sizing our 12 v system for RV

Tradiscantia
Tradiscantia Solar Expert Posts: 35 ✭✭
Because we want to run an AC fridge rated 850 watts (it used 830 in our Kill A Watt meter test over 24 hours on a very hot day), as well as the normal laptop, cell phone charger, and 12 v lights, we were advised to get panels somewhere between 400 and 500 watts. We picked out two 245 watt panels and an MPPT charge controller and 4 6v batteries. We are putting this system on an RV. Now I am concerned about this:

The large panels run about 28 v but only 8.7 amps, but the controller has to lower the voltage down to 12. It seems like we are wasting voltage, and not getting much more amperage out of these panels than we would with two 120 watt panels that run 17v and 8 amps.

Generally amps x volts = watts but if the charge controller is lowering the voltage, don't we get lower wattage than the panels are rated?

The bottom line is, are our larger panels going to be able to charge up the batteries in the morning while we are running the fridge even though they only output 8.7 amps?

I don't want to end up having to buy 12v batteries and a new inverter. (Our inverter BTW is rated 800 continuous watts per hour, 1600 surge, and our fridge is rated 850 watts, 11 amp startup but much lower continuous run, <2.)

I'd really like to understand this. thanks to anyone who can explain it.

Janis

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: sizing our 12 v system for RV

    Welcome to the forum.

    Let's back up and get the terminology and application right.

    Your refrigerator uses 850 Watt hours per day. This is a good refrigerator (depending on size); it's manageable. :D

    This has nothing to do with the size of the panels, really. It does have a bearing on the capacity of the battery bank: batteries run inverter, inverter runs refrigerator (and other AC loads). What the panels do is try to recharge the batteries.

    Being in an RV application you're pretty much limited to a 12 Volt system. Let's say you could get by on 1200 Watt hours per day. That's roughly 100 Amp hours @ 12 VDC (not including losses). As such you would go for a battery bank of at least twice that. Two of the standard "golf cart" batteries would give you 220 Amp hours @ 12 Volts.

    Now you want to recharge them. Normally you'd size the array according to peak charge current (a quick rule-of-thumb that saves a lot of math). So you'd shoot for 10% (22 Amps) @ 12 Volts nominal for 264 Watts, less typical derating: 343 Watt array. If at all possible round up to the nearest available panel combination. If not, round down a little and take your chances. This assumes using an MPPT type charge controller.

    That's where you were getting confused about Volts & Amps: the MPPT function allows the higher Voltage panels to be used on a lower Voltage system; it will convert the 'extra' Voltage into additional charging current. So you start out with, say, 5 Amps @ 35 Volts (175 Watts) but when the converting is done you could have 10 Amps @ 17.5 Volts (175 Watts). What you actually get in charging current is a bit more complicated. More like Watts * efficiency / Volts = current.

    Did that help?
  • Tradiscantia
    Tradiscantia Solar Expert Posts: 35 ✭✭
    Re: sizing our 12 v system for RV
    Welcome to the forum.

    Let's back up and get the terminology and application right.

    Your refrigerator uses 850 Watt hours per day. This is a good refrigerator (depending on size); it's manageable. :D
    It's small, too small for our needs, but it's what we have for now. It's really a poorly insulated thing, compared to our wonderful, too small Engel.

    Being in an RV application you're pretty much limited to a 12 Volt system. Let's say you could get by on 1200 Watt hours per day. That's roughly 100 Amp hours @ 12 VDC (not including losses). As such you would go for a battery bank of at least twice that. Two of the standard "golf cart" batteries would give you 220 Amp hours @ 12 Volts.

    Now you want to recharge them. Normally you'd size the array according to peak charge current (a quick rule-of-thumb that saves a lot of math). So you'd shoot for 10% (22 Amps) @ 12 Volts nominal for 264 Watts, less typical derating: 343 Watt array. If at all possible round up to the nearest available panel combination. If not, round down a little and take your chances. This assumes using an MPPT type charge controller.
    Yes, we did the battery bank of 4, 2 wired in series, 2 in parallel to get our 220/225 aH
    I'm not sure how many watt hours per day we need, as people online suggested I convert to aH to compute our needs, and it was hard to figure out how to compute the watt hours for the 12v lighting. So that's what I did to get 166 aH per day, and on that amount, both the saleswoman and an installer/blogger suggested we need at least 400 watt panels since, with a fridge and the panels mounted flat, we'll need to be able to keep our food from spoiling in less than ideal conditions (e.g. cloudy days, very hot days, lower sun in spring and fall, shade tree here or there)
    That's where you were getting confused about Volts & Amps: the MPPT function allows the higher Voltage panels to be used on a lower Voltage system; it will convert the 'extra' Voltage into additional charging current. So you start out with, say, 5 Amps @ 35 Volts (175 Watts) but when the converting is done you could have 10 Amps @ 17.5 Volts (175 Watts). What you actually get in charging current is a bit more complicated. More like Watts * efficiency / Volts = current.

    Did that help?
    YES! That was my concern. I didn't understand that the charge controller not only reduced the voltage for a 12v system but also increased the amount of current going into the batteries. THANK YOU
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: sizing our 12 v system for RV

    If you've got four "GC2" batteries in series/parallel you'd have 440 Amp hours @ 12 Volts. With that much battery you'd want a much bigger array: around 685 Watts.

    You can use the Icarus Formula to get an approximate number of AC Watt hours from an array:

    Watts * hours of good sun * 0.52 (over-all efficiency)

    So a 400 Watt array with 4 hours (typical minimum) good sun will supply around 830 Watt hours.
    There are things that will improve this, and things that will make it worse.

    One problem RV's often have is a lack of "roof real estate". Quite often it is necessary to buy the more expensive monocrystaline panels which are somewhat more efficient than polycrystaline (more Watts per square inch).

    Another problem RV's have is imperfect insolation (less than ideal sun exposure on the panels). Especially if you like to park in the shade so the A/C works better. Some have found the answer to be moveable panels that can be placed in the sun while the RV is in the shade.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: sizing our 12 v system for RV

    you could opt for a remote pv mounted on the ground so you can move it into the shade, but a separate mppt cc would be wanted to be used here. maybe put all of them remote ground mounted so they are all in the sun and you find the shade for the rv.

    as far as pv prices go i see monos priced less now than some poly was 6 months ago.
  • westend
    westend Solar Expert Posts: 46
    Re: sizing our 12 v system for RV

    I just went through this on a smaller scale but am using an AC fridge with inverter in a TT. Your choices for the system support your use as far as I can see but you may want to do a complete and thorough audit of your anticipated use.
    Here is a spreadsheet that will assist your calculations of use:http://www.quartzfest.org/php/main.php?RV_Solar_Battery_bank_and_Solar_systems_by_N8GS
    BTW, substituting LED's in place of the stock incandescent bulbs will lower your lighting consumption by 80%.
  • ajhs
    ajhs Registered Users Posts: 3
    Re: sizing our 12 v system for RV

    Wow. 80%! I knew they took less power, but that is amazing. Time to find some LED lights!
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: sizing our 12 v system for RV

    For those who like math:

    A 12 Watt LED is the "equivalent"* of a 60 Watt incandescent. Difference of 48 Watts.
    48 * 100 /60 = 80%

    *Equivalent can be argued. Lighting area, direction, and colour will vary and may not be to everyone's tastes. The fact the LED's cost more should also be considered. Up here it's 46 times more, so whereas they may save electric they won't necessarily save you money.
  • Rybren
    Rybren Solar Expert Posts: 351 ✭✭
    Re: sizing our 12 v system for RV

    FleaBay is a great source for LED bulbs. The 4W & 5W MR16 and GU10 tracklight bulbs make great reading lights. They're generally around $3-5/ea
  • westend
    westend Solar Expert Posts: 46
    Re: sizing our 12 v system for RV

    I'm using these 48 SMD 3528 panels for the most part. They came with three adapters to fit common base sizes. Since my fixtures were old and beyond hope, I used these dollar store card frames in a few spots. Total cost was about $3.80 per lamp, IIRC. The LED's were ordered in "warm white" and the color is close to the incandescent's they replaced.

    IMG_0004-40.jpg
  • Shadowcatcher
    Shadowcatcher Solar Expert Posts: 228 ✭✭✭
    Re: sizing our 12 v system for RV

    Our trailer was designed from the beginning for serious boondocking. There are two components to this unless you wish to run a generator. Conservation is the first component and the trailer was designed using marine LED lights through out. I also used Antec three speed computer case fans instead of a Fantastic Fan a substantial savings I also use an Espar diesel heater which is 1A on medium heat.
    I mistakenly bought a used 185W solar panel meant to be used in a grid tie system, read high voltage. This meant the necessity of an MPPT controller, Morningstar was the choice, the unintended consequence is that it works under low light situation and puts out more usable current. Part of this is monitoring the battery goes into and comes out of current. I used a Vectron but other prefer a Trimetric.