Help with sizing up a PV and Battery for 12.5 watt draw

gqrootgqroot Registered Users Posts: 22
Hi folks,

It's been awhile since I posted. Over the weekend, I've been testing out a wireless repeater that uses Power Over Ethernet (POE). I powered it with a pair of UPS batteries (in series for 24v) and a combined 14.4 AH to a step-up device of 48v. That lasted for ~ 20 hrs. When I hook the wireless repeater to a power supply for an AC outlet, I used a kill-a-watt to see what the draw was - it was using up ~12.3 watts. Do you think a 120 watt 12v PV with a 50 AH battery would be sufficient for a 24x7 operation? The catch is sunlight might only be 2-3 hours/day. I am planning on deploying this in Seattle. I'm still wondering if I should go with a 24v and have the MPPT controller lower it to 12v. The controller to PV is about 15 feet apart.

Thanks in advance for your help!


  • BB.BB. Super Moderators, Administrators Posts: 29,521 admin
    I would look very closely at the power required for POE (Power Over Ethernet)... I believe that the standard is 48 volts, but the repeater itself may operate at 24 volts or less (at least that is how I would design such a system).

    If you can ditch the AC inverter (usually around a 6 watt minimum DC load just to turn the inverter on) to save power. And possibly run something like 24 volt on separate copper cables (much heavier that Cat 5 or similar cable)--Get rid of the voltage drop (save more power). And you may get down towards 6-10 watts @ 24 VDC to run the repeater.

    Just to finish out the numbers (plug in your own)--Say 10 Watts @ 24 VDC:
    • 10 watts * 24 hours per day * 1/24 volts * 2 days storage * 0.50 maximum discharge = 40 AH @ 24 volt battery bank
    Two calculations to figure out solar array for battery bank charging... First based on 5% to 13% rate of charge:
    • 40 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 75 Watt array minimum (seasonal/weekend use)
    • 40 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 151 Watt array nominal (daily use)
    • 40 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 196 Watt array "cost effective" maximum
    And based on hours of sun per day:
    • 10 Watts * 24 hours per day * 1/0.77 panel+controller derating * 1/0.80 battery eff * 1/2 hours of sun per day = 195 Watt array
    So--A 10 Watt DC load--I probably would be running close to a 200 Watt panel if I wanted 24 hours per day 365 days a year--And you still might have to run a genset/shut down after 2 days of very poor weather.

    If, however, you really need a 12.3 Watt @ 120 VAC load--Your numbers would look something more like (using a MorningStar 300 Watt TSW 12 VDC inverter or similar):
    • (12.3 watts load + 6 watt inverter tare) * 24 hours per day * 1/0.85 inverter losses * 1/12 VDC battery * 2 days storage * 1/0.50 max discharge = 172 AH @ 12 volt battery bank
    Note this is a 12 volt battery bank and would be same storage as a ~86 AH @ 24 volt battery bank (i.e., adding the AC inverter makes the battery bank about 2x larger than my "minimum" DC power POE device--If I am correct).

    The array size numbers would be about double (based on 5% to 13% rate of charge).

    And based on hours of sun per day:
    • 18.3 Watts * 24 hours per day * 1/0.52 AC system eff * 1/2 hours of sun per day = 422 Watt array
    I really like AC inverters instead of playing the whole "24 VDC battery bus" game--But it is going to cause you to run about 2x larger power system for that privilege if you cannot find a "low power" DC solution to your problem.

    My guess anyway.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • gqrootgqroot Registered Users Posts: 22
    Hi Bill,

    Thank you so much for the info and calculations. This really helps out. I'm glad I asked because I was going to build it with 120 watt PV. I'll need to splice the CAT5 to see how much load it is using. Would you recommend putting a 24v PV (instead of a 12v PV) and the MPPT controller to a 24v battery (2 in series)?

    Thanks again Bill!
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