A Bathtub of Water
- Bathtub holds 9 cubic feet, 0.25 cubic meters, 250 kilograms
person displaces 50 kg? -> 200 kg of heated water
- Inlet water 10C, tub water 40C
30C * 2e5 g * 4.184 J/calorie -> 25 MJ
- Gas is $1 per therm, or $1 per 63 MJ at 60% heating efficiency
- $0.40 per full bathtub, $160/year for a bath every night
Electric Resistance
$0.12 per 3.6 MJ, 90% efficiency -> $1 per 27 MJ
- $0.93 per full bathtub, $340/year for a bath every night
Electric Heat Pump
$0.12 per 3.6 MJ, 240% efficiency -> $1 per 72 MJ
- $0.35 per full bathtub, $130/year for a bath every night
est $2000 purchase and install plus one service call -> 67 year payback
- If electric rates rise because of renewables, no savings at all