Homework Help: Energy required to heat water

1. May 26, 2014

1. The problem statement, all variables and given/known data
A family uses 150 L of hot water each day. The hot water system is set to 75°C and the
supply water has a temperature of 20°C. How much energy is required? If the hot water
consumption was spread out evenly over the whole day (24 hrs), what is the total daily cost
(assuming 14 c/kWh)?

2. Relevant equations
$Q=mC \Delta T$

$C = 4.184 kJ/kg (HeatingCapacity)$
3. The attempt at a solution
Hi all, I have this question and I thought I was doing it correctly, but for some reason I'm not getting the required answer.
$density of water = 998$

$Volume = 0.15 m^{3}$

$m = 149.7 kg$

$\Delta T = 55$

$Q=(149.7)(4.184)(55)$
$Q=34424.26 kJ$

$seconds/day = 86400s$

$p = \frac{dQ}{dT} = \frac{\Delta Q}{\Delta T}$

$p = \frac{34424.26 kJ }{86400s}$

$p = 0.39843kW$

$p kWh = 0.39843kW (3600)$

$p kWh = 1434kWh$

$cost = (0.14c) (1434kWh)$

$cost = 200c = 2.00$

However the answer states that it is 1.35/day I checked my work three times through, but I think I have a fundamental misunderstanding of something which is stopping me from getting the correct answer. 2. May 26, 2014 DrClaude Staff: Mentor This is where you go wrong. In that second line, you are actually calculating how much energy is used every hour: $$0.39843\ \textrm{kW} \times \frac{3600\ \textrm{s}}{\textrm h} = 1434\ \textrm{kJ/h}$$ Instead, find the conversion factor to go from kJ to kWh: \begin{align} 1 \textrm{kWh} &= 1 \textrm{kW} \times \textrm{h} \\ &= 1 \frac{\textrm{kJ}}{ \textrm{s}} \times \textrm{h} \\ &= 1 \frac{\textrm{kJ}}{ \textrm{s}} \times \textrm{h} \times \frac{3600\ \textrm{s}}{\textrm h} \\ &= 3600\ \textrm{kJ} \end{align} 3. May 26, 2014 miniradman Ahh, I plugged that conversion factor in after I found Q, then got1.33... close enough?

Cheers for the response