Scotty first step is determine how many watt hours you need each day. Real simple Watts x Hours = Watt Hours. So lets say you want to run that 40 watt bulb 10 hours, then 40 watts x 10 hours = 400 watt hours.
To find the right battery is pretty simple once you know how many watt hours, and what battery voltage you are going to operate at. With 400 watt hours a 12 volt battery is practical. So the battery Amp Hours capacity [Daily Watt Hours x 5] / Battery Voltage. So using the example above [400 wh x 5] / 12 volts = 166 AH battery @ 12 volts. So you are looking for a 175 to 200 AH 12 volt battery. A good fit would be a pair of 6 volt golf cart batteries wired in series in this example.
Panel wattage is easy to determine once you know how many watt hours you need each day, and how many Sun Hours you have during the day. There are a lot of ways to find Sun Hours for your area. PV watts is about the best using you rzip code or Lat Lon. For a battery system you have to use worse case for time of year use. In your case December and January an din winter you only get anywhere from 1 to 3 Sun Hours depending on your location. For this drill let's say 2 Sun Hours.
OK to find panel wattage we need a 3rd peice of information, which is charge controller type. From experience I know you will want to use a MPPT controller because it will be the least expensive option. So if using MPPT you need a correction factor of 1.5, and 2 if using PWM.
OK Panel Wattage = [Daily Watt Hours x Controller Correction Factor] / Sun Hours. So in this example [400 wh x 1.5] / 2 sh = 300 watts.
Last is controller size in amps. For MPPT Amps = Panel Wattage / Battery Voltage. So 300 watts / 12 volts = 25 amps.
Done.
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Help Off Grid Chicken Coop
We are having a problem with the water freezing at the chicken coop and there is no way to get electricity to it. I think if I could get a 40 watt bulb to burn at night near the metal watering can it would keep it from freezing. What would I need to make this happen? How many panels and what size, what size batteries and how many? In another lifetime I was an electrician but it was all A/C current and certainly not solar. I have a huge open field to set the panels so plenty of sun. I am in VA.
Thanks,
Scotty
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