Best setup for DC-only 9w system?

[ksten]

Hi, I'm having trouble getting a straight answer on this, and most of the online info assumes large systems with inverters, so...

I have an LTE internet hotspot that I have to relocate to the top of a hill in order to get decent reception. The hotspot itself draws about 3w and takes 12v. To beam the signal back to the house, I'm planning to use a pair of Ubiquity Nanobeam M5s, one of which needs to be on top of the hill, of course. It draws 6w and takes power via POE at 24v (I have a simple POE injector that you connect 24v DC to).

So in short, 9w power draw 24/7, DC only (no inverter). I'm in BC, Canada, roughly 50th parallel. Worst case sun is about 2.5h in deep winter, and temperature can get down to -20c on rare occasions (more often it hovers around freezing during winter).

I've had quotes for solar panels ranging from 100w to 360w, batteries up to 200ah, with some people arguing 24v and others 12v, but I can't seem to find any agreement, and these solar system dealers are really striking me as a shady bunch. From my searches online, it seems 12v panels are the most common and fairly inexpensive.

So, my questions:
- What panel wattage do I actually need? From my calculations it seems 100w would be enough, but I'm getting such conflicting info that I'm not sure.
- Is 75ah of battery good enough for this?
- Is a 12v system (with a 24v DC-DC converter for the Nanobeam) acceptable, given the greater availability of 12v systems? Or am I better off with 24v (stepping down to 12v for the LTE hotspot)?
- If I go 12v, is a single 12v battery acceptable? 2x6v? or a 24v system made of 2x12v or 4x6v?

Bear in mind that the load on this system will not change. 9w DC power draw (mixed 12v and 24v), 24/7, now and forever.

[Sunking]

Read this and and the other Stickies in the Off-Grid section and you will know what it takes.

But lest's cut to the chase. If you read then you know what it will take. So without any further delay let's start.

First thing is the battery. Let say you use 10 watts for wiggle room. With such tiny Toy Wattage 12 volts is all that is required. Higher power = higher voltage. Higher wattage is 500 watts or more. You are no where close to that. OK in a day you use 10 watts x 24 hours = 240 Watt Hours. Minimum Battery Capacity is 5 days which give you 3 days of usable run time without sun. So AH = [5 days x 240 watt hours] / 12 volts = 100 AH. So 12 volts @ 100 AH is adequate where Summer is more than 1 month long and no frozen ground. You need to account for the which means you need a 150 AH battery. Secondly it will require a AGM battery because of the expose cold. So battery a 12 volt 150 AH AGM battery.

OK for panel wattage, again if you lived someplace that gets sun in winter like 4 Sun Hours would only take a 240 wh x 1.5 / 4 Sun hours = 90 watts. You you only have 2.5 Sun Hours which means if you used a MPPT Controller:

240 watt hours x 1.5 / 2.5 hours = 144 or 150 watts.

If you want to go the expensive route with a cheap PWM controller and battery panels will require

240 watt hours x 2 / 2.5 hours = 192 watts or a pair of 100 watt battery panels.

Last is the charge controllers. It will require a 15 amp controller.

Smart Money buys a inexpensive 200 watt Grid Tied Panel, 15 amp MPPT Controller, and 12 volt @ 150 AH AGM battery.

Now if you need 24 volts, two things change. Battery will be 24 volts @ 75 AH AGM, and Controller drops to 7 amps which you cannot find. You would stil use the same 15 amp Morningstar Controller. One more note here for 24 volts requires a 36 volt panel.

[ksten]

Perfect! Thanks so much. I'd messed up the battery calculation (didn't account for 50% SOC).

BTW what do you mean by grid tied panel? As in tied to mains? I can't run power up to the top of the hill, unfortunately (it's about 1 mile away from the house).

[Sunking]

Perfect! Thanks so much. I'd messed up the battery calculation (didn't account for 50% SOC).

BTW what do you mean by grid tied panel? As in tied to mains? I can't run power up to the top of the hill, unfortunately (it's about 1 mile away from the house).

GT or Grid Tied Panels are higher voltage, higher power, and cost a lot less than Battery Panels. $/watt

Battery panels are antiquated technology. They are 36 cell panels which limit the voltage to 18 volts, and limits power to around 160 to 180 watts. They are used with inexpensive PWM controllers. They cost 2 to 8 times more than GT panels $/watt.

GT panels are 200 watts and up. They go up to 70 cells or 35 volts which is much more efficient and allowing much higher wattages up to 300+ watts per panel. You must use MPPT Controllers on a battery system

It takes a 300 watt PWM System with Battery Panels to equal a 200 watt MPPT system with GT panels. Now here is the kicker a 300 watt PWM system cost roughly $700, A 200 watt MPPT system roughly $500. The MPPT system wiring and racking will also be less expensive as you will only have one panel to rack and smaller wiring. You can use a 600 volt panel to charge a 12 volt battery using MPPT.

You are welcome. What I recommend is a slight bit of over kill you and I can both sleep well over. 10 months out of the year it will be over sized. It is those months of December and January you have to design for. A 200 watt Panel will warm up an AGM enough to thaw it out if you went dark for a few days.

[Mike90250]

i would only add that a pair of 6V golf cart AGM batteries wired in series, would be easier to manhandle up the hill, than on giant 12V 150ah battery.

Use a 12V - 24V DC-DC up-Converter to get the 24v you need for the radio. Might have to look a bit for one that can manage the temperature ranges you need.

[Logan5]

I use an ubiquity m2 and it can operate from 11 to 30v. I would check the documentation for voltage input parameters, it may run fine from a 12v battery w/o any step up converter. since it may have a wide input value you should not even need an inline voltage reg.