solar power for security alarm system

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I'm new to the solar aspect of powering devices, so be gentle with me. But one of our customers recently asked us to install an alarm system on a mini-storage complex that has been broken into a number of times and has no available grid power. It would be possible to get grid power to the site, but would be exceptionally expensive. I opened my mouth and said, "Why don't we use solar?". Customer said okay, so now I have to figure out how to make it happen.

Situation: A typical alarm panel receives its power through a 16.5VAC transformer (typically a 40VA transformer), which steps down the power from 120VAC to 16.5VAC. The alarm panel then converts the incoming 16.5 VAC to 12VDC for charging the internal backup battery (12V/7Ah SLA) and supplying 12VDC power to hardwired detection devices. I know that I can use an inverter to boost the 12VDC from solar backup batteries up to the 120VAC required by the 16.5VAC transformer and I'm guessing that such a small inversion draw would create a minimal drain on the backup batteries (compared to, say, a blow dryer) even though it's pretty inefficient to convert from 12VDC to 120VAC to 16.5VAC. And at this point you're probably asking "why not just power the whole thing off 12VDC and forget the 120VAC inversion and the 16.5VAC step-down?" Great question. The answer is that I could, but the alarm system loses a lot of functionality when it loses the 16.5VAC input voltage and essentially the sytem, now operating only on 12VDC input, shuts down every non-essential function and just keeps the system functional until 16.5VAC is restored. That's why it looks to me like I'll need to use a 120VAC inverter.

Here's my problem...being a novice and this being an important customer, I don't have the luxury of taking a lot of time and experimenting on his dime. When I quote him a price, I need to be pretty sure I can pull it off for that price. The unknown factor is I'm not sure how to figure the size of panels and storage batteries I'll need for this application. If there is someone is this forum that can help a solar noob get going with this project, I would be forever indebted. Thanks.

[Sunking]

It can be done. But I have to ask why would you want to convert 12 volt DC to 120 VAC when the system only needs 12 volts? It is not necessary.

OK th efirst thing you need to do is determine how many watt hours the system uses in 24 hours. If the transformer is 45 Va then the system obvisiouly uses less than 45 watts. Let's say for example it actually uses 30 watts. So the system would use 30 watts x 24 hours = 720 watt hours.

Next peace of information is the location so we can determine the winter Sun Hours to determine the panel wattage required. So for example let's say 3 Sun Hours in December and January.

So if it uses 720 watt hours in a day with 3 Sun Hours in winter you would need:

400 watt Solar Panel
40 Amp MPPT Charge Controller
12 volt 300 AH Battery.

So your job now is to give use the daily watt hours and location. Or just do it all yourself and read this post and it will tell you most everything you need to know.

[inetdog]

It can be done. But I have to ask why would you want to convert 12 volt DC to 120 VAC when the system only needs 12 volts? It is not necessary.

True only if you manage to defeat the AC-input failure sensor of the system so that it will still operate at full functional level instead of just low-function backup mode.

And at this point you're probably asking "why not just power the whole thing off 12VDC and forget the 120VAC inversion and the 16.5VAC step-down?" Great question. The answer is that I could, but the alarm system loses a lot of functionality when it loses the 16.5VAC input voltage and essentially the sytem, now operating only on 12VDC input, shuts down every non-essential function and just keeps the system functional until 16.5VAC is restored.

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Thanks to both of you, sunking and inetdog. I read the posting you supplied the link for, sunking, and did the calculations myself and was pleased when my calcs matched yours. And inetdog, you understood what I was saying when I explained the need for inverting to 120VAC before stepping back down to 16.5VAC. So a big thanks to you both...

My location is Stuttgart, Arkansas, which is virtually the same latitude/elevation/weather as nearby Little Rock, Arkansas. According to http://www.solardirect.com/pv/system...sun-hours.html, Little Rock gets 5.29 hrs/avg summer sun, 3.88 hrs. winter sun, and a yearly average of 4.69. Assuming the worst-case scenario and figuring for winter months, the formula would then be:

30W/h x 24hrs. = 720 Watts/day
720 watts x 1.5 overhead = 1080 Watts/day
1080 watts/day divided by 3.88 Sun Hours (winter, Little Rock, Arkansas) = 278.35W, rounded up to a 400W panel

720W-H x 5 days = 3600W-H
3600W-H divided by 12V = 300A/h Battery

400W panel divided by 12V battery = 33.333A, rounded up to a 40A charge controller.

I think I just earned a gold star. Not that I would mistrust your calcs, sunking, but I needed to understand the calcs, not just take someone's word for it. Now, a couple of follow-up questions:
The author of the tutorial suggested never using 12V, opting for 24VDC with panels 300W or higher. He also cites pros and cons of using FLAs versus SLAs. In your experience, what are your opinions on 12VDC versus 24VDC and FLAs versus SLAs in an application such as mine?
Thanks again...

[inetdog]

Thanks to both of you, sunking and inetdog. I read the posting you supplied the link for, sunking, and did the calculations myself and was pleased when my calcs matched yours. And inetdog, you understood what I was saying when I explained the need for inverting to 120VAC before stepping back down to 16.5VAC. So a big thanks to you both...

My location is Stuttgart, Arkansas, which is virtually the same latitude/elevation/weather as nearby Little Rock, Arkansas. According to http://www.solardirect.com/pv/system...sun-hours.html, Little Rock gets 5.29 hrs/avg summer sun, 3.88 hrs. winter sun, and a yearly average of 4.69. Assuming the worst-case scenario and figuring for winter months, the formula would then be:

30W/h x 24hrs. = 720 Watts/day
720 watts x 1.5 overhead = 1080 Watts/day
1080 watts/day divided by 3.88 Sun Hours (winter, Little Rock, Arkansas) = 278.35W, rounded up to a 400W panel

720W-H x 5 days = 3600W-H
3600W-H divided by 12V = 300A/h Battery

400W panel divided by 12V battery = 33.333A, rounded up to a 40A charge controller.

I think I just earned a gold star. Not that I would mistrust your calcs, sunking, but I needed to understand the calcs, not just take someone's word for it. Now, a couple of follow-up questions:
The author of the tutorial suggested never using 12V, opting for 24VDC with panels 300W or higher. He also cites pros and cons of using FLAs versus SLAs. In your experience, what are your opinions on 12VDC versus 24VDC and FLAs versus SLAs in an application such as mine?
Thanks again...

if going to 24 volts would allow you to use batteries in series instead of parallel, that would be a good thing all by itself. (300 AH at 12 volts is a large battery)
As to FLA versus AGM, the deciding factor may be whether you will be able to do the periodic maintenance (checking) on the batteries which FLA will need or have to have something (AGM) which is more maintenance free but costs twice as much and will require replacement twice as often. Avoid GEL batteries.
Sunking will probably also tell you that if you have limited solar hours during the day to recharge the batteries, AGM will be able to handle more panels (higher maximum charging rate) than the same capacity FLA batteries.

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if going to 24 volts would allow you to use batteries in series instead of parallel, that would be a good thing all by itself. (300 AH at 12 volts is a large battery)
As to FLA versus AGM, the deciding factor may be whether you will be able to do the periodic maintenance (checking) on the batteries which FLA will need or have to have something (AGM) which is more maintenance free but costs twice as much and will require replacement twice as often. Avoid GEL batteries.
Sunking will probably also tell you that if you have limited solar hours during the day to recharge the batteries, AGM will be able to handle more panels (higher maximum charging rate) than the same capacity FLA batteries.

I'm diagramming this out in my head as I go. If I run two 12VDC/160a/h AGM batteries in series, I get 24VDC @ 160a/h. If I run two 12VDC/160a/h AGMbatteries in parallel, I get 12VDC @ 320a/h. So should I run a parallel/series configuration to get 24VDC @ 320a/h? Did I even figure that right? And even hope I can remember how to wire a parallel/series configuration? Yikes, that's roughly $2500 worth of batteries, not to mention the cost of the panel, controller, inverter, cabling, and labor. My client may have a cow and decide to hire an armed guard. Ah, well, I'll run it by him and then duck. A lesson from the past remembered..."Engage brain before putting mouth in gear". Seems like a whole lotta expense for a little old alarm system that runs off a 16.5VDC 40VA transformer.

[Sunking]

I think I just earned a gold star. Not that I would mistrust your calcs, sunking, but I needed to understand the calcs, not just take someone's word for it.

No problem. First calculation is straight forward energy use in a day. 30 watts x 24 hours = 720 watts hours. Just like miles per gallon in a car that gets 30 Mpg and you gotta go 720 miles. No matter how you spin it you will use 24 gallons.

The adjustment fact is to account for all the system losses in the wiring, charge controller, battery charge efficiency, and inverter conversion losses. All totaled up a Off-Grid Battery system with MPPT controller is 66%, and 50% for PWM. So if you need to use 720 watt hours, the panels have to make more than that. So at 66% efficiency to use 720 watts you got to make 720 / .66 = 1090 watthours.


With any battery system you have to use worse case which is winter. In your case 3.9 Hours right. Just simple 5th grade math to find the watts. WATTS = WATT HOUR / HOUR. 1090 wh / 3.9 h = 279 watts. Round that up to 300 watts and you are done with panel calculations.

Battery is easy fall off the log math. You need a battery with 5 days reserve capacity. So 5 x 720 wh = 3600 Watt Hours. . Now select battery voltage and you say 12 volt? Find the Amp Hours. Again simple 5th grade math, or 10 year college math in Arkansas. AH = WH / Battery Voltage. 3600 wh / 12 volts = 300 Amp Hours.

If using a MPPT charge controller the formula is Panel Wattage / Battery Voltage so 300 watts / 12 volts = 25 amps

All of it is very straight forward 5th grade math.

[Sunking]

what are your opinions on 12VDC versus 24VDC and FLAs versus SLAs in an application such as mine?

For this application you are going to want AGM batteries. DO NOT confuse that with Gel batteries. They will last about 2 maybe 3 years, then you get a follow up call and more Jingle in your pocket for replacement.

[inetdog]

I'm diagramming this out in my head as I go. If I run two 12VDC/160a/h AGM batteries in series, I get 24VDC @ 160a/h. If I run two 12VDC/160a/h AGMbatteries in parallel, I get 12VDC @ 320a/h. So should I run a parallel/series configuration to get 24VDC @ 320a/h? Did I even figure that right? And even hope I can remember how to wire a parallel/series configuration? Yikes, that's roughly $2500 worth of batteries, not to mention the cost of the panel, controller, inverter, cabling, and labor. My client may have a cow and decide to hire an armed guard. Ah, well, I'll run it by him and then duck. A lesson from the past remembered..."Engage brain before putting mouth in gear".

Good idea, but one which is amazingly uncommon among people who actually buy part of a system and then come here for help on the rest.
You came for help before you started, although only after getting your friend excited.


Yes. Two 12 volt 160 AH (by the way, it is AH, as in amps times hours rather than A/H as in amps per hour) batteries in series would be 12V, 320AH.
In parallel they would make 24V, 160AH. Four in series parallel (make two strings of two in series and then parallel the ends of the two strings. No connection across the middle) would give you 24V at 320AH for twice as much stored power as two batteries would give you.

If you go for FLA batteries instead of AGM, the cost will be about 1/2 as much. The first issue is to get a very accurate idea of what total power you need for the system.
By Sunking's estimate and calculation, you would only need two batteries, not 4, just using a 24 volt instead of a 12 volt inverter to get your 120 volt AC.

If the real power requirement of the alarm system is less than 30 watts, then you would also need less battery. You can determine the real power drain by using a Kill-a-Watt meter on the transformer input if you can get access to a working system OR by looking at how long the system is supposed to last on its internal battery and calculating the standby power drain. Then double it for a rough estimate of the AC operation power.

You also need to keep in mind that when the inverter shuts down from low battery, the system will still keep going for some time on its internal battery. Hopefully it will generate a trouble signal at that point so you could cart the batteries off to be recharged from AC and then bring them back.

[Sunking]

I'm diagramming this out in my head as I go. If I run two 12VDC/160a/h AGM batteries in series, I get 24VDC @ 160a/h. If I run two 12VDC/160a/h AGMbatteries in parallel, I get 12VDC @ 320a/h. So should I run a parallel/series configuration to get 24VDC @ 320a/h? Did I even figure that right?

Yes, again 5th grade math.

AH = WH / V
3600 wh / 12 v = 300 ah = 3600 wh / 24 v = 150 ah.

So you buy 2-12 volt 150 AH batteries either way. or 1 single 12 volt 300 AH battery. Pretty easy puzzle to put together huh?

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Good idea, but one which is amazingly uncommon among people who actually buy part of a system and then come here for help on the rest.
You came for help before you started, although only after getting your friend excited.


Yes. Two 12 volt 160 AH (by the way, it is AH, as in amps times hours rather than A/H as in amps per hour) batteries in series would be 12V, 320AH.
In parallel they would make 24V, 160AH. Four in series parallel (make two strings of two in series and then parallel the ends of the two strings. No connection across the middle) would give you 24V at 320AH for twice as much stored power as two batteries would give you.

If you go for FLA batteries instead of AGM, the cost will be about 1/2 as much. The first issue is to get a very accurate idea of what total power you need for the system.
By Sunking's estimate and calculation, you would only need two batteries, not 4, just using a 24 volt instead of a 12 volt inverter to get your 120 volt AC.

If the real power requirement of the alarm system is less than 30 watts, then you would also need less battery. You can determine the real power drain by using a Kill-a-Watt meter on the transformer input if you can get access to a working system OR by looking at how long the system is supposed to last on its internal battery and calculating the standby power drain. Then double it for a rough estimate of the AC operation power.

You also need to keep in mind that when the inverter shuts down from low battery, the system will still keep going for some time on its internal battery. Hopefully it will generate a trouble signal at that point so you could cart the batteries off to be recharged from AC and then bring them back.

Again, thanks to both of you for your prompt and patient replies, although I must correct Sunking in that 5th grade math elsewhere is post-doctoral study here in Arkansas. I'll take the advice and get a more accurate estimate of power drain at the panel and should be able to downsize the system (and the cost). Kill-a-watt meters are not very expensive and I'm certain I'll use it elsewhere. We have hundreds of alarm systems installed, so access to one (my home and my shop, for example) won't be a problem. Typically a new, fresh, fully-charged 12VDC 7AH SLA backup battery will power a panel in "backup" mode for about two days without AC input, so I should be able to figure the drain based on that. I'll revise my panel and battery calculations...

[inetdog]

Again, thanks to both of you for your prompt and patient replies, although I must correct Sunking in that 5th grade math elsewhere is post-doctoral study here in Arkansas. I'll take the advice and get a more accurate estimate of power drain at the panel and should be able to downsize the system (and the cost). Kill-a-watt meters are not very expensive and I'm certain I'll use it elsewhere. We have hundreds of alarm systems installed, so access to one (my home and my shop, for example) won't be a problem. Typically a new, fresh, fully-charged 12VDC 7AH SLA backup battery will power a panel in "backup" mode for about two days without AC input, so I should be able to figure the drain based on that. I'll revise my panel and battery calculations...

So the questions on "Are You Smarter Than a Fifth Grader" are considered really hard out there?

Just to jump start your calculations: The backup battery you describe holds 84 watt-hours of energy if you drain it to zero (not a good thing, but it does not happen very often in a real system).
If we figure 48 hours, that would be about 1.75 watts. It could use a whole lot more in full operation, and you will have some drain on the inverter's battery just to service the zero power idling current in the transformer. The Kill-a-Watt will tell you what that amounts to. Compare the Watt reading to the Volt-Amp (VA) reading. Even worse, the inverter itself will use a significant amount of power (a few watts at least) just to keep itself running. \

Looking forward to seeing the results.

PS: If you can figure out whether the AC is actually powering anything but the battery charger inside the alarm unit, you may find that you are able to fake it into thinking it is getting AC and so avoid the double conversion losses.

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PS: If you can figure out whether the AC is actually powering anything but the battery charger inside the alarm unit, you may find that you are able to fake it into thinking it is getting AC and so avoid the double conversion losses.

We've been using DSC alarm systems for years and there are programmable options that allow trouble conditions such as AC failure to be ignored (not displayed on the keypad with associated beeps), for turning off keypad backlighting, for blanking the keypad when not in use, etc., all of which would help conserve power. Further, while the detection devices to be used will be wireless (and thus use no power from the panel, having their own internal batteries), the panel will have to supply 12VDC power to the receiver for the wireless devices and the system's backup battery will have to supply continuous 12VDC operational power to the attached cellular alarm communicator, and that cellular communicator can draw up to 2 amps from the panel's backup battery (for a few seconds) when the system goes into alarm and the cellular transmitter communicates with Central Station. Sirens draw a lot of 12VDC power as well when the system goes into alarm, a large part of which is augmented by the panel's battery backup, but I can program the 40W siren to only sound for one minute rather than the default setting of four minutes.

I think I need to retrench a bit here and hook up a system at my shop with the configuration as it will be in the field (i.e., new fully charged battery without a 16.5VAC power supply) and do some testing. If it turns out that I have enough functionality running strictly off the system's 12VDC backup battery, then I see no reason to have to invert to 120VAC and then step back down to 16.5VAC.

PS: I can also run several panel batteries in parallel to give myself much more battery capacity. It may be simplistic thinking, in that I'm a noob, but what would prevent me from connecting the charge controller to a bank of 12VDC/12AH batteries in parallel and simply using the panel's existing 12VDC battery inputs? Assuming, of course, that I can retain enough functionality running off battery?

[Sunking]

PS: I can also run several panel batteries in parallel to give myself much more battery capacity.

Do no trun batteries in parallel, you are just asking for trouble. If you need 100 AH, then use a 100 AH battery

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Do no trun batteries in parallel, you are just asking for trouble. If you need 100 AH, then use a 100 AH battery

Thanks, Sunking, but please clarify why running batteries in parallel is more problematic than a single large battery. I ask this because the battery pigtails supplied by the manufacturer of the alarm panel have two sets of terminals, allowing for two 12VDC batteries of pick-your-AH-rating to be connected in parallel for longer reserve power. I do understand that if one of the batteries in a parallel setup goes bad, it will affect the other batteries as they will attempt to equalize across the array, but in my mind this will still keep the system somewhat functional until the bad battery in the array is replaced and max storage capacity is restored, whereas the failure of a single, large battery will render the system useless.

Please point out the flaw(s) in my logic.