Is my logic correct?

System Posts: 2,511 admin
If I have a 80 watt panel putting out 4.63 amps max and I want to fully charge a 100ah battery that is at 50% then I divide the 50 amps I need to recharge by the 4.63 amp rating of the panel and come up with 11 hours of charging time needed?

Is this how it works?

Project is to have a panel charge a battery in which I can run my cpap for 2 days or so for a total of 16 hours before needing to recharge battery. cpap using a constant 36 watts dc into a small 400 watt cobra inverter.

Just thinking about this small project to enable me to stay alive during the night should I loose power to the house :)


  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
    Re: Is my logic correct?

    Yes, the basic math is correct... We do have various "fudge factors" too...

    A flooded cell lead acid battery can be 80% efficient. Solar panels and charge controllers only output (very roughly) 77% of their rated output (name plate rating).

    Also there are different types of charge controllers... The less expensive PWM (pulse width modulation) or the more expensive MPPT (maximum power point tracking).

    And, weather plays a huge role in how much your panels outputs... My Grid tied system will run from a maximum of 20 kWHrs per day down to less than 1kWhr per day, depending on weather and time of year.

    Solar power does work, but it does require work on your part to monitor the battery bank charge levels (hydrometer, battery meter) and replacing batteries every 3-8 years or so, etc...

    Using the PV Watts website, we can estimate out much power a 1kW panel will output for your use (sorry, 1kW is the smallest panel supported). Picking Los Agneles, using a derating factor of 0.77 * 0.80 = 0.62 (solar panel * charge controller * inverter derating factors):
    "Station Identification"
    "Lat (deg N):", 33.93
    "Long (deg W):", 118.40
    "Elev (m): ", 32
    "PV System Specifications"
    "DC Rating:"," 1.0 kW"
    "DC to AC Derate Factor:"," 0.620"
    "AC Rating:"," 0.6 kW"
    "Array Type: Fixed Tilt"
    "Array Tilt:"," 33.9"
    "Array Azimuth:","180.0"

    "Energy Specifications"
    "Cost of Electricity:","12.5 cents/kWh"

    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
    1, 4.44, 80, 10.00
    2, 5.35, 88, 11.00
    3, 5.62, 101, 12.62
    4, 6.06, 105, 13.12
    5, 6.19, 110, 13.75
    6, 6.17, 105, 13.12
    7, 6.48, 113, 14.12
    8, 6.68, 117, 14.62
    9, 5.78, 97, 12.12
    10, 5.43, 96, 12.00
    11, 4.84, 84, 10.50
    12, 4.46, 79, 9.88
    "Year", 5.63, 1175, 146.88

    if you assume 79 kWhrs per month as the minimum energy you will collect (per 1kW of solar panels):
    • 79 kWH per month / 30 = 2.633 kWhr per day = 2,633 Watt*Hours per day
    • 36 watts * 8 hours per day / 2,633 WH per day per 1,000 Watts of panels = 110 Watts of panel minimum (winter)
    Of course, you will need a backup generator (Honda eu1000i + 20 amp battery charger + stored fuel + fuel stabilizer) and battery charger for that week of bad weather (if there is a power outage.

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