Solar Sanity check (SSC?)

prepilot_3ck wrote:
Fellow drivers of the Fonzie-Class RV,


This is a google sheet, but you shouldn't need a google account to use it. The fields highlighted in GREEN can be changed to effect the results. Feel free to change those fields and see how it changes stuff.

What I'm looking for:
I need a quick sanity check on the math there, and the results. There are enough "Black boxes" inline there that I'm not sure what effect they have on the math (charge controllers, inverters, etc).

What I'm hoping to determine:
I hope to determine, if in full, 8+ hours, of sun (I camp 99% of the time in the desert southwest of the US) how many panels do I need to run my residential fridge (1-3amps on the AC side) and still have enough juice to charge my batteries up from say...50% so when the sun goes down, I have full batteries to start with.

My conditions:
My RV is new to me, and came with brand new 6V batteries, arranged in a standard series/parallel setup to deliver 440A/Hrs. When these die, I'll swap them with something better, probably lithium but that's a few years out, so for now I'm dealing with what I have.

During the day, I need my panels to charge my batteries and run my fridge. At night, I'll turn off my fridge (by turning off my inverter) and just use the batteries for lights, water pump, phone/laptop charging and other somewhat minimal loads. I don't expect a ton of drain overnight, but I figure I'm probably under estimating there a bit. So, I expect to have mostly dead batteries by morning. I'll also be doing the initial cool down of the fridge on the generator while I'm driving. So when on the battery/solar is should be just maintaining temp, not cooling it down from ambient air temp.

So... what do you all think? Did't the 80's hair band Slaughter write a song about this? "Drain all night, charge all day!" I think those were the words anyway. :)

Thanks!
Brian

WAFlowers wrote:
As azrving noted it might be better to leave the inverter on and run the fridge all night. With the cooler overnight temperatures the fridge shouldn't cycle as often or as long as during the day.

azrving wrote:
Frig may warm up more than you plan on. Each time mine is turned back on it does a defrost.

My resi uses about 75 AH per day. I was running on four GC2 with a gas fridge, LED bulbs, 42 in led television, fans etc and did fine on 520 watts tilted.

When I went to the 10 cu ft resi I added 480 watts of panels and two more GC for six total. It can pretty much carry itself but we usually run the Genny in the am for coffee maker so some of the heavy amps are going back in.

WAFlowers wrote:
Remember that a residential refrigerator figured by EPA to run 1/3 of the time, so figure 4-5A for 20 minutes per hour. Best to err on the high side.

As azrving noted it might be better to leave the inverter on and run the fridge all night. With the cooler overnight temperatures the fridge shouldn't cycle as often or as long as during the day.

Before starting I would install a Victron battery monitor and figure out exactly what you use. Then you can size your system appropriately.

Although you might be in "full sun" for 8 hours many of those hours the sun will be at a lower angle (and going through more atmosphere) reducing your solar efficiency. Even here in Florida the standard calculation is 4.4 hours of full (direct) sun despite 8+ hours of sunlight. Also take into account the angle of the sun to the panels unless you install tilting mounts.

Without knowing all the details which a good battery monitor will give you about your current usage (pun intended) it is difficult to say if 400W on the roof will be enough. It might be. And maybe you need 600-800W. You could look at installing four 180-200W panels and use a (more expensive) MPPT charge controller.

time2roll wrote:
I would be aiming at 800+ watts. Clear space without shadows can get to be an issue when going big.

pianotuna wrote:
I there is enough solar, then running the fridge during the day is better as the it may draw the bank down for 20 minutes, and then the solar can recharge for 40 minutes. If 600 watts is used, then there may be little or no discharging of the bank for the shoulder hours from 2.5 hours before and after 'solar noon'.

WAFlowers wrote:
As azrving noted it might be better to leave the inverter on and run the fridge all night. With the cooler overnight temperatures the fridge shouldn't cycle as often or as long as during the day.