We suggest starting with a bedroom. Look around your bedroom and see what devices you have that run on electric. As I look around my room, I see one table lamp, two floor lamps, wall lamp, digital TV, Stereo, satellite box, portable dvd player, vcr, and XBox. I also have a battery powered clock, powered by rechargeables, so I’ll add the battery charger to our list. There is also a curling iron, hair dryer, and ceiling fan with lights, but we will leave those items off our list for now.
The first thing I need to do, is plug these devices in, one at a time, to my Kill-A-Watt ($30 or £16.20), to determine individual loads and daily run times. This will determine the battery capacity, and size and number of Photovoltaic (PV) panels necessary to support these loads.
Lets assume I need 100 watts to power any devices used simultaneously, and I use that load 6 hours a day. I would then need 100 x 6, or 600 watt-hours (wh’s) per day to power that equipment. I would only need a 100 watt inverter, but there is little price difference between that and a 300, so I’ll upgrade this item to a 300 watt inverter ($40 or £21.60).
I decide I want to have 1 days worth of power in a battery bank, in case of no sun, so I convert those watt hours into amp-hours (ah’s) by dividing by 12, the voltage of my battery pack. 600 / 12 = 50 ah’s. I do not want to discharge my battery pack more than 50%, to ensure long life, so I want a 100 ah pack. One deep cycle type 27 battery from Walmart is 115ah and cost $55 (£29.70).
To keep that battery charged, I need to be able to put 600 wh’s per day back into the pack. My area of the world gets on average 2.5 sun hours daily, so 600 / 2.5 = 240 watts of PV. A 240 watt array (2 * 120w panels) ($480 or £259.50 each panel) would need a 30 amp (240 / 12 + 50%) charge controller ($180 or £97.30) to keep the array from overcharging the battery.
So there you have it, for less than $1300 (£702.70), you have taken one room off grid, and eliminated $0.09 / day (600wh’s x $0.15 kWh) of grid electric, giving you a 40 year payback ($1300 / $0.09 = 14444 days / 365 = 39.6 years). And if electricity prices rise faster than inflation, which they will, the payback is much faster. In fact you get the double benefit of having made one room Kyoto-compliant, AND saving money.
This article first appeared in the Ecologist April 2008
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