I was shopping around for a pair of headphones to use at home and for no particular reason I decided to try 'active noise canceling' headphones. Headphones that have microphones to pick up external sounds and circuitry to invert these external waveforms and play them through the headphones in effect 'canceling' the outside noise.
I had no idea what to expect as far as how much external noise would be removed. I was disappointed at first to learn that not ALL of the external noise was eliminated, but it was clear that the deeper low frequency sounds were filtered out pretty well. At my office I'm located next to an air conditioned room full of hardware, the muffled sound from the well insulated room never really bothered me but after getting these headphones (Sennheiser PXC 450) I was amazed at how much of a difference the 'extra silence' could make. At home I've become aware of just how much noise my desktop computer fan puts out. Now that I've experienced the refreshing effects of auditory quiet, I can't go back.
Silence is Golden, but it comes at a price. The headphone's sound wave generating circuit is powered by a single AAA battery. I've been powering the headphones with rechargeable Nickle-Metal-Hydride batteries which keeps the cost down. I'd recently noticed that with the prices of Solid State Solar Electricity Generating Cells coming down, that a small battery charger could be had for under $20. I decided to get one to use for charging the batteries that power the headphones.
I did some quick but very conservative calculations to get an idea of how long it would take to 'earn back' the cost of the charger when compared to using AAA alkaline batteries.
A Duracell AAA Alkaline battery has a listed capacity of 1150 mAh.
Cheap bulk AAA batteries can be had for about $0.50 each.
Assume that an AAA NiMH 700 mAh cell costs $2 and can be recharged 200 times. That puts the cost of the NiMH battery at $0.01 per use.
That means that the savings per Alkaline cell is: 1 1150 mAH Alkaline @ $0.50 - 2 (700 mAH NiMH use @ $0.01) = $0.48
So we'll get back the cost of the charger in: $20.00 charger price / $0.48 savings * 2 NiMH cells = 83 AAA NiMH cells
The charger charges 2 AAA cells at a time so it will take ~42 charging sessions to charge 83 cells
To charge a pair of NiMH cells conservatively will take about: 1600 mAh (some current wasted) / 120 mA (average charging current) / 6 hours sunlight = 2.2 days
Since not all days are sunny, we'll say that we can charge 2 sets = 4 batteries per week.
That makes the time to recoup the cost of the charger at:
83 cells / 4 cells per week = 21 weeks or 1/2 year.