Dad just bought the panel listed below and installed it on our camp. The price 
on that setup varies WILDLY on Amazon, I knew that (I've been watching them for 
a year) so we bought when it was low at $150. He's got 2x 35ah wheelchair 
batteries for about $100 I think plus a 400w inverter (we're powering a ceiling 
fan and range hood plus charging phones etc) so we're in maybe $300 total. 
Dad reports (I spoke to him last night) that the charge controller starts 
blinking with incoming power just as soon as dawn breaks and stays going until 
the sun is fully down. He's got a Kill-A-Watt on the output so he knows his 
output power but neglected to get anything for the incoming side which is 
something we'll add on the next trip. Overall he's satisfied with the system, 
the ceiling fan pulls around 50 watts, the range hood with light is 90w so 
we've got decent capacity (35ah or say 400 watt hours) for what we've got now. 
He's planning on another panel and 2 more batteries next year, with 5 guys at 
deer camp charging phones and limited sunlight we'll want the extra capacity.
      From: fmiser via Mercedes <>
Cc: fmiser <> 
 Sent: Tuesday, September 8, 2015 3:37 AM
 Subject: Re: [MBZ] OT: Practical solar application
> 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:

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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