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re: sola tubes frugal???
5 mar 2003
dean wrote:
>> >as an energy saving measure? no! not even close.
>> would you have any numbers to support this article of faith?
>the solatube is "on" an average of 12 hours per day. in my climate,
>a good 15w cf lamp will provide about the same median light for an
>energy cost of 66 kwh/year, or about $5.50/yr.
steve baer's 1984 treatise on skylight economics begins:
a 1 sq ft skylight might allow one to turn off a 150 w light bulb
for 2000 hours a year. worth 2000 hrs x 150 w = 300 kwhr/year @
0.06/kwhr -- a value of $18/sq ft yr...
>at a life expectancy of 10,000 hours, that lamp should last 2.5 years,
>so a $10 lamp should amortize to about $4 per year. total lamp and
>energy cost is about $9.50/year if the lamp is run full-time. if i
>really used a lamp for this purpose, it would be "on" only 25% of the day,
>costing $2.25/yr.
hmmm.
>my local vendor sells solatubes for $275 as kits, or $429 installed.
>best case, using the kit to offset a full-time cf, the payback would
>take 29 years, assuming the top dome never needs replacement for uv
>or tree damage. more typically, paying an installer for a tube to
>offset a part-time cf, payback is 190 years, excluding maintenance.
you might prefer a pv-powered heliostat in the garden.
>this does not include the extra heat loss through the breached attic
>insulation. this seems more like your territory, nick.
we have 4954 (f) heating degree days in phila. an r1 square foot would
lose 4954x24hx1ft^2/r1 = 119k btu/yr worth about $1 (more lately...)
it would also have useful solar gain, esp if it's in an insulated attic
floor with a transparent south roof and a reflective north roof, under
a foil-faced foamboard cover, hinged on the north edge, that folds down
at night.
>when the sky is clear, these tubes provide a much nicer light than
>any lamp, and are quite an improvement to the home atmosphere. i'd
>buy them again. but not for the purpose of frugality.
times change.
nick
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