R = 018 Windows in a House Are Replaced with R =

Question

Examlex · Accepted Answer

The Answer is The reduction in power loss per m2 which results from replacing the windows is

\Delta P = \left[ \left( 18.3 ^ { \circ } \mathrm { C } - 5 ^ { \circ } \mathrm { C } ight) / 0.18 ight] - \left[ \left( 18.3 ^ { \circ } \mathrm { C } - 5 ^ { \circ } \mathrm { C } ight) / 0.52 ight] = 48.3 \mathrm {~W}

Over one year this amounts to

( 48.3 \mathrm {~W} ) imes \left( 3.15 imes 10 ^ { 7 } \mathrm {~s} ight) = 1.52 imes 10 ^ { 3 } \mathrm { MJ }

At

\$ 0.03

per MJ, the cost of the energy saved is

\left( 1.52 imes 10 ^ { 3 } \mathrm { MJ } ight) imes ( \$ 0.03 / \mathrm { MJ } ) = \$ 45.60 ext { per year }

The payback period for a

\$ 250

window is

( \$ 250 ) / ( \$ 45.60 / \mathrm { y } ) \approx 5.5 \mathrm { y }

.

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