Deep cycle battery.
15 amp Solar panel from Norther tools.
Thermostat from GQF.
200 watt inverter.
2 x100 watt lamps. or GQF 200 watt heating element
Will the panels actually keep the system up and running around the clock?
No.
Sorry, I planned to come back, as soon as I did the math. Ran out of toes.
15 amp charger will keep up for 11 hours, if you have 12 hours of sun, and charger is 100% as claimed. In 7-10 days, battery would be flat. That is assuming heater is 200 watt, and on half of the time. Switching battery with fully charged every week would about keep up, if everything went as planned. When has that happened? Two cloudy days could kill your chicks. Warm enough weather, and you might make it.
With extreme insulation you might do OK. Most energy lost to conversion turns to heat, putting the inverter inside brooder would help. There are some pretty efficient inverters available. I know a guy who uses them. $700-$800 for 2000 watt continuous duty. I assume same quality is available in 500 watt size for about half price. I wouldn't count on cheap brands.
100 watt 12 volt bulb (incandescent) should produce about same heat/light as 100 watt 120 V AC. GQF element should be same wattage, regardless of voltage (I may be missing something). You may not have enough force behind it with 12. Ehw Ehw Ehw Ehw Ehw Ehw (remember Horshack on welcome back Kotter) 12 volt ceramic heaters to pre heat cars on cold mornings. Wal-Mart I remember thinking "That's cheap, but I don't need one".
I'd build the first one "horse high, @$$ tight, and bull strong". Like the first Amana Radar Ranges. They had a magnetron that would fry a woolly Mammoth. We designed DC refrigerator units in '69-'71 for ice cream vending trucks. Two prototypes were still running as of 2003.
Use at least 45 amps of solar panels, you will then have 270 watt capacity on 12 hours daylight. Two true deep cycle batteries (connected in series, not parrelell). Inverter (if you use one) inside the brooder. With the brain I'm using, now, that should give you 4.5 days of continuous operation with no sunlight, assuming 200 watt draw. But that sounds preposterous (so did walking on the moon). Figuring escape velocity, thrust, and trajectory with a slide rule? Sure you did, uh huh!
Build it! Run it with solar panels, and thermostat disconnected (full on) and check remaining voltage continually. An amp meter would help a lot. If voltage drop is less than .20 volts per hour average (rate will accelerate) you will make it. If drop rate is higher, connect thermostat, and set temperature at 99F. With full charged (slow charge) battery, start unit at sundown. Check, and record voltage when possible, but especially right at sun up. If voltage drop averages less than .1 volt per hour, you're in great shape! At .1-.2 volts per hour, you're OK. This test should be done on fairly cool day, or with main unit (not panels) in the shade.
Connect solar panel at sun up. Check voltage hourly. Now is when things can get really subjective, so record sun conditions, and ambient temperature, so you can compare varied days. On a sunny day, with panels pointed properly, you should return to 13.75 volts by noon. or shortly after. Create a partly cloudy day, using sunshade (63%, or darker). If on partly cloudy day, you are at full charge at sunset, you're on your way. If you reach full charge by noon, you're doing great! If you return to full charge on a cloudy day before sundown, put some birds in it! If you get two or three cloudy days in a row, or sudden temperature drops go check on it. Take jumper cables, or generator/charger.
Once you've been through several bad times, weather wise, with excellent results, you can try to engineer your creation down to more affordable proportions for the next one.
Use caution, and check my figures with charts, or someone with recent experience. I'm faltering a bit. Can't even call back BTU conversion, or electrical horsepower (I think 720 watts, but not sure). 200 watts would be enough heat in a barn. Outdoors, in the wind? Too many variables for me to calculate.
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