gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
130922-2147 EDT
Question on electric water heater energy loss.
Are the following assumptions reasonably valid?
No water is drawn from the tank.
The ambient temperature is 70 F.
The average water temperature is 120 F.
Tank size 40 gallons.
When the tank is in normal use 12 kWh are used per day and this includes losses.
Losses are 1.2 kWh per day when no water is drawn.
A single heating element of 3 kW is used for heating.
Three times per day the heating element is energized to maintain the average temperature.
The heating times are equally distributed thru the day, and are asynchronous relative to sleep time.
Sleep time is 8 hours.
Total on time hours per day is 0.4 when no water is drawn. Or 0.1333 hours or 8 minutes every 8 hours. Average power within an hour 3*0.1333 = 400 watts. Thus, if the on time is fully contained within an hour, then an 0.4 kWh bump should occur within one hour in the 8 hour sleep time. The hour of occurrence will randomly occur from day to day. Thus, averaging over 30 days sleep time energy use can be expected to rise from other base load by 0.4/8 = 0.05 kWh. Hard to see from other variations.
Any suggestions to make for better values to use, and/or are there any mistakes?
.
Question on electric water heater energy loss.
Are the following assumptions reasonably valid?
No water is drawn from the tank.
The ambient temperature is 70 F.
The average water temperature is 120 F.
Tank size 40 gallons.
When the tank is in normal use 12 kWh are used per day and this includes losses.
Losses are 1.2 kWh per day when no water is drawn.
A single heating element of 3 kW is used for heating.
Three times per day the heating element is energized to maintain the average temperature.
The heating times are equally distributed thru the day, and are asynchronous relative to sleep time.
Sleep time is 8 hours.
Total on time hours per day is 0.4 when no water is drawn. Or 0.1333 hours or 8 minutes every 8 hours. Average power within an hour 3*0.1333 = 400 watts. Thus, if the on time is fully contained within an hour, then an 0.4 kWh bump should occur within one hour in the 8 hour sleep time. The hour of occurrence will randomly occur from day to day. Thus, averaging over 30 days sleep time energy use can be expected to rise from other base load by 0.4/8 = 0.05 kWh. Hard to see from other variations.
Any suggestions to make for better values to use, and/or are there any mistakes?
.
