# Math/setup questions

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Solar Expert Posts: 71 ✭✭✭✭
This is in relation to a thread I had posted some time ago about solar-powering some 12 volt incandescent landscape lights. I was looking through some old posts trying to firm up some ideas when I saw THIS solar lighting controller mentioned in another thread and it looked pretty interesting. Anyhow, I still have some questions...
If I wish to run (20) 7-watt lights and (4) 11-watt lights for a total of 184 watts, what would I need as far as solar panels and batteries (ideally to keep them on until dawn). If I used that controller, then they would all have to be on the same string, right? So would voltage drop come into play? Finally, what math would I need to do if I wanted to add more lights later on as far as solar panel requirements and battery capacity? I feel like there's something else I'm forgetting but those are the key points for now.
Thanks!

• Solar Expert Posts: 71 ✭✭✭✭
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Re: Math/setup questions

...already I see that I am exceeding the wattage for that controller and would need to go with the 20 amp version... :roll:
• Solar Expert Posts: 10,300 ✭✭✭✭
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Re: Math/setup questions

like so many previous threads what you need depends on what you use and for how long. the battery then needs to store what you need and have at least 50% more in capacity as what you need as few will use their pv power only during sunny times. yes, resistive losses can and do come into play as do charging losses and if you use one, inverter losses. are the clfs you plan on using dc or ac? are they to just replace the incandescent bulbs you already have? i don't remember, but did you supply alt energy to the landing lights or not? ac has lower losses to get to the distant areas along the landing/takeoff areas, but loses in its inversion. if sending dc out for dc cfls then the lower voltages will present a higher resistive loss percentage.
• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
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Re: Math/setup questions

Jet,

Knowing your load (184 W DC) is part of the equation. We can assume battery coulombic efficiency (Ah out / Ah in) of 90%. We can guess at worst-case (winter) time estimates for the loads and sunlight, but you’ll need to research the real numbers.

Assuming an overnight “on” time of 16 hours, the lights will consume 184 W x 16 hours = 2,944 Wh. Assuming a 12 V battery, that’ll be 245.33 Ah. Assuming 3 nights of autonomy and limiting discharge to 50%, the 12 V battery will have to be sized at (245.33 Ah/night x 3 nights ) / 50% = 1,472 Ah. That would be six Group size 8D batteries wired in parallel.

Yikes!!

Assuming 90% coulombic efficiency for the battery, the “12 V” PV array will have to generate 245.33 Ah/day / 90% = 272.6 Ah/day. Assuming an average 3 hours/day of “full” Sun in the winter, the array would have to be rated at 272.6 Ah/day / 3 hrs/day = 91 A, which in turn means large diameter wire from the array to the controller. Assuming an STC Vmp of ~17.2 V, the array size would have to be ~1,565 W STC.

This will be a big and expensive installation.

The SunLight is a good PWM controller… I’ve used one for several years now to charge a flooded-cell battery and operate the path lights for my front walk, rear deck, and garden shed. However, I dumped the 4 W and 7 W incandescent bulbs and replaced ‘em with LED clusters I designed and built. Each LED cluster is rated at 12 V x ~20 ma, or ~1/4 W, and I think there are 23 clusters. Power source is a single 40 W PV module, and the battery is a Group 29 size 12 V model rated at 120 Ah.

HTH,
Jim / crewzer
• Solar Expert Posts: 71 ✭✭✭✭
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Re: Math/setup questions

Wow, that IS a lot of batteries...and panels!
Perhaps then I will just add a few lights at a time and then convert them to LED bulbs before adding more. That should keep my battery requirements a bit more realistic. They would be the Malibu landscape lights, but without using a transformer. I believe there are LED bulbs available for them as well. This should also decrease my solar panel wattage requirements, correct?
• Solar Expert Posts: 71 ✭✭✭✭
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Re: Math/setup questions

Well, I found some 1.3W LED 12 VDC replacement bulbs so let's see if I understand the math...

If I use 20 fixtures with the 1.3 W bulbs, that is a total of 26W. So for 16 hours of overnight time, that is 416 Wh. Using a 12 V battery, that is 35 Ah. Limiting discharge to 50% at 3 nights would be a battery bank size of 210 Ah.

For the array, 35Ah/day/90% = 39Ah/day. With 3 hours sun/day that would be 13 A. An STC Vmp of 17.2 V wouold make an array of 224 W STC.

Do I have this correct? If so, I may have to slim things down more with less units or lower wattage bulbs. 20 units was a high guess but I mostly wanted to see If I understood the formulas correctly.
• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
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Re: Math/setup questions

Looks good to me!

Regards,
Jim / crewzer
• Solar Expert Posts: 71 ✭✭✭✭
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Re: Math/setup questions

Thanks! Now to try to trim off some of the fat...