Inverter Power Draw

mjp24cohomjp24coho Solar Expert Posts: 104 ✭✭✭
This is hopefully a basic question, but I need to ask regardless. I'm working on the specs for my off-grid cabin solar panel & battery system I'm designing, and I want to make sure I get my power consumption as precise as possible. I plan on the system running a few things while I'm not there, including a small refrigerator and a satelite internet system (to run wireless IP cameras to monitor the property while not there). I'm looking at an outback inverter (so I have the charge capability from my generator while I'm there). The inverter has a ~20W full idle power draw. My question is whether I need to account for that 20W power draw for 24 hrs a day? Or, since I'm already designing it to run the satelite internet and refrigerator 24X7, would that power draw already be accounted for with the ~90% charger efficiency. I want to make sure I'm not double-counting the watt consumption as I'm designing the system, and as we all know, every watt counts.

Thanks!

Comments

  • john pjohn p Solar Expert Posts: 814 ✭✭✭
    Re: Inverter Power Draw

    The inverters efficiency of 90% has no relationship to its 20w idle use
    It will use 20w at idle if it was 30% efficient or 99.99 % efficient
    the efficiency is related to its conversion of its input to output and its not linear, It may be 90% efficient at 80 to 95% of its output but only 60% efficient at 40% of its output. that is why its not a good idea to use a 1000w inverter to power a 100w lamp.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Inverter Power Draw

    Furthermore, Outbacks have a "standby" mode wherein power consumption drops to about 6 Watts when no power is used. It will then go up to full power when load demand it present. If you have loads 24/7 or the loads are too light to trigger the change this will save you nothing. You have to change some programming to set this stand-by mode. I found with my 'frige on the savings were minimal because it runs 1/3 of every hour anyway, and everything else was on all day.

    Refrigerators, even small ones, have big power demands in terms of start-up and over-all Watt hours. You can see some real data in this thread: http://forum.solar-electric.com/showthread.php?t=12272 That might help you pick a viable unit.

    Otherwise the other things you mention could all probably run off a Morningstar 300. It's power draw is tiny compared to big inverters, but it has no built-in charger.

    This is why it's very important to know what you're going to power before designing the system.
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Inverter Power Draw

    If you need 110 VAC 24/7 for your sat/camera system, then consider a small inverter such as Morningstar's PS-300. It has relatively low self consumption and is an excellent little unit. Then use your bigger inverter on standby mode to run your refrigerator. Having that big inverter on 24/7 adds up to almost half a KWH per day that you'll have to come up with, power that does nothing but keep that inverter running. Yes, inverter idle draw is power that will be taken out of your batteries and must be replaced from solar, generator, or whatever.
  • mjp24cohomjp24coho Solar Expert Posts: 104 ✭✭✭
    Re: Inverter Power Draw

    I like that idea - I only need a small inverter for the majority of the time. Could the two inverters be configured together, so the larger outback only kicks in when needed (when the compressor on the refrigerator kicks in)?
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Inverter Power Draw
    mjp24coho wrote: »
    I like that idea - I only need a small inverter for the majority of the time. Could the two inverters be configured together, so the larger outback only kicks in when needed (when the compressor on the refrigerator kicks in)?

    That's the system I use. A PS-300 is always on, supplying 110VAC for small to med loads that need power 24/7. My large inverter is in "search" mode, so uses very little power until a load appears, such as the water pump, or in your case the fridge, at which time it powers up for as long as the load remains, then goes back into search. The outputs of the two inverters are NOT connected together. Each is wired to their own loads and thus operate independently of each other. The only thing they have in common is the battery bank.
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Inverter Power Draw

    I believe some of the Outback inverters have (or had) a stacked mode with a primary and secondary inverter... The secondary would power up when addition output was needed.

    However, they still are larger inverters for larger systems... Wayne's (and other's) method of having separate inverters (and branch circuits) for different loads can be a nice solution... Put one small inverter to power the 24x7 and a second, larger inverter, to power the other, larger loads that may not need 24x7 power.

    You do have to be careful, some of the appliances (like most refrigerators) need power 24x7 to keep track of the defrost cycles.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Inverter Power Draw
    mjp24coho wrote: »
    I like that idea - I only need a small inverter for the majority of the time. Could the two inverters be configured together, so the larger outback only kicks in when needed (when the compressor on the refrigerator kicks in)?

    As mentioned, something like the Outback could run in standby mode and only fire up to full power when the 'frige turned on. That's 6 Watts vs. 20; a savings of 14 Watts when not needed. If the refrigerator runs 1/3 of the time that would be saving 112 Watt hours per day.

    One other thing: if your "big" inverter needs to be really big you will have two separate systems entirely, as the Morningstar is only available in a 12 Volt version. Still, a 12 Volt 1-2 kW inverter should be able to start most any refrigerator.

    Morningstar 300: http://www.solar-electric.com/mosu300wasiw.html
    and
    Outback 2012: http://www.solar-electric.com/fx2012t.html
    or
    Magnum 2012: http://www.solar-electric.com/maenms2020wa.html

    Just as a couple of examples. You may find something with better specs out there somewhere.
Sign In or Register to comment.