> Andrew wrote:
> There is no heat requirement.  I use low wattage (?)
> seedling heat pads...

A heating pad is a heater.

> ... that draw almost no current.

"almost no current" when there is 115 VAC in the wall outlet
is not the same as battery power! 

> Typically these:
> http://www.amazon.com/Hydrofarm-MT10006--19-1-2-Inch-Seedling/dp/B0001WV010/

That page reports "The Seedling Heat Mat uses 17 watts".

With a 100% efficient inverter and 12 VDC battery, that will
draw about 1.5 amps.  

Figuring on inverter losses, etc., lets round that off to 2 A.

To run one for 24 hours would require (2 x 24) 48 Amp-Hours (Ah).

Since this would be about the time of the spring equinox,
there should be about 12 hours of sunlight.  That is NOT 12
hours of max output from a photo-voltaic panel, but I'll
ignore that for now.  I expect you will want the pv panel to
run the heater _and_ charge the battery, so it will need at
least 4 A output - or about 50 W.

Since it is never good to let a wet cell run all the way
down, and to allow for decreased capacity over time, that
would suggest a 50 Ah battery to effectively power one
heater overnight.  (24 Ah draw from a 2 A load for 12 hours -
rounded up.)

That amounts to a 20 W inverter, a 50 W pv panel, and a 50 Ah
battery - for each heating pad.  A quick look at Amazon for
prices results in almost $500 to run two heat pads.

How many do you use?  How much to run an AC power cord?

 #############  DISCLAIMER  #####################
        This is just a mental exercise on my part
        In NO way do I recommend, endorse, or 
        make any claim as whether these products
        are appropriate to work together or will
        achieve any implied performance target.

$220 for a 100 W pv panel and charge controller

$245 for a 100 Ah battery

$20 for a 50 W inverter


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