System Design for Dock Lights

[SunEagle]

Hey SunEagle, I was wondering if you could refresh my memory on why you said the following:
"Next to size your panel you need to use 1/10th the battery system Ah rating which would be 20 amps."

What's the math behind the 10% figure? From what I remember you should hope for 4 hours of sunlight a day and therefore multiply the sizing by 4 to get the panel wattage. I feel like I looked up your reasoning when you suggested it but I can't remember it now. I also couldn't figure out how to Direct Message you, hence the zombie post Here.

To size your pv wattage you first need to be able to have enough amps to charge your battery system. For a Flooded Lead Acid type (FLA) the amount of charging amps should be between C/8 and C/12 where C = your battery Ah rating. C/10 is the sweet spot which calculates to 10% of your battery system Ah. If you use an MPPT type CC then you can multiple the battery voltage x the amount of charging amps needed to get an approximate wattage of your solar array. That is half the formula.

Now to make sure you have enough time in the day to replace what you have used in your battery system will depend on where you live and the lowest value insolation hours of the year. So if you use 1000 watt hours in a day your pv system needs to put that back and about 15% more due to losses.

So for example if you need 1000wh a day and you have say 2 hours of use full sunlight then you can calculate the panel wattage. ie. 1000wh / 2hr * 1.15 = 575 watts.

To see if that panel wattage gets you the amount of charging amps then the approximate wattage, which with an MPPT CC, will yield 575w / 12volt ~ 48 amps of charging. That should be able to support a 480Ah 12volt battery system.

[Datech]

Hey SunEagle, I was wondering if you could refresh my memory on why you said the following:
"Next to size your panel you need to use 1/10th the battery system Ah rating which would be 20 amps."

What's the math behind the 10% figure? From what I remember you should hope for 4 hours of sunlight a day and therefore multiply the sizing by 4 to get the panel wattage. I feel like I looked up your reasoning when you suggested it but I can't remember it now. I also couldn't figure out how to Direct Message you, hence the zombie post Here.

[SunEagle]

Well the project is finished and working well! I ended up making a post on reddit with a link to the Imgur album here: https://www.reddit.com/r/SolarDIY/co...wer_on_a_dock/

Looks like you did well sizing and pricing your system. Nice job. Keep us informed on how it preforms.

[Datech]

Well the project is finished and working well! I ended up making a post on reddit with a link to the Imgur album here: https://www.reddit.com/r/SolarDIY/co...wer_on_a_dock/

[SunEagle]

Oh nonononono, that's not the plan. The dock's existing system was *designed* to be hooked up to the grid. It is not, and will not be hooked up to the grid (otherwise I wouldn't be chatting with you guys). This dock was purchased used and moved to a new location. The Corps of Engineers will not grant us the permit to extend grid power to the dock's new location.

My "splice" will simply be removing the connections that extended out along the gangway, and wiring in the inverter output. It's expecting 120VAC, and that's what I intend to give it. Sorry for the confusion!

Sorry. My misunderstanding.

Now your question is how to wire from your "inverter" to a distribution panel. I will tell you that backfeeding a receptacle is not a good idea so if I was doing it I would install a circuit breaker rated for just under the maximum output of that inverter into the dock's panel box and run the wire between the inverter and circuit breaker in a raceway to protect it.

While this should work it does open you up to allowing anything that is powered from the docks panel to be turned on at the same time and overloading the inverter or quickly draining the battery.

I would feel a safer set up would be to use a rated extension cord plugged into the inverter and then plug the load you want to use into the other end of that extension cord. That could be any of the lighting or that vacuum cleaner. That way you can control your "loads".

[Datech]

Ahhh. Hold the phone. You want to build and off grid system that is "spliced into" a grid fed power panel.

Please do not do this because in most cases it would be illegal and unsafe without using the proper connecting system and expertise to use it. Also anytime you connect an electrical generation system that could "back feed" the grid you have to have approval from your POCO and local permitting people.

Oh nonononono, that's not the plan. The dock's existing system was *designed* to be hooked up to the grid. It is not, and will not be hooked up to the grid (otherwise I wouldn't be chatting with you guys). This dock was purchased used and moved to a new location. The Corps of Engineers will not grant us the permit to extend grid power to the dock's new location.

My "splice" will simply be removing the connections that extended out along the gangway, and wiring in the inverter output. It's expecting 120VAC, and that's what I intend to give it. Sorry for the confusion!

[SunEagle]

I like the idea, but I'm not sure if I could make that work with the electrical system that's already on the dock. The dock expects an input from the land at the end of the gangway, and our plan was to have this system contained in a deck box near one of the bays. We would then splice the inverter output into the breaker just before the first receptacle. I'm having trouble comprehending how I would split that between two inverters.

Also, I had another post yesterday that had a bunch of Amazon links that are probably awaiting moderation. I'll post if here without the links for one final look. Aside from further considerations about two inverters, I think this might be my final system: [TABLE="border: 1, cellpadding: 1, width: 500"]
...

If I have a 30' run from the panels to the CC I would need a 10# wire, correct? I believe that's the gauge of those 10ft Renogy cables and I would string 3 together. Otherwise I'll source that wire (and all of the other wire for CC to battery, battery to battery, and battery to inverter) locally and attach the MC4s as appropriate.

Final thoughts?

Ahhh. Hold the phone. You want to build and off grid system that is "spliced into" a grid fed power panel.

Please do not do this because in most cases it would be illegal and unsafe without using the proper connecting system and expertise to use it. Also anytime you connect an electrical generation system that could "back feed" the grid you have to have approval from your POCO and local permitting people.

[Datech]

Consider 2 inverters, a small, low tare, 300w for the lights and fan, and a larger 1,000w that you only switch on for the vacuum or other large load.
And there is a lot you can do with 12V LED malibu style garden lights, leave the inverter off.

I like the idea, but I'm not sure if I could make that work with the electrical system that's already on the dock. The dock expects an input from the land at the end of the gangway, and our plan was to have this system contained in a deck box near one of the bays. We would then splice the inverter output into the breaker just before the first receptacle. I'm having trouble comprehending how I would split that between two inverters.

Also, I had another post yesterday that had a bunch of Amazon links that are probably awaiting moderation. I'll post if here without the links for one final look. Aside from further considerations about two inverters, I think this might be my final system:

Part	Cost	Quantity	Total Cost
12V 100Ah Deep Cycle Battery	$ 155.87	2	$ 311.74
MPPT TRACER 30A Charge Controller	$ 158.19	1	$ 158.19
Renogy 100W Solar Panels (3pc set)	$ 415.91	1	$ 415.91
Zamp 600W Pure Sine Inverter	$ 202.96	1	$ 202.96
MC4 Assembly Tool	$ 6.99	1	$ 6.99
14W Flood Lights (4 pack)	$ 46.80	1	$ 46.80
DC Voltmeter	$ 9.80	1	$ 9.80
Renogy Mounting Brackets (set of 4)	$ 44.99	1	$ 44.99
Renogy 5 Pair MC4 Connectors	$ 9.99	1	$ 9.99
10ft Renogy Solar Extension Cable	$ 14.99	6	$ 89.94
7ft Dock Box from Overton	$ 634.99	1	$ 634.99
		Total:	$ 1932.30

If I have a 30' run from the panels to the CC I would need a 10# wire, correct? I believe that's the gauge of those 10ft Renogy cables and I would string 3 together. Otherwise I'll source that wire (and all of the other wire for CC to battery, battery to battery, and battery to inverter) locally and attach the MC4s as appropriate.

Final thoughts?

[Mike90250]

Consider 2 inverters, a small, low tare, 300w for the lights and fan, and a larger 1,000w that you only switch on for the vacuum or other large load.
And there is a lot you can do with 12V LED malibu style garden lights, leave the inverter off.

[SunEagle]

Thank you! I can use my limited electrical engineering background and all of the calculations I want but it's hard to replace experience.

I'm good with the batteries in parallel. I was hoping I could add some headroom, but I wasn't sure if adding the battery meant scaling up the rest of the system too so it's good to know that's the recommendation. For the sine inverter, if it's a little high then can I get away with a 500W? It seems like there are a lot more 500W inverters out there with reputable reviews than there are 600W.

Would throwing a simple meter on the batteries be sufficient to monitor charge, or are you suggesting I should ensure I leave the system open to adding another battery in parallel later? Obviously adding another panel would also change my CC, but if that's needed it should be a simple rip/replace of what I've already specc'd, right?

Well I own a pretty good 600w pure sine wave inverter made by Xantrex. It is a little big for my 200Ah battery system but I also have a kill a watt meter which can track how much I use.

Two other little items you need to know about.

The first is that if you wire more than 2 batteries in parallel you can easily end up with unequal charging and discharging. That can kill one or more batteries much faster then it's rated life. A good system consists of all the batteries wired in series. That means you use lower voltage (2v, 4v, 6v) batteries with higher Ah rating. In my case I used 4 x 50Ah 12volt batteries wired in parallel to make a 12v 200Ah system when I should have used 2 x 6v 232Ah batteries wired in series for a 12v 232Ah system. The first way (4 in parallel) has a chance of the batteries failing sooner than later as well as costing much more (~ $400) then the second way (~ $250).

Second issue is that you can't just add new batteries to an existing system. Old batteries will "pull down" the performance of new ones that are not the same age or size. Sort of like one bad apple spoils the bunch idea. So expanding a system usually means replacing all the batteries at the same time while you increase the panel wattage and if necessary the Charge controller.

[Datech]

Your calculations are close but discharging your battery to 50% daily will shorten it's life and leave you in the dark if you get more than 2 days without sun light, so let me help you.

Thank you! I can use my limited electrical engineering background and all of the calculations I want but it's hard to replace experience.

I'm good with the batteries in parallel. I was hoping I could add some headroom, but I wasn't sure if adding the battery meant scaling up the rest of the system too so it's good to know that's the recommendation. For the sine inverter, if it's a little high then can I get away with a 500W? It seems like there are a lot more 500W inverters out there with reputable reviews than there are 600W.

Would throwing a simple meter on the batteries be sufficient to monitor charge, or are you suggesting I should ensure I leave the system open to adding another battery in parallel later? Obviously adding another panel would also change my CC, but if that's needed it should be a simple rip/replace of what I've already specc'd, right?

[SunEagle]

Your calculations are close but discharging your battery to 50% daily will shorten it's life and leave you in the dark if you get more than 2 days without sun light, so let me help you.

Your daily load is 603wh. You should only discharge your battery about 25% a day so you would multiply that 603wh x 4 = 2412wh battery system. That gives you a couple of days to use your battery without needing to recharge it fully.

Divide that value by your battery voltage of 12v. 2412 wh / 12v = 201 Ah. So you will need 2 of those 12v 100Ah batteries wired in parallel.

Next to size your panel you need to use 1/10th the battery system Ah rating which would be 20 amps. Multiply that by the battery voltage of 12v. 20A x 12V = 240watts. So 300watt of panels should be able to get your 200Ah battery charged up if you use an MPPT type CC rated at least 25 amps which I have been told that 30a Tracer is.

And that 600w pure sine inverter is sized a little high for a 200Ah battery so you will need to manage what you plug in and try to make sure you do not exceed that 603 daily watt hour usage.

Finally while you may have properly calculated your daily watt hour usage most people do not include losses between the solar panels & charge controller & battery & inverter. Every item is not 100% efficient so you lose a little output at each stage. More than likely you will end up using more than 603 watthours which means you will discharge your 200Ah battery more than 25%. So be aware of your system limitations and how easy it will be to kill your batteries if you do not keep an eye on them.

[Datech]

Ah, that's the info I wasn't paying attention to, thanks!

Maybe I should approach this from the other end, by starting small and then building in the head room.

Device	Units	Watts	Hours	Total
Flood Lights	4	14	1	56
Accent lights	4	14	5	280
Overhead Lights	2	9	4	72
Vacuum	1	450	1/6	75
Fan	1	30	4	120
Total		517		603

I switched to a small 30W fan, and changed the vacuum to 10 minutes, putting me at 603Whr.

Here's the rest of the calculations

Battery Capacity
Watt Hours (W*hr): 603
Battery Voltage (V): 12
Battery Capacity (Ah): 50.25
Battery Capacity @ 50% DOD: 100.5
Specc'd Capacity (Ah): 100
12V 100Ah Deep Cycle: $149.95

Solar Panel
Power Output of Panels: 1200
Effective Hours/Day: 4
Power Generated Per Hour (W): 300
Number of Panels: 3
Power per Panel (W): 100
3x Renogy 100W Panels: $419.97

Charge Controller
Charge Controller Rating (V): 12
Current Rating (A): 25
MPPT Tracer CC 30A: $158.19

Inverter
Max Load (W): 517
Zamp 600W Pure Sine Inverter: $202.96

If this is the absolute minimum:
1. Do my calculations look so I can scale up from here?
2. Any comments on the materials chosen?
3. I understand the majority of the wiring between the CC, batteries, and inverter, but how do the panels get wired to the CC?

[Datech]

Ah, that's the info I wasn't paying attention to, thanks!

Maybe I should approach this from the other end, by starting small and then building in the head room.

Device	Units	Watts	Hours	Total
Flood Lights	4	14	1	56
Accent lights	4	14	5	280
Overhead Lights	2	9	4	72
Vacuum	1	450	1/6	75
Fan	1	30	4	120
Total		517		603

I switched to a small 30W fan, and changed the vacuum to 10 minutes, putting me at 603Whr.

Here's the rest of the calculations

Battery Capacity
Watt Hours (W*hr): 603
Battery Voltage (V): 12
Battery Capacity (Ah): 50.25
Battery Capacity @ 50% DOD: 100.5
Specc'd Capacity (Ah): 100
12V 100Ah Deep Cycle: $149.95

http://www.amazon.com/100Ah-SOLAR-WIND-CYCLE-BATTERY/dp/B00S1RT58C/ref=pd_sim_sbs_23_4?ie=UTF8&dpID=51DUgUkzajL&dpSrc=sims&preST=_AC_UL160_SR160%2C160_&refRID=1A9BC25VYX5B7EC2BNMB

Solar Panel
Power Output of Panels: 1200
Effective Hours/Day: 4
Power Generated Per Hour (W): 300
Number of Panels: 3
Power per Panel (W): 100
3x Renogy 100W Panels: $419.97

http://www.amazon.com/Renogy-Watts-Volts-Monocrystalline-Solar/dp/B009Z6CW7O/ref=sr_1_1?ie=UTF8&qid=1463511602&sr=8-1&keywords=renogy+100w

Charge Controller
Charge Controller Rating (V): 12
Current Rating (A): 25
MPPT Tracer CC 30A: $158.19

Inverter
Max Load (W): 517
Zamp 600W Pure Sine Inverter: $202.96

http://www.amazon.com/Zamp-solar-ZP600PS-Solar-Inverter/dp/B00O5BZRYE/ref=sr_1_16?s=lawn-garden&ie=UTF8&qid=1463511791&sr=1-16&keywords=solar+inverter

If this is the absolute minimum:
1. Do my calculations look so I can scale up from here?
2. Any comments on the materials chosen?
3. I understand the majority of the wiring between the CC, batteries, and inverter, but how do the panels get wired to the CC?

[SunEagle]

I picked out some bulbs on Amazon to make it simple, so we'll use this going forward:
- 4x 14W (100W eq) LED floodlights, total of 1 hour a day since they'll mainly be security lights: 56W
- 4x 14W (60W eq) accent lights, probably brighter than I need, for 5 hours at night: 280W
- 2x 9W (65W eq) overhead lights, for 4 hours at night: 72W
- 1x 450W vacuum for 15 minutes: 112.5W
- 1x 100W fan for 8 hours (overkill): 800W

That adds up to 1320.5W, which would be the very high end of use. Since I would be very surprised if we ran the fan for 8 straight hours, I feel like that's a pretty safe estimate for 50% DOD for sizing. I'm considering just closing off the receptacles and saying lights-only in order to keep costs down.



I didn't mention it, but I originally had a 45A MPPT charge controller. Did I miss something that would tell me I should get the 60A? Also, since this is a boat dock in Georgia that will almost exclusively be used during the swimming season (or at least 55 degrees and up), could I generate some cost savings by going PWM?

With 640 watt of panels and using 12volt battery your MPPT "could" see as much as 53amps (640w / 12v = 53.3a) so to be safe going with a 60A CC would be better then overloading a 45A CC.

But if you do not get maximum sunlight your panels may only generate less than 50 amps so you could be ok depending on how much that 45A CC can accept for input wattage.

Once you exceed 200 watts a PWM CC basically "steals" wattage from you. Since a PWM is Amps in = Amps out then you will only get the total amps of the 4 panels if you wire them all in parallel. That will probably come to less than 10 amps each or less than 40amps total. Which should ok for your PWM CC but basically turns that 640 watt system into a 450watt system. If that is ok with you then that is your choice.