My what expensive electricity you have...
That's ten times what I pay.
Back of the napkin calculations...
40 watt panels only produce 40 watts under perfect conditions, and conditions are generally less than perfect. Let's say you can mount them at a good angle and in real life, you can collect 34 watts for an average of 4 hours per day.
34 watts x 4 hours = 136 watt-hours per day
but we need KILOwatt-hours, so
136 watt-hours/1000 = 0.136 kilowatt-hours per day
Again, round figures, let's multiply that times your rate of a buck a kilowatt-hour (holy crap, where to you live
)
0.136 kilowatt-hours x $1.00 = $0.136, or let's call it 13 cents a day.
That's 13 cents x 30 days = $3.90 a month you'd save.
Let's say a hundred bucks for the panels and 70 dollars for a battery like this:
http://www.amazon.com/dp/B00K8E0WAG?psc=1
a thirty dollar, 400 watt inverter that you use sparingly. It's more efficient to use the 12 V directly than it is to convert it up to 120V:
http://www.amazon.com/Cobra-400-Watt-12-Volt-120-Volt-Inverter/dp/B001RNOHBC
Twenty bucks for other bits and pieces like wire, a fuse, some sockets,
100 + 70 + 30 + 20 = $220
$220 / $3.90 per month = 56 month payback period. Just a touch under 5 years, at which point the battery will be pretty much worn out, and possibly the cheap chinese inverter as well.
But you gain considerable independence and self reliance, and a whole new skill set, and a tiny bit of saving the planet, so there's that.
Bigger systems get quite a bit cheaper per kilowatt hour, but cost more up front.