Grid tie system question...

Discussion in 'Off Grid Living' started by newbloom1, Dec 21, 2014.


  1. newbloom1

    newbloom1 Monkey

    Well you guys are likely the experts. I have a 600 watt system, 30amp charge controller and 7 deep cycle batteries. Everything is set up great and it works well plugged into the grid (I run about 30% of my house on it). I noticed my batteries drain fast when I'm connected to my service panel. So I decided to run the inverter off the load on the charge controller to see how much its drawing. My entertainment system is drawing 25-30amps? That doesn't seem right. Could it be the distance to the system (about 35 feet)? Not sure why its using up so much power? When i use a meter, it shows its using about 6 amps for that outlet strip when on grid power. I'm still in the newbie stages of solar/battery systems. What am I doing wrong?
     
    HK_User likes this.
  2. BTPost

    BTPost Old Fart Snow Monkey Moderator

    You leave out the most important piece of t he puzzle.... What is the System Voltage of the Inverter? I will assume that it is 12 Vdc, so 30 *12= 360 Watts which is about normal for a BIG A/V System.. You also do not tell us the much about the Inverter you are using. SineWave or Modified SineWave? You also do not give us any information about the Battery Bank except that they are "Deep Cycle" which is mostly a Marketing gimmick, what we really need to know is the AmpHour Capacity of the Battery Bank, and w oh manufactures the Batteries. Your Power Strip Amp readings tell me that (6*120= 720 Watts) TE LS me that you are taxing your inverter, a lot, running it off the Inverter/Battery System... We really need more information....
     
    sec_monkey likes this.
  3. Airtime

    Airtime Monkey+++ Site Supporter

    The disconnect I think you might be experiencing is between amps and power. 5 amps at 120 bolts is not the same thing as 5 amps at 12 volts. To normalize this stuff, you need to look at power. Power is meadured in watts and equals voltage times amperage. So if your entertainment system consumes 25 amps at 12 volts that is 300 watts (sounds about right) which is the same thing as 2.5 amps at 120 volts. Now an inverter isn't 100% efficient and will loose some power to heat. So if it's say 90% efficient then 300 x .9 = 270 watts which may be what the stereo is consuming and the inverter is consuming 30 watts.

    AT
     
    sec_monkey likes this.
  4. newbloom1

    newbloom1 Monkey

    This was great information and got me thinking about the inefficiency of the inversion process. Is this just what we have to deal with when looking to run on a battery system or is there a better way? I built a trailer camper and everything runs on 12V (fridge, tv, lights, etc). Maybe the best solution is having a 12v system in your home? Thoughts?
     
  5. Airtime

    Airtime Monkey+++ Site Supporter

    Let's give you a couple fundamental principles (and equations) and I strongly suspect you'll start putting the pieces together.
    Power (P) is energy consumption measured in watts (w)
    Electromotive Force (E) is the force to make electrons move, like psi of water pressure, measured in volts (V)
    Current (I - comes from the Latin for current) is the flow of electrons, like gallons per min, measured in amps (A)
    Resistance (R) - is the opposition to or impeding of the flow of electrons measured in ohms (Greek upper case symbol for omega )
    So, as previously noted:
    P = I x E
    And the other fundamental of Ohms law:
    E = I x R
    Note, we can combine the two equations substituting for different parameters:
    P = I squared x R
    P = E squared / R

    If we need 120 watts of power for lights or something, we can accomplish that by supplying 1 amp at 120 volts or 10 amps at 12 volts.

    Now to address your question about running a 12 volt system to avoid the inefficiency of the inverter, that can often be very desirable. However, if the juice needs to run a long distance from the batteries to the loads (lights, etc.) then the resistance of the wire can become a factor. You will get a voltage drop in the wire as it's resistance will consume power (the current squared times the resistance of the wire getting both there AND back to the batteries). This can be reduced by using thicker wire which has lower resistance, but that adds cost. Sometimes adding another solar panel and battery and sticking with an inverter might be more cost effective. The short answer is for short distances, 12 v system may be more efficient. But also note that 12 volt appliances, phone and computer chargers, etc. may be more expensive and harder to find so be sure to evaluate total system costs, convience and flexibility.

    You can now start answering your own question specific to the distances you may need to run electricity.

    Have fun
    AT
     
    Last edited: Dec 29, 2014
    kellory and BTPost like this.
  6. BTPost

    BTPost Old Fart Snow Monkey Moderator

    Rule of Thumb is:

    <1500 Watts = 12Vdc
    <>3000 Watts = 24Vdc
    >4000+ Watts = 48Vdc
     
    kellory likes this.
  7. ghrit

    ghrit Ambulatory anachronism Administrator Founding Member

    Don't forget the extra cost for heavier gauge wire.
     
    kellory likes this.
  8. newbloom1

    newbloom1 Monkey

    Such great information here, thank you. Airtime you explain things very well and it makes so much more sense now. As far as upgrading to a 24V or 48V system, is the inversion process more efficient as the difference in volts is closer to the 120V AC system? Do they make an inverter that can handle 12V & 24V systems? I'm planning to build a hillside/underground home within the next year or two. Its not going to be huge (around 1000sw/ft) so I've debated going with a 12 lighting system. I'll have to do the math on distance and volt loss before I get to far into it. Glad I'm planning ahead!
     
    kellory likes this.
  9. Airtime

    Airtime Monkey+++ Site Supporter

    Just to be clear, I understood you were talking about wiring the house/bunker to use 12 volt appliances and lights. When setting up a solar or wind system charging batteries and operating an inverter, those can be set up to use a bank of batteries configured to operate at 12 volts, or they can be setup for 24 v or 48 v (some of the batteries are in series instead of parallel to increase the system voltage) However, finding lights, blenders and computer charges that operate at 24 or 48 volts aren't easy to find, so you'd wire the house for 12 vdc if not going 120 vac with an inverter.

    You could do both. Have a minimal 12 vdc for some LED room lighting and just a few things like a radio, security system/cameras, small TV and it would be available 24/7. Then power some larger appliances like the entertainment system, refrigerator, etc. with an inverter but not run it 24/7, just limited times when needed for those things before you go to bed. This could give you most of the advantages of both with modest cost increase for some redundant wiring.

    The 24 and 48 volt solar systems generally are more efficient for several reasons. There will be less power loss in the wiring at higher voltages but more significantly the transistors in the charger controllers and inverters have a bit of voltage drop and resistance through them. You reduce power loss in these with higher voltage systems (remember power loss will be proportional to the current SQUARED so cut the current to 1/2 by doubling the voltage and you reduce the power loss to 1/4!) There is a generally a bit of cost for higher voltage rated parts so hence BT's rules of thumb which tend to balance these trade offs.

    Have fun.
    AT
     
    chelloveck, kellory and BTPost like this.
  10. newbloom1

    newbloom1 Monkey

    So if I'm figuring this correctly, if my solar is bringing in 30amp... through the inversion process that's only 3amp at 120v (pre-inverter efficiency)?
    Thanks for all the information on the house. Yeah, your right most appliances are 12v (rv based). Great idea to have some running via the inverter at times of the day. Right now I have a timer set to charge all chargeable items (phone, computer, vacuum, batteries, etc) during peak sun. I'd probably do something like that.
     
  11. BTPost

    BTPost Old Fart Snow Monkey Moderator

    @newbloom1 You might look at what I wrote in Alaska Wilderness Building | Survival Monkey Forums Blog for some ideas, and Practical Information, on how we do things, in the Bush of Alaska.... I have a 12Vdc System that powers my Domestic Water System, as well as a Light at the head of my Bed, for Night Reading. This supplements the 24Vdc Inverter/Charger/1200 AmpHour Battery Bank that keeps the Cabin 120Vac, running 24/7/365.
     
  12. Cruisin Sloth

    Cruisin Sloth Special & Slow

    BTPost likes this.
  13. newbloom1

    newbloom1 Monkey

    Great posts guys, thank you very much for the information! I think this is making much more sense now. I've built two RV's from empty spaces so moving to a house requires a bit more thought and planning. This forum is a great resource!
     
  14. azrancher

    azrancher Monkey +++

    +1 on the Northern Arizona Wind & Sun site, that's where I go for answers.

    Like how to make a Grid Tie Solar System to be off Grid.
     
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