX easily accepts multi-C charge rates (sub-hour full charge if charging wattage is available), while Pb shouldn’t charge faster than 1/10 C for longevity or ever 1/4C if aggressively charging. Similarly, a smaller LiXX battery can drive aggressive inverters at insane amperages and be 95% discharged without damaging the battery.

Lithium Envy is an RV-communicable disease… 🙂

Bill T.
1 year ago
Reply to  19KC69

More than likely, your factory equipped charger/converter will need to be replaced as well and this is additional bucks that need to be factored in to lithium upgrade. I agree with Wolfe below, unless the bottom drops out of the lithium market soon, I will continue to budget for FLA batteries, most of which you can buy in any small town if you have to.

Wolfe
1 year ago
Reply to  Eddie

In my estimation, “too many” solar panels is a question of budget, roof real estate (room to mount them), and average vs. peak usage. Some folks figure their worst day uses 20KWh, so they need to cover their whole RV with panels — but on a more typical day, or removing very brief extreme loads, they use 10% of that and waste most of their generated power. So, for my money / space / effort, I’d fire up a generator while running AC for long, and only count on the solar for longer drains. I *can* run AC or nuke from batteries for short runs, but it’s punishing to batteries.

As far as batteries, 2-4X expected daily rated usage (2 days all-battery, only draining to 50%). For me, this means about 400Ah. This also gives enough of a bank that I can absorb a lot of amps during the bulk-charging phase. Again, if I surge my usage I could run down — but I shouldn’t constantly without abusive loads.

If you keep in mind that your worst loads are during daylight hours, hopefully not all your solar power has to bounce through the batteries first. Charging 400Ah requires 600-800W, any extra W for live-feeds.

As far as the actual panels, I’m planning to build out my array sometime next year. It will be portable, foldable tripod(s) because I don’t expect to get too much longer on my well-worn 12yo TT. The other reason is that I fully intend to try having it 2-axis solar-tracking (not as hard as you probably think) so that all panels WILL track 90* to the sun all day.

Don
1 year ago
Reply to  Eddie

Eddie,

Do lots of research on the flexible panels. Most of what I have read on them is disappointing in the long run with longevity and power output.

With two 100W 12v panels an mppt solar controller would be a waste of money. A quality pwm controller from Morningstar would be a much better choice.

Bill T.
1 year ago
Reply to  Eddie

Hi Eddie. Excellent analogy. 2 cups is 2 cups. My wife and I boondock periodically for about 4 days at a time. That’s about as much time as I can take before the wild loses it’s appeal and we need to resupply. IMO, on my rig I have only 1 group 27 160 amp/hour battery. This gives me about 50 useable amp hours before the battery is at 50% charge. With using the LED lights, LP fridge electronics, pump, and CO detector, I use about 30 amps a day max. Remembering that the only things running “all the time” are the fridge electronics and the CO/LP detector, using 700 milliamps/hour.The rest of the stuff is only used periodically. I have 160 watts of portable solar, so we can stay in the shade and still charge the battery and this seems to work fine for us. So as long as I can fill my 2 cups, or in this case 30 amps, I’m good. I believe it’s all about what you realistically need and not what Youtube, solar, or battery salesmen say you need.