Formula for Calculating Loads on PV'S & Bat

tcvb
tcvb Solar Expert Posts: 44
Can anyone give me the formulas for calculating my loads i will need to put my system together:

Load for PV'S and Batteries

Thank's

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Formula for Calculating Loads on PV'S & Bat

    This starts with: what do you want to run, and for how long?

    And are you planning on a 120 VAC system or a straight 12 VDC system?

    Gotta know what you want to get out of it before you can figure out what you need to put in.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Formula for Calculating Loads on PV'S & Bat

    For simple AC plug-in loads... The Kill-a-Watt meter is the way to go. For DC loads, a cumaltive DC Amp*Hour / Watt*Hour meter is very nice to have.

    For "simple" DC and AC loads--The basic equations are:
    • Volts=Current * Resistance (Amps * Ohms)
    • Power in Watts = V*I = V^2 / R = I^2 * R
    • Work = Power * Time = Watts*Hours (for "our needs")
    • Work = 1,000 Watt*Hours = 1 kilo Watt*Hours = 1 kWH
    • Amp*Hours => is Watts without the voltage
    • Work = Amp*Hours * Volts
    For a start--a cost effective Off-Grid home would max out about 100kWhrs per month:
    • 100 kWhrs per month / 30 = 3.3 kWHrs per day = 3,300 WH per day
    • 3,300 WH per day / 24 hours = 138 Watts average
    We can look at your location for how much sun you get... But for most places in North America, you can start at 2 hours of "full noon time sun" per day in the winter and 5+ hours per day in the summer.

    A typical off grid system is (very) roughly 52% efficient at gathering energy based on Name plate rating... So to calculate panel size (by the way, I toss out the low three months of sun--typically winter--as most people are OK with using a generator during bad weather). using 3,300 WH per day (largish off-grid system):
    • 3,300 WH per day * 1/2 hours of sun per day * 1/0.52 derating = 3,173 worth of solar panels for deep winter
    • 3,300 WH per day * 1/5 hours of sun per day * 1/0.52 derating = 1,269 worth of solar panels for most of summer
    Battery sizing is based on 0.85% inverter efficency, 3 days of no-sun, and 50% maximum battery discharge. Note, the actual stored power is the same in a 12, 24 or 48 volt bank--just the series/parallel arrangement of the batteries differ:
    • 3,300 WH per day * 1/0.85 inv eff * 1/12 volt bank * 3 day * 1/0.50 max disch = 1,941 AH @ 12 volt bank
    • 3,300 WH per day * 1/0.85 inv eff * 1/24 volt bank * 3 day * 1/0.50 max disch = 971 AH @ 24 volt bank
    • 3,300 WH per day * 1/0.85 inv eff * 1/48 volt bank * 3 day * 1/0.50 max disch = 485 AH @ 48 volt bank
    More than happy to work through each step for you (add exact location, generator requirements, etc.)--but the above is a good start at sizing the system.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • DC Builders
    DC Builders Registered Users Posts: 13
    Re: Formula for Calculating Loads on PV'S & Bat

    Hi Bill,

    Been working on battery sizing for school. Need about 14kW per day. Need to do 3 days of autonomy.

    Keep coming up with 16 Trojan T-105 6 volt in 2-48 volt strings. The example shows twice as many batteries as I get.

    What am I missing on the rated usage. Is that considering no power from the panels and a grid failure?

    Thanks
    Dan
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Formula for Calculating Loads on PV'S & Bat

    Dan, we actually use 6x daily average load--3 days of no-sun and 50% maximum discharge = 6x daily load.

    Is that the missing factor?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • DC Builders
    DC Builders Registered Users Posts: 13
    Re: Formula for Calculating Loads on PV'S & Bat

    Thanks Bill,

    Guess that's why you got a good sine wave gen-set.

    For some reason the Honda sounds way better, anymore nowadays than two tons of dead batteries.

    Dan
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Formula for Calculating Loads on PV'S & Bat

    Dan,

    When I do posts like above--I keep revisiting my own system and wondering about swapping in an XW system... But--I keep coming back to the fact that we may lose power for 1-2 hours every year or two (and maybe a couple few minute outages during the year).

    My "big one" is a once in a hundred year earthquake... I keep my loads low--and a small 1,600 watt inverter/genset running a fridge/freezer and a few lights for 10 days on less than 20 gallons of gasoline. And a siphon hose for our cars can give me more--I just cannot justify the costs of a battery bank.

    And that stored fuel can be used for cooking or in the car for evacuating the area if needed. And the genset is very portable in an emergency--can't say that for a solar PV off-grid system.

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