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do my sums add up for a 125w supply

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  • do my sums add up for a 125w supply

    hi just trying to set up a simple 125w 240v supply for a small tube heater for night time use ,using a 800w inverter ive measured the current from the battery at 10 amp,which for my 36ah batteries means approx 2 1/2 hours use (i think),ive got a 50w solar panel which puts just over 1 amp back in my battery so it would need approx 20 hours to recharge for just 2 1/2 hours use at full sun(i know the panels should be giving me more),also my inverter is draining .6 amp in standby.Any advice appreciated or suggestions ,thanks
    Last edited by iwols; 04-13-2022, 05:17 AM.

  • #2
    Hello iwols and welcome to Solar Panel Talk

    In one answer your solar pv system is too small for your loads. You are correct an inverter will drain the battery even if there are no loads running so you have to figure in it's watt hour usage as well.

    Also for most battery systems you need about 1/10th the Amp Hour rating to get a the correct charge. So figure at least 3.6 amps of charging. But the biggest issue is that your battery is too small to run your load for any length of time.

    If your heater uses 125 watts every hour then multiply that figure by how many hours you want to run it. Then to size your battery to run your loads when the sun isn't shining you divide the number of watt hours you need each day (or night) by the battery voltage and then by 25%. That should get you close to what your battery needs to be in Amp hours rating.

    So a quick napkin calculation could be: 125 watts x 8 hours / 12 volts / 25% = 333 amp hour 12 volt battery. And then you would need about 33 charging amps and a Charge controller rated for at least 40 amps as well as being an MPPT type. A PWM type CC will lose about 33% of the panel wattage and charging amps. With a MPPT CC you would need at least 400 watt of panels (33amps x 12v = 396 watts) but with losses you may need to go to 500 watts and max out that 40amp CC.

    I know that this may be a lot to understand but most people that build a solar/battery system to run their loads usually undersize it and kill off the battery quickly. Then they end up having to get a new battery and more panels, a bigger CC, etc. That can hurt your finances unless you design your system to the correct size.

    Hang in there and someone else will provide additional info for you to build the proper sized system to run your heater.
    Last edited by SunEagle; 04-13-2022, 10:39 AM.

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    • #3
      The most efficient way to small scale heat, is to devise a directly battery
      operated heater, no inverter. For larger scale, heat pumps do very well.
      Bruce Roe

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