High powerd Solar yard lights - Check my calculations.

shift · February 2012

I'm building a high power solar land scape lighting system. Can you please let me know if i am working this out correctly.

2x 30W solar panels
Pmax 30W
Vpm 18v
Ipm 1.66A
Voc 21.6v
Isc 1.83A

1x 147AH deep cycle ups battery (UPS12-540MR)
1x mppt charge controller (10A cheaper ebay one)
1 x 12v timer

15-20 x 1.2W led Landscape light bulbs

So 1.2W/12V = .1 Amps/bulb
20 bulbs x .1A = 2 amps.

So running the lights would consume 2amps per hour.

If i run them for 6 hours a day i will consume 12 amp hours from my battery.
To replenish the batteries i need to put back in 12amps + 20% = 14.4 amp hours.

If each solar panel is rated for Ipm 1.66A x2 pannels would produce 3.32 amps per hour.
which should mean....

Sun hours | output
1 => 3.32A
2 => 6.64A
3 => 9.96A
4 => 13.28A
5 => 16.60A

So according to this i would just over 4 sun hours/day for the system so sustain itself. (or use bigger panels)

Now are these calculations correct? Is that 4 peak hours so 6 non peak hours would be sufficient?
One one graph it averaged 2-6 "peak sun hours" depending on the time of years.

Any comments/suggestions would be appreciated.. This is my first real project to start getting into solar.

Cheers,

vtmaps · February 2012

Re: High powerd Solar yard lights - Check my calculations.

You seem to be off to a very good start with your calculations. I think you ought to consider different batteries. UPS batteries are meant to spend their lives in float mode and usually are not a wise choice for a renewable energy system with daily discharge cycles. Also, I would be worried about a cheap ebay charge controller... there have been a number of reports on this forum about problems with cheap ebay electronics. Another thing to consider is a low voltage shutoff which will protect your battery from serious damage in the event that it becomes too discharged. Another important feature to look for in a charge controller is a battery temperature sensor. Spending a bit more for quality electronics and suitable batteries will save you money and aggravation. Our forum host is a good place to start looking for these items.
--vtMaps

shift · February 2012

Re: High powerd Solar yard lights - Check my calculations.

The batteries were free, so i couldn't complain.. Ill invest in a different one when it finally bites the dust
Here are the 2 charge controllers i got MPPT and PWM and 12v timer

Now with my calculations are the 60W of panels enough or should i use 100W? (i have 2x30w and 2x50w).

I'm not overly worried about a cutoff (yet.. will in the future) since i am using a timer to control the lights. Are cutoffs built in to the "better" chargers?

niel · February 2012

Re: High powerd Solar yard lights - Check my calculations.

i too think it's all correct except that you won't have enough pv power all of the time. we generally go with a pv efficiency factor of about 77%. the 1.66a stc output current times 2 pvs is 3.32a. at 77% is 2.5564a. now even with 5hrs of full sun every day this gets you 12.782ah. most likely in the winter you will see 2 or 3 hrs of full sun in the northwest, weather permitting, and so this tells me you need 2.5x to 3x the pv current to be semi-reliable. if it's meant to be unmonitored or inaccessible for long time periods then go 3x or more than the presently proposed current from pvs. and yet the only losses we've addressed are the pv efficiency losses for there's charging losses and wire resistance losses so it will go up from here in requirement.

that was 1 aspect of looking at it and here's another take on it. a battery of the proposed 150ah would require a charging rate in the range of 5% to 13%. this means that minimally it will be .05 x 150 = 7.5a. for a pv to deliver this at the 77% efficiency it needs to be at least 9.74a imp which is in the neighborhood of 3x the original proposed pv output of 3.32a. with the other losses and resistances this could also go to the 3x to 4x range. this puts the pvs' total imp in the range of 10a-13a and multiply that by the vmp typically in the 17.5v-18.5v area means this is a high output pv or array of smaller pvs minimally at 175w stc and most likely over 200w stc. there are pvs out there that are good for this range of specs without needing an mppt controller, but the numbers are dwindling as most with high power pvs go with the mppt controllers making nominal voltages for 12v in the 17.5-18.5v range unnecessary. you don't need an mppt controller for your project as a pwm will suffice, but there's nothing wrong if you decide the mppt controller too as it will recover some of your lost power over a pwm type cc. (10% average)

you could probably elect to get a battery down to about the 100ah area, but you must remember the battery should not go below the 50% point making a 100ah battery good for 50ah and a 150ah battery good for 75ah. remember that a long stretch of days will little sun could sap the extra amphours in no time flat so the better the cushion the less likely of going too deeply into the battery's capacity and the longer life it will have. (if batteries are cold this saps the ah rating and could go up to 5x the capacity needed for the same output current ability)

edit to add: i would question the quality and claims for those controllers at those prices.

shift · February 2012

Re: High powerd Solar yard lights - Check my calculations.

Thank you for the detailed reply.. im slowly understanding the indepths considerations!

I will give the controllers a test and let you know how they work. I didn't want to spend a ton for the backyard project.
Taking into consideration all your points I think i may use the 2x50w panels instead., they may be better suited for this project.

the 50w specs
pMax 50Watt
Vpm 17V
Ipm 2.94A
Voc 22V
Isc 3.23A

so 3.23x2 = 6.4a isnt quite the 7.5a but its closer.

I'm assuming its not advisable but could i mix the 30/50 watt panels together to create 160w? alternatively could one use two separate systems (2x30w + 2x60w) each with their own controllers feeding the same batter(s)?

On a side note i'm located in Southern BC/Canada (just above Washington)

westbranch · February 2012

Re: High powerd Solar yard lights - Check my calculations.

use two separate systems (2x30w + 2x60w) each with their own controllers feeding the same batter(s)?

On a side note i'm located in Southern BC/Canada (just above Washington)[/QUOTE]

YES, each controller will measure the battery independently.

niel · February 2012

Re: High powerd Solar yard lights - Check my calculations.

6.4a would be closer, but you would be slowly losing at some times of the year.

yes, you should be able to have 2 of each on their own controller. just make sure the battery is the only thing that is common in both systems. no pv should be hooked up to more than 1 controller. so assuming your ccs are good then i'd put 2-50w pvs to the mppt and the 2 30w pvs to the pwm and then they go from the ccs to the battery.

one of our moderators is from that area too. cariboocoot.

shift · February 2012

Re: High powerd Solar yard lights - Check my calculations.

This seems slightly overkill for the backyard solar mood lighting but it will be a fun project

now the panels do not have a blocking diode. Do i need to add them in or would the charge controllers take care of that?
- if yes, would i put it right off the positive of the pannel or could i wire the panels in parallel and put it just before the charge controller.(or before the battery?)

On a side note, if it did slowly drain it.. is it worth hooking two of those batteries in parallel for extra capacity giving me 294Ah or is one sufficient?
I was going to use them for separate projects.. but it looks like it may all be needed for this one!

P.S. Thanks for all the quick replys

niel · February 2012

Re: High powerd Solar yard lights - Check my calculations.

i think one battery for this unless you keep it in freezing temps.

you can check if it would need one by connecting the cc to the pv and the pv is covered or facing down so as to not produce power like at night. you would then be able to verify if reverse current is there or not, but do be sure there isn't any power production from the pv before doing that by measuring for the pv voltage. if reverse current is present you can readily buy suitable diodes at radio shack, but be sure the diode can take the imp of the pv it is being put to. if you can't find one locally then they are available mail order and there are some with better specs and higher ratings available from our host.
http://www.solar-electric.com/blocdiod8amp.html

shift · February 2012

Re: High powerd Solar yard lights - Check my calculations.

How much do i have to worry about voltage drop?

Each run would be consuming 1-2amps.

12v-2amps is at 14 gauge (which i believe is the 12v Malibu lighting standard..14 or 16awg) about 84' what happens if I run longer than that? also would that be one way.. or there/back. so 42'?

If i put the panels on the roof of the house than down and around that yard it will defiantly exceed that distance.

---
with a side question, is there any issue in using a few cheaper pwm chargers? Is there any advantage on using a single 30A vs 20a +10A or 3x10A? ect..
my assumption is that the cheaper ones 30a ect would have alot more issues with heat dissipation, so 10-20a may be safer..

Thanks again

niel · February 2012

Re: High powerd Solar yard lights - Check my calculations.

ok for a wire run that is indicating the one way distance that 2 wires would travel. that means for an 84ft run this is 168ft total wire. now this is not good for going from pv to cc to batteries, but from batteries to lights it won't kill the production even though there are some losses.

i will give the results for the total wire lengths given at 12v and 2a with #14.

42ft---2.25%
60ft---3.22%
84ft---4.51%
168ft--9.01%

if going to #12 for 12v at 2a it will look like this:

42ft---1.42%
60ft---2.02%
84ft---2.83%
168ft--5.67%

if going to #10 for 12v at 2a it will look like this:

42ft---.891%
60ft---1.27%
84ft---1.78%
168ft--3.57%

shift · February 2012

Re: High powerd Solar yard lights - Check my calculations.

So as long as the panels/charge controller/batteries are all in close proximity the batteries to load is not as big of a deal?

If you were to break up the run abit, would it be more efficient to have the larger run from panel to controller or controller to battery?

Also any input on the cheap ebay charge controllers? multiple smaller ones or one or two "larger ones".

PS. Thanks for prompt responses neil!

niel · February 2012

Re: High powerd Solar yard lights - Check my calculations.

what i'm saying is that the losses presented by the lights will not be as critical as losses presented to charge a battery. if you have say 12.5v from a battery and there is a .5v drop that the lights will all still operate fine. that same .5v drop to a battery could stop the proper charging of that battery and would lead to its quick demise.

i stated pv-cc-battery only to illustrate that this is considered all one run as the v drop %s are additive from the pv to the battery. it is not a case that you can lengthen the battery to cc if you shorten the cc to pv length by the same or visa-versa. in fact lengthening the battery to cc is more critical as the voltages there are a bit smaller in being regulated.

if the length of the run is that 84ft then you must also consider the losses presented by the cc to batteries and add that v drop percentage to the pv to cc run v drop percentage. if the total v drop percentage is wrong for the length of wire you need to have then the only alternative is to use a wire gauge that will present a good v drop percentage.

with ebay it's iffy. you might get something good and then again it might be something bad.

High powerd Solar yard lights - Check my calculations.

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