Need help on Solar panels to use

fultimer · June 2007

Being a newbie to solar power I need some help in determining the amount of solar panels to get. My required usage is as follows:

1 Small freezer.....~68 watts......24hrs/day
2 TV....................~125 watts....~3-4hrs /day
3 Direct TV Rec.......~22 watts....~3-4hrs/day
4 DVD...................~20 watts.....~1-2hrs when not using TV & DTV Rec

I have 4 Trojan T-125 batteries in series Parallel giving me ~480AH. I also have a Xantrex Pro 1000 inverter (1000Watts).
Am thinking about getting a 30 amp charge controller and a Kyocera Kc-130 panel to mount on the roof of my RV. I also have a 5.5KW Generator. We generally spend the entire Summer in the RV and do a fair amount of boondocking. Will this be enough solar power to keep from having to use the generator so much.

Any and all recommendations will be appreciated.

Dick

mike95490 · June 2007

Re: Need help on Solar panels to use

So, using your numbers, and rounding up, I come up with about 2300 watts/hr daily (2.3KWH)
( I'm sure someone will tell me if I've done this wrong. )

How do you use the DVD, without the TV, or is it a self-contained portable player ?

How much do you want to run the gen ? The least would be 2x a week, to get a quick charge into the batteries in the morning, and let the solar top them off. (adsorb charge by solar)

You can count on 4 hours daily for solar, and that leaves you with about 700 watts of panels, to replace the previous days consumption. (inc 20% charging loss) You will have days of more sun, you may park in a better spot, or even make a angled mount, and get better sun angle, and get by with less panels.
So that's the starting point I found.

fultimer · June 2007

Re: Need help on Solar panels to use

You are right, I forgot about needing the TV with the DVD. However the DVD/Tv combo is usually used very sparingly. Like only when we can't get Direct TV. The generator can be used as needed however I'd like to use it as little as possible. The main problem is keeping the freezer going. I think there might be a misunderstanding about the freezer. It won't be pulling 68 watts the entire time as it will cycle on and off to maintain temperature. I'm not sure I understand why only 4 hours of solar daily. Also all the watts in the original message are AC Watts. Sorry about not being more thorough

niel · June 2007

Re: Need help on Solar panels to use

it's a bit higher if i did this right at 2,510wh. now if it is only to supplement the generator you may wish to use that generator to bulk charge the batteries while it's running the freezer and such allowing the top 20% of the charge to come from the pv. this would also insure the batteries won't get too far depleated, but it does mean buying a charger for the batteries though and you should get at least a 2 stage charger that is regulated with a 3 stage being better yet. using standard automotive chargers would work, but are risky if not watched.

mike95490 · June 2007

Re: Need help on Solar panels to use

fultimer wrote:

I'm not sure I understand why only 4 hours of solar daily.

See the charts on my page here http://www.mike-burgess.org/images/MB_May-3-2007.htm I have 4,500watts of panels, but get really good power output for about 4, maybe 5 hours, and none of it is at 4,500 design laboratory watts.
Mounted on an RV roof, you will not likely get nameplate rated output either.

With revised freezer guesstimate, take 840 watts off - puts you at 1,500 Wh daily /4 solar hours is about 450W of panels needed.
You will still need to run the gen for half hour, 2x a week to top off the batteries

crewzer · June 2007

Re: Need help on Solar panels to use

I'm not sure I understand why only 4 hours of solar daily.

I suspect Mike meant the equivalent of four hours of full Sun, but we can get more specific. You've already indicated you spend the summer boondocking in your RV. Can you tell us a bit more about where you camp?

Or, create your own insolation map from this site. Just select (1) "Average", (2) "July", (3) "Horizontal Flat Plate", and (4) "View the Map".

Also note that the PV array needs direct sunlight to be of much use, and that'll mean either parking your RV in the Sun (and not in the shade), or you'll need a way to park the array in the Sun while the RV is in the shade.

More later,
Jim / crewzer

fultimer · June 2007

Re: Need help on Solar panels to use

We generally camp in the central to northern western states from Wyoming to the pacific coast. According to the insolation map we should get ~6-7 hrs/day in July and August. Does that sound right? I do have installed in the RV an Intellipower 9100 converter that includes a three stage charger that charges the batteries when using shore power or the generator. If i understand what all you are saying it means that using 2 KC-130 panels and a 30 amp controller is not going to be enough to cover my needs.

crewzer · June 2007

Re: Need help on Solar panels to use

FT,

When you factor in losses from hot PV modules, wiring, the controller, the batteries, and the inverter, overall system efficiency in the summer may well be only ~60%. You could expect average usable energy production from two 130 W modules to be 2 x 130 W x 6.5 hours/day x 60% = ~1,014 Wh/day. Sunny days will be better, but cloudy and rainy days will be worse.

HTH,
Jim / crewzer

mike95490 · June 2007

Re: Need help on Solar panels to use

More panels = less generator run time & longer battery life [generally]

fultimer · June 2007

Re: Need help on Solar panels to use

Newbie question; would anyone know ~ how many DC watts the freezer would use per hr or per 24hr day? Its spec says its 110 V AC at 68 watts. Being a freezer and set to a specific temp it will only draw power from the inverter part time. No load inverter draw is <5watts. Mike and Neil above came up with some numbers but are they dc or ac numbers and did they take into account the freezer and all the other devices are rated as ac watts.

I'm not doubting anyone, just trying to get a clear picture in my newbie mind, which sometime takes a lot.

crewzer · June 2007

Re: Need help on Solar panels to use

how many DC watts the freezer would use per hr or per 24hr day? Its spec says its 110 V AC at 68 watts.

Can you give us an estimate on the duty cycle? For example, 15 minutes on and then 15 minutes off?

More later,
Jim / crewzer

mike95490 · June 2007

Re: Need help on Solar panels to use

Watts are watts. A 10V 1A Bulb is the same watts as a 100V .1A Bulb

Conversion losses are another thing though, No converter is 100% efficient, including PWM chargers, flooded cell batteries, DC-DC converters, and DC-AC inverters.
Flooded cell batteries are about 20% loss (electric conversion to chemical)
DC-DC converters are about 20% loss also
Sine inverters range from 25% loss for cheap units, to 6% loss for multi-thousand $ units.

They all list their efficiencies in the fine print somewhere. I try to bury the extra figuring, for rough back of the cuff guesses, by rounding everything up, and only counting 4 hours sun, and let the leftover cover my ass. I'm assuming you will be getting a fairly nice, efficient inverter, maybe in the 1500w class? It has to be able to handle the motor start surge of the fridge, and other appliances you have.
A 24V system will have less losses that a 12V system (ohmic losses in the cables)

crewzer · June 2007

Re: Need help on Solar panels to use

~ how many DC watts the freezer would use per hr or per 24hr day? Its spec says its 110 V AC at 68 watts

FT,

OK, now that I'm awake, I think you're asking how much energy the freezer, rated at 68 W from 110 VAC and running via an inverter, will consume daily from a DC energy source such as your battery bank.

Assumptions:
1) I've interpreted your question correctly
2) A 66.7% fridge duty cycle (40 minutes on and 20 minutes off each hour)
3) A sine wave inverter (is yours a Prosine or a ProWatt?)
4) 5 W inverter standby power
5) 80% inverter efficiency (big inverters are pretty bad with small loads)

The fridge's daily energy requirement would be 68 W x 24 hours/day x 66.7% / 80% = ~1,360 Wh/day
The inverter's standby energy requirement would be 5 W x 24 hours/day x 33.3% = 40 Wh/day

The total requirement from the battery bank would be ~1,400 Wh/day. From a nominal 12 V battery bank, that would be 1,400 Wh/day / 12 V = ~117 Ah/day.

Continuing along this thread, the coulombic efficiency (Ah out / Ah in) of flooded-cell batteries is about 90%, so you'd need to generate 130 Ah/day to replace the 117 Ah/day used. Borrowing the 6.5 hours/day spec we used earlier, you'd need an array rated at 20 A -- ~three KC-130's wired in parallel -- to meet the fridge's energy need described above.

The actual duty cycle and efficiency values might be different. You can substitute other values into the calculations above. A modified square wave inverter (i.e., the ProWatt) might indicate better efficiency, but they can also cause loads to draw more power, so be careful with their specs. Also, a smaller inverter would probably operate more efficiently with such a small load. For example, check the efficiency graph for Morningstar's new SureSine inverter.

HTH,
Jim / crewzer

fultimer · June 2007

Re: Need help on Solar panels to use

The inverter I have is a Xantrex Pro 1000 (1000Watts). The manual says it has 90% efficiency. I have no idea what the duty cycle of the freezer is. I would imagine it depends on the insulation in the freezer walls as to how it dissapates the cold. I do have a fan in the compartment where the freezer is to draw off as much heat as possible.

crewzer · June 2007

Re: Need help on Solar panels to use

The inverter I have is a Xantrex Pro 1000 (1000Watts). The manual says it has 90% efficiency. I have no idea what the duty cycle of the freezer is.

FT,

The closest "Pro 1000" model I can find on Xantex' website is this one. It's a MSW model with peak efficiency of 90% and typical efficiency of 85% - 90%. Considering your fairly light loads, I'd be surprised if it ever operates at better than 85%.

The fridge's summer duty cycle is important in accurately determining its summer energy requirement. A close estimate would be to time its operation over an hour: how many minutes on, and how many minutes off. Alternately, you could buy a Kill-a-Watt meter and accurately record its energy use over a longer period, say, a week.

HTH,
Jim / crewzer

fultimer · June 2007

Re: Need help on Solar panels to use

Crewzer

On the Xantrex home page its listed under Boat Products. Its an XM1000 model. Also I just might get one of those Kill-a-Watt meters.

Thanks again for all the good info to you and all who replied.

crewzer · June 2007

Re: Need help on Solar panels to use

8-)

Need help on Solar panels to use

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