Load calculation for elect lights

I live in Florida and would like to have a small emergency power source. I would like to supply my home 115v electric lights with these pv panels. I need to know the watts I need and the batteries. Also is there a switch that could be installed that could manually switch from the grid to the pv? Thanks.


  • BB.BB. Posts: 26,204 admin
    Re: Load calculation for elect lights

    The grid switch is called a "transfer switch"... There are manual and automatic switches--and some inverters that have internal transfer switches.

    How big of panel and battery system depends on how much power you which to use.

    Using conservative rules of thumb--

    Basically, how many watts and how long... So--an 11 watt light running for 10 hours will take 110 Watt*Hours.

    If you want the system to work for winter and summer (2-3 hours of full sun in winter, 5+ hours in summer)... So, solar panel wise--the rough size of panel would be:

    Panel Size = Watt*Hours per day * 1/hours of sun * 1/system efficiency.
    Panel size = 110 Watt*Hours per day * 1/2 hours of sun per day * 1/0.52 system efficiency = 106 watt solar panel minimum.

    Battery size should roughly be (based on 3 day of no sun, and 50% max discharge and 85% inverter efficiency, and 12 volt battery):

    Battery Amp*Hour = 110 Watt*Hours per day * 3 days of no sun * 1/50% max discharge * 1/12v battery = 55 Amp*Hours (20 Hour Rate)

    A good way to measure your power use (120 VAC, 15 amps max) would be to get a Kill-a-Watt meter.

    wind-sun_2068_2764407Kill-A-Watt AC Power Monitor Meter
    P4400 Cumulative Killowatt-Hour Monitor

    This the starting point and gives you a rough idea of what it would take... The more power you need, the larger the panel, inverter, charge controller, and batteries). If you want 110 watts of power (light, laptop, radio)--then the system would be 10x as large as the calculations above.

    Obviously, there are still many questions to ask and choices to make... Let us know what you need.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Posts: 17,615 ✭✭
    Re: Load calculation for elect lights

    It's a bit more complicated than that.

    To have a battery-based back-up power system you need not only the batteries, but an inverter to convert DC to AC. And you need a way to re-charge the batteries at the proper rate.

    One example would be to use a small inverter, stand-alone battery charger, and batteries.
    A step up from that would be to have an inverter with charger capability: it would take AC in and 'feed' your lighting circuits all the time while keeping the batteries up. Power goes down: batteries take over for that circuit only.
    Since you're on grid and looking at buying panels, you might consider a grid-tie inverter with battery back-up. That way any time the batteries are fully charged and the sun is still shining you sell your 'extra' power back to the grid. I understand Florida has some pretty fair incentives for that sort of system.

    If money is an issue, than a small, efficient generator might be your best choice for emergency power situations (although you have to keep fuel on hand and keep the gen maintained).

    And yes, any back-up power system MUST disconnect from the grid when the grid goes down: you don't want to 'back feed' loose wires on the ground and risk electrocuting someone.

    To start planning your system, get a Kill-A-Watt meter (or similar) and measure the actual loads you want to keep running in an emergency.

    Don't worry: it takes a long time and a lot of posts to get this stuff all sorted out. We're here to help! :D
  • 3toe3toe Posts: 5
    Re: Load calculation for elect lights

    I figure we use 4 75w lights a day for about 2 hrs ea.
    75 x 4 x 5=1200 w. Is this correct? I didnt understand the math on the 106w panel. Thanks.
  • CariboocootCariboocoot Posts: 17,615 ✭✭
    Re: Load calculation for elect lights

    (4) 75W lights = 300 Watts.
    Used for 2 hours per day = 600 Watt/hrs.

    To supply that you'd need:
    1. an inverter capable of supplying 300 Watts: http://store.solar-electric.com/sa300wa12vos.html minimum
    2. batteries to run it for 2 hours: 300/12V = 25 Amp/hrs * 2 hrs = 50 Amp/hrs * 2 (for 50% Depth Of Discharge maximum) = 100 Amp/hr battery.
    3. something to recharge the battery with at approximately 10 Amps charging current, i.e. a standard automotive battery charger or, say, a 120W solar panel and a charge controller.
    4. some type of transfer switch to keep it separated from the main power grid.

    Those are rough calculations; not an actual system design. It doesn't take into account system losses and variations in actual loads. Just to give you an idea of what's involved.

    You should be considering what all you need to keep running in a power outage - such as refrigeration.
    And think about compact fluorescent lights too.
  • BB.BB. Posts: 26,204 admin
    Re: Load calculation for elect lights

    600 Watt*Hours is the correct answer.

    That is about 5x larger than the numbers I gave you above...

    Normally, you would want to use very small lights (instead of 75 watt filimant, use an 11 watt compact florescent lamp). Solar RE power is very expensive (about $1-$2.00+ per kWhr vs $0.10-$0.20 per kWhr for home power--for a system that is used 365 days a year--and emergency system costs even more).

    The size of the panel depends on the amount of sunlight it receives and the overall system efficiency (solar panel-charge controller-inverter-load). The average full sunlight hours is 2-3 hours per day in winter and about 5-6 hours per day in summer. System efficiency (solar panels + charge controller are about 77%, 80% for standard lead acid battery, and 85% for a DC to AC inverter--numbers are very approximate but close enough for a rough estimate of a cost effective system).

    So, to estimate the size of the solar panel, based on your load.

    Panel Size=Watt*Hours per day * 1/number of hours of sun * 1/system efficiency

    In general, for systems that have more than a few hundred watts of solar panel are very expensive to run as an emergency only system.

    We can go into more details based on your needs and where you are located.

    But, unlike a generator which can power some pretty large loads for a few gallons of gasoline--Solar RE on the other hand really is best able to supply a minimal amount of power for emergency use.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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