What size of solar panel would you get?

[Lanarkwanderer]

Originally Posted by Craig Alan

I decided to write a bit of an overview for those who are just starting out with solar. To those brave souls who are just departing on this adventure, I say take your time. There is a lot to learn.

It’s important to understand some basic electronics before undertaking a solar conversion project. The most important term to understand is the amp-hour. An amp-hour, abbreviated AH, is described as the use of 1 amp of current for one hour of time. If you use a 4-amp device (like a CPAP machine) for 6 hours each night, your draw is 24 AH/day. If your device draws 0.5 amps (like a pair of LED lamps would) and you use them for 6 hours each night, you used 3 AH/day. The formula for calculating number of AH’s is Amps x Hours/day = AH/day. Some authors will simply say “AH’s,” meaning AH/day.

If you are using an 85 AH, Group 27 battery (that, I believe, is the battery currently provided by Casita) and you only want to discharge 50 - 80% of its capacity, you can only use 43-68 AH before recharging. (If you wanted to, you could spend a day or two reading about that 50-80% figure, but you have more important things to learn.) You are probably safer discharging only 50% of the battery capacity. I’ll use that figure (43 AH) going forward.

Think of AH’s as the currency of electricity and this all becomes simpler. Your battery stores AH’s. Your solar panel fills the battery with AH’s. When you run your water pump, LED lights, or anything else, these powered devices use AH’s. It’s like a checkbook. Make regular deposits, keep a positive balance, and everyone’s happy.

We can calculate our solar system needs by finding the amps used by every device that runs on the 12V battery and multiply each amperage by the number of hours used by that device to find our AH need. Consider the following estimates of the AH needs of a “frugal solar RV’er” (my words/definition):

Furnace (propane): 3 A * 0.5 hr/day = 1.5 AH/day
Fridge (propane): 0.3 A x 24 hr/day = 7 AH/day
Water Heater (propane): 0.5 A x 1 hr/day = 0.5 AH/day
Water pump: 6 A x 1 hr/day = 6 AH/day
Fantastic Fan (on Low): 1 A x 3 hr/day = 3 AH/day
LED Lights (4): 1 A x 4 hr/day = 4 AH/day

These values may not be exact, but they are what I’ve measured and are close to what I’ve seen reported. If we add up the AH/day and add 10% for inefficiency, we find we need about 25 AH to recharge our battery. This is less than the 43 AH limit we spoke of earlier. This is good news.

You can tell by examining this list that we are sort of "roughing it." That is, 25 AH/day is a fairly small quantity. Note that there is no cell phone/iPad charging, no 12V-to-120V Inverter to run a laptop, hair dryer, or other AC devices, no CPAP, and I’ve left off a bunch of other items that have limited power needs (bath fan, fridge vent fan, etc.). By the way, don't even consider using your air conditioner or microwave. These are energy beasts! Also keep in mind that many propane devices draw current for their control circuitry. In my list above, the fridge is a big-ticket item because it runs 24/7, even though it only draws a small current. The furnace runs on propane, but it has a powerful fan.

To summarize: We’ve calculated that we use about 25 AH per day and our battery will hold about 43 AH. This sounds like we will need to charge our battery about every other day. This is only true if we have a big enough solar panel.

A 100 Watt (W) solar panel is quite common and can be purchased for about $75. This panel will deliver roughly 6 amps/hour in “good sun.” Good sun is often spoken of in terms of “peak sun hours” and there are roughly 5 peak sun hours per day. With good sun, our panels will deliver 6 amps/hour ........

Craig, excellent job ! I think it helps those without an electrical background a start understanding what the advice they are getting is all about. Plus the fact that it's about more than the electrics, weight, roof real estate, personal needs etc all come into it.

I also chose the Bogart PWM controller and battery monitor. A great system for me. The programmability of the charge controller to match what the battery manufacturer specifies was one of the factors in my choice. Plus the information available from the battery monitor, historical, unused power available etc all good if you are inclined in that direction. I can understand why an engineer would buy this product.

I intend to use the Bogart solar charge controller with a 20 Amp DC supply to safely and more quickly charge the battery bank using our Honda generator when our 190 W panel can't keep up.

We are also new to RV's & boon docking but have lived on boats for extended periods. I installed solar on all of those so much of what I learned there I'll apply to our Escape. The boat's were more power hungry because the fridge and freezer were 12 V !

Bob

[gordon2]

Originally Posted by John in Santa Cruz

really? only 1/2 hour per day for the furnace? If its much below about 50F outside, we tend to set our furnace for about 60F when we go to bed, and its running off and on all night long. ...

But Glen only runs his furnace for five minutes a day (first thing in the morning) no matter how cold it is!

Even if you don't believe that, it is true that there is much variation in power use between different people. That is why I like the idea of a power monitor that tracks amps in and out (Bogart TriMetric, Victron Energy, etc). Once you have used one, you will likely never go back to using only a voltmeter. But it is also true that with a lot of experience, you can guess the state of charge pretty close.

Another good point is to keep your battery as full as possible. Its better for the battery, and it gives you more reserve in case of unexpected increase in energy demand and/or decrease in energy supply. So its not a bad idea at all to deploy your solar panels well before you need to, unless its just too much effort, there is a risk of theft, or something other reason not to.

[Perryb67]

Originally Posted by Craig Alan

If you are using an 85 AH, Group 27 battery (that, I believe, is the battery currently provided by Casita) and you only want to discharge 50 - 80% of its capacity, you can only use 43-68 AH before recharging. (If you wanted to, you could spend a day or two reading about that 50-80% figure, but you have more important things to learn.) You are probably safer discharging only 50% of the battery capacity. I’ll use that figure (43 AH) going forward.

.......................

Most solar RV'ers get around this limitation by using two or more batteries and these often provide a total of 225 AH, not the much smaller 85 AH that is provided with a single Group 27 battery. This new battery pack will require that we charge our batteries every 3-4 day. A pair of 6 V/225 AH golf cart batteries sound like a smart purchase.

Unfortunately, those two batteries don't fit in the Casita battery bay and must be stored elsewhere, like on the tongue of the camper. How are we going to do that? Will this extra 125 pounds overload the tongue? Do I have to move the propane tanks? Hmm?

Sorry, but 50-80% is an invitation for disaster. I've never, ever, read anyone with knowledge say or imply anything over 50% discharge. You'll quickly ruin your batteries. We quickly went through two batteries in our Scamp before we purchased a series 31 battery with 110 ah and understood not to use more than 50% of capacity.

By cutting off the big post on a series 31 battery I was able to get a series 31 into our Casita too. Those 55 ah were just enough to get us through a two night weekend in the spring or fall with the furnace running. OTOH in the summer those 55 ah would last all week.

I would never plan a system for 50% discharge. The 50% is considered the maximum you can go before creating problems. We currently have two 6 volt AGM batteries giving us 220 ah of theoretical capacity, or 110 ah to the theoretical 50% capacity. We also have a Victron 712 battery monitor. Our goal is to try to keep our system 75% charged (25% discharge) or better. A number of times last winter, in 25-30 degree nights, we went as far as 37% discharge (82 ah).

_____________________________

If you read enough solar articles, you'll see every competent installer recommends the solar controller as close to the battery as possible to prevent line loss. Twelve volt power needs a minimum of 10 gauge wire to the battery (technically from the controller it's 13.2 to 14.6v). The higher the voltage the smaller the cable needed. Our Renogy 100 watt portable puts out between 18 to 21 volts depending on the wattage harvested, so the 10 gauge wire can handle a decent distance to the controller.

The Renogy was purchased without the integrated controller. We want to harvest as much energy as possible, so to avoid line loss our Renogy does NOT have the controller mounted to the panel 15 to 30 feet from the camper. We purchased a Victron Smartsolar 100/20 controller ($150) that is mounted next to the battery, so only use 18" of 8 gauge wire from the controller to the battery to minimize line loss.

Many times we need to go over 20 feet to the camper and then even 10 gauge will have line loss. We have 45' of cable (three 15' cables) and have needed that 45' to get to get the panel in a sunny spot. Some manufacturers only use 12 gauge wire. Avoid a portable with 12 gauge cables.

If you have a hitch mounted battery, some have purchased or built a waterproof box to mount the controller next to the battery. Others have placed the controller inside the camper with 2' of 8 gauge wire going out to the battery on the hitch.

We use Zamp cables and a Zamp port. Others use PowerPoles or MC4's, but both can easily be damaged if stepped on, and I found MC4's a PITA to disconnect, unless you have and can remember where you put the tool. All three work well and have their advantages and disadvantages.

There's a lot of good information here. Spend a few pennies more and you'll have a system that works and is expandable.

Enjoy,

Perry

[Spongelander]

If space / weight are premiums, this panel may interest you. We ordered ours without a controller, because the one that is bundled with it isn't waterproof. These are not glass panels; rather, they're heavy plastic. But the 100W panel has worked quite well for us, and it's very portable (the panels, controller, and 25' 10AWG cables fit in the box the panel came in). https://ca.renogy.com/renogy-100w-ec...ut-controller/

[garyhall]

Installed my system today.

Acopower 120W portable/foldable this one

Renogy 10A controller this one

I did not go with a kit because the cable configuration was to put the controller at the panel and use alligator clips at the battery.

I mounted the controller inside the Casita. The controller wire to the 27 battery is 10 feet of 12 awg and connected to the battery using ring terminals. I would have liked to use 10 awg but the Renogy connectors would not allow anything any larger than 12awg.

The panel is putting out 25 volts in the sun and 13 volts with cloud cover. I am guessing my AH need will avg around 30 per day.

My only concern is we usually adventure out of the camping area for about 4 hours per day so there will be no moving the panel for optimal sunlight. I guess I will leave it flat on the ground or see if I can rig a quick temporary attachment to the roof of the casita. Any suggestions?

[Raspy]

Gary,

It's probably best to aim the collectors to the south and adjust them up to about 20 degrees, if you are going to be gone in the middle of the day. I wonder if you'll need to tie them down or weight the legs to keep it from blowing over?

[John in Santa Cruz]

never had any issues with my renogy solar suitcase panels even in pretty stiff winds.

at night, I aim it about SE so it gets full light after sunrise, mid morning, I aim it due south, mid afternoon, I aim it SW, late afternoon due west. once plance in particular where I camp every summer for 4-5 nights is notorious for heavy afternoon winds from the W-NW nearly every day.

[Craig Alan]

Raspy: You provided some good info here that warrants further thought. Thanks for responding.

I did not mean to imply that I or anyone should wait until the "magic 50%" battery AH deficit before charging. Charging every day is ideal. However, it might not always happen either due to weather or the combination of a low AH battery/high AH needs. I am limited by a single battery and 85 AH. I fear that until I upgrade my battery, I'm going to be down to 50% by morning on most days.

The PWM vs. MPPT debate rages on. There are many in the community (including Bogart Engineering - manufacturer of the Trimetric system) that use PWM for a variety of reasons. I've read enough to know that an controller is more expensive but it's benefits are hyped by the sellers of this technology. The single instance that I know that MPPT is advantageous over PWM is when using non-voltage matched panels.

I disagree with your stated entry route for newbies into the world of solar boondocking (purchase suitcase system, see if it works, upgrade when needed). This sounds expensive and, frankly, likely to lead to disappointment. But this is just my opinion. I hold three engineering degrees and I'm wired a little differently than many other folks. I spent two months reading all I could before taking the plunge with my Trimetric system. I learned a lot from boards and blogs (especially, the blog of the solar guru Handy Bob). Still, I feel I have a lot to learn.

Thanks again, Raspy.

[Eggcentric]

We have a 100 watt panel flat on the roof, and never give it a thought, as long as it runs the inverter to make her highness's morning coffee we are happy with that. We do have two 6V golf cart batteries tho' to bottle the suns rays.