Measuring power produced by panels

 Hi all! New to the forum here.

 I'm looking for a way to measure the power produced by my panels. I have two 235 watt 24 volt panels going into a xantrex c35 controller with 4 trojan t105 batteries and a magnum ?2424? Inverter.

 I recently hooked up a 3 cubic foot chest freezer and now it seems like my panels can't keep up. The fridge and the freezer are only using 600 watts combined in a 24 hour period measured with a kill a watt meter. Other then that, I have a couple of led lights and my phone charger running of off it. Even with bright sunny days, I have to run the genny at least once a week. 

 So anyways, how can I measure how much power is actually being produced? Thanks all!


  • Raj174Raj174 Solar Expert Posts: 493 ✭✭✭✭
    edited July 2016 #2
    Hello TheOffGridMan 

    Welcome to the forum. I see a couple of issues that I would like to bring to your attention. First, as you suspect, the current number of panels are insufficient to support the loads described. The batteries need 18 to 25 amps, so more wattage need there too. Additionally, the 2 235 watt panels are not well suited for the c35 PWM controller. Your current panels probably have a VMP of about 29V. A 24 volt PWM controller needs 35 to 36 volts to charge a 24v battery bank properly. That is usually a 72 cell panel, not a 60 cell panel, which is usually called a grid tie panel.

    What is the solution? I can only think of one. Replace the c35 with a 30 amp MPPT charge controller. Wire the panels in series and this will allow you to get more wattage out of the panels you have.  

    12 x 300W Renogy PV, MNE175DR-TR epanel modified, MN Classic 150, Outback Radian GS4048A, Mate3, 51.2V 195AH HI Power LiFePO4 no BMS, 4000W gen.
  • BB.BB. Super Moderators Posts: 26,958 admin
    edited July 2016 #3
    Your 600 Watt*Hours per day (not just Watts) is not too bad for a small refrigerator (they tend to be pretty inefficient--A full size refrigerator/freezer may run as low as 1,000 to 1,200 Watt*Hours per day).

    Just to run you a couple of quick solar calculations. First, you should have 5% to 13% rate of charge for the battery bank. And 10% or more for a full time off grid system is highly recommended. Assuming your T-105 batteries are 225 AH, the solar array needed to support this would be:
    • 29 volts charging * 225 AH battery bank * 1/0.77 panel+controller deratings * 0.05 rate of charge = 424 Watts minimum array
    • 29 volts charging * 225 AH battery bank * 1/0.77 panel+controller deratings * 0.10 rate of charge = 847 Watts nominal array
    • 29 volts charging * 225 AH battery bank * 1/0.77 panel+controller deratings * 0.15 rate of charge = 1,102 Watts "cost effective" maximum
    And then there is sizing the array to support your loads. Say you are near Agusta Georgia, have the array tilted to 33 degrees from horizontal, fixed array:
    MonthSolar Radiation
    ( kWh / m2 / day )

    If you toss the bottom 3 months (assume genset usage), use February at 4.68 hours for "break even" month (of course, you can size to January as your minimum month--if you wish), and you want 1,000 WH per day:
    • 1,000 WH per day * 1/0.52 typical off grid system eff * 1/4.68 hours (Feb) sun = 411 Watt array minimum (based on loads)
    Besides guessing at your total load, as Rick says--You most likely have a miss-match between your solar panels and the C35 PWM charge controller. To charge a 24 volt battery bank, you need Vmp-array of ~35 to 40 volts (STD rating) for use with a PWM type charge controller... And there are very few 2xx solar panels that are 72 cell Vmp~36 volt panels these days.

    Most likely, you would need to put your two panels in series and get a MPPT type charge controller to take the Vmp~60 volt array and down convert to the 24 volt battery bank. And it would not hurt to get 2x more panels (2 in series, 2 parallel strings) to beef up your solar array and avoid genset run time (and keep your battery bank much happier--You are probably "deficit" charging your battery bank and it will lead to early death via sulfation because of the present 2x panels in parallel with a PWM controller--And during winter, you really are close on having enough solar array for full time off grid living).

    If you do go with 4 panel total, the minimum rated MPPT controller would be around:
    • 4 * 235 Watt panels * 0.77 typical panel+controller losses * 1/29 volts charging = 25 amp "minimum" recommended MPPT controller
    A 30-45 amp minimum MPPT charge controller would work very nicely for you.

    And, you asked about measuring your panel's output power... You can get a "good enough" for our use DC Current Clamp DMM from Sears for $60 or so.


    PS: I would also highly suggest a remote battery temperature sensor too (for the solar charge controller). Especially if you get upwards of 13% or more rate of charge.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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