Calculating required V & I from batteries through inverter

I am stumbling on a single calculation, which I hope you maybe able to help with. I will begin with a simple description: (Numbers are just for descriptive purposes)

Solar panel 12v - 50W, this charges 2 - 150Ah batteries through a charge controller. This means that when the batteries are fully charged, I will have 150 hours, of running a device that consumes 1 Amp of current. But this is at 12v, simply put: 12v x 1 = 12W, so I can run - say a 12V, 12W light buld for 150 hours with this capacity.

Now the issues arises, when you add an inverter to the mix, that is now supplying an AC voltage and current to a device, lets say a 1kW heater. I want to be able to calculate how long the battery will last, if I run this heater through the inverter for a given period of time. To calculate for the heater is easy, 1,000 / 110 = 9 Amps.

But this is 9 Amps, and 110v, and my calculations earlier were for the battery, let alone they are for DC and not AC too. Is there a method that I can use to calculate, for the given output of the batteries through the inverter (less the inverters effeciency of course (~80%), that will give me the draw that the inverter will draw from the batteries? I just want to be able to calculate, for a given AC draw on the AC side of the inverter, the inverter will draw X amount from the batteries - which then converted into AH, will tell me roughly how long the batteries will last when they are fully charged.

After reading a whole bunch of postings here, I have not seen this come up...


  • RoderickRoderick Solar Expert Posts: 253 ✭✭
    Re: Calculating required V & I from batteries through inverter

    I think it's easier to think in terms of watt-hours for this kind of calculation, as the unit of energy will be independent of voltage and current.

    So, 12V x 150 Ah = 1800 watt hours.

    Trying to run a 1000 W heater, 1800 Wh / 1000 W = 1.8 hours run time.

    Subject to the other real-world considerations of efficiency, battery discharge curve, etc. that you mentioned.

    BTW, this is not too bad an illustration of why insulation is very cheap compared to generating heat.
  • System2System2 Posts: 6,290 admin
    Re: Calculating required V & I from batteries through inverter

    Hey terribletrouble
    I'm sure you know by now, after reading a lot of these posts, that you should never discharge your battery past 50%, therefore, your run time would be cut to 0.9 hours, now calculate in the inefficiencies of the batteries, inverter etc., and you're down to perhaps 30 minutes, more or probably less.
    Also, your 50 watt PV, will put out less than 3 amps (a trickle charge) in full sun, and again looking at inefficiencies, it could take over 30 hours of full sun to bring the batteries back from 50% state of charge.
    Assuming 5 hours full sun every day, that would be 6 days of full sun, to get 30 minutes of running your 1000 watt heater.
    But don't give up on solar, just change your focus to other more practical uses, such as running CF lights. Let us know how you make out and don't be afraid to ask questions.
    Cheers and good luck
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Calculating required V & I from batteries through inverter

    another and better way to look at the inverter is to see it just as another dc load that can vary. it has a maximum and a minimum current draw at whatever dc voltage it was designed for. for practical purposes we go by the maximum figure to design the rest of the system to handle it.
    now to roughly figure it from the ac loads translated back to the battery this would be the ac load plus the efficiency losses. inverter efficiency is different at different draw percents of the inverter capacity. most inverter manufacturers will tell the best efficiency you can expect, but the rest will be guess work if they don't publish at all load levels. i would imagine that could vary a tiny bit also from one inverter to the next for identical models, but shouldn't be by much. do know you can effect the efficiency too by using wire that is undersized for the job.
  • System2System2 Posts: 6,290 admin
    Re: Calculating required V & I from batteries through inverter

    Thanks guys for all the words of wisdom, I am just getting into this, so no matter what, I will win, maybe I won't get my 1kW heater, but I will get something:-)

    My recent purchase of an inverter (mod sine wave - so not the best) yielded some rather interesting details in the instruction booklet that came with it. Here is what it says:
    1. Your battery as an AH rating, their example is 45 AH.
    2. To estimate the maximum battery power the inverter will require to run a piece of equipment, divide its continuous load wattage requirement by 10. Therefore an appliance with a 450 watt continuous load requirement will create a 45 Amp draw from the battery through the inverter (450 / 10 = 45)
    3. Conclusion, a battery with 45 AH, can satisfy the continuous load demand placed on the inverter for a maximum of about one hour. (45 AH / 45 Amp draw = 1 hour).

    Of course as Wayne points out, don't run the batteries below 50%, which is a very valid point indeed. I am planning on getting two batteries, Sears Optima Deep Cycle Batteries, CCA - 500, therefore 150 AH. Two of these will give me 300 AH. My solar panel is indeed a 50W panel. My intention is to run a smallish heater - throughout the months of winter, for about 4 hours a night, it does not have to be 1 kW, that was a ball park figure, I will look for some thing efficient, something to take the chill out, I am up in the North East.
  • BB.BB. Super Moderators, Administrators Posts: 33,140 admin
    Re: Calculating required V & I from batteries through inverter

    If you are after heat, then forget solar electricity. Go either hot air or hot water panels (assuming that you can physically mount the system appropriately). And Solar Thermal panels probably require ~1/4 the collector space as would solar electric panels of the same rating.

    I personally like Grid Tied solar because of low maintenance and the power company acts like an unlimited battery with up to 1 year of storage. I purchased a small Honda eu2000i for backup.

    Solar hot water or hot air is going to probably 1/3 or 1/4 the price of solar electric for the same "kWatthrs" of heat. Solar Electricity and Batteries for heat is a waste of money.

    Use the solar electrics for lights, fans, electronics and such. And spend on conservation (insulation, low power devices & appliances, etc.) before you spend much money on solar power/heat.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: Calculating required V & I from batteries through inverter

    Bil is correct - Solar electric for electric heat is a non-starter, a no-go.
    Solar electric truly shines in many areas, but for electric heat, it's a disaster and a HUGE black hole for all your savings and all the money you will make for the rest of your life and still you'll have no satisfaction. There is no compatability between the two.
    Solar hot water (direct solar heating, NOT electric) works great however. I built my own hot water panel and have been swimming in "free" hot water all Summer! Of course, "swimming in free hot water" does NOT include 2 hour long showers that through away 100 gallons of hot water.
    We in North America have generally been brought up to be very wastefull in all we use and all we do.
    Going solar requires a whole different mindset and way of thinking. A way that requires us to work with nature. On sunny days, you'll produce lots of hot water, or electricity, not so on cloudy days and not at night. No problem, just plan your energy use for when nature gives it to you and save for cloudy weather. It works!
    Again, don't turn your back on solar, but learn what it can do for you and work with what it can give you.
    Cheers and good luck.
Sign In or Register to comment.