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Sun Volt Solar

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Clean Energy, Clean Environment

We are at a cross roads in human history, we can choose to continue on as we have been, or we can make a change to improve our future and quite possibly the future for several generations to come. We are here to promote energy independence, a better environment, a secure future and a higher return on investment for your hard earned dollar. It is what I believe in, it is why I am in the solar business.

Calculating energy needed to heat water

05 May 09 | Solar Hot Water, solar thermal

In order to properly size a Solar Domestic Hot Water (SDHW) system, a few pieces of information are needed:

  1. Current and future occupants of the house or average hot water use.
  2. Water supply temperature
  3. Desired hot water temperature
  4. Stand by loss of heating unit

We know that in this area (Mid Hudson Valley) ground water temperature averages 53 degrees.  I know this because I have personally measured the well water temperature at all of our SDHW installations.  This is a good starting point.

Most people desire their hot water temperature to be between 110 to 120 degrees.  There are some applications where hotter water (laundry, dish washers, etc) is desired.  For general purposes 115 degrees is a good ending point.

We also can base average hot water useage on the number of occupants of any house.  The rule of thumb is 20 gallons per person for the first two people, 15 gallons per person for any additional people.  This means that the average family of four uses 70 gallons of hot water per day (20+20+15+15 = 70).

Standby losses for water heaters generally range from 5-10% for electric and oil fired systems and 40% for natural gas or propane water tanks.

For the purposes of Solar Hot Water, an appropriate unit of energy would be the BTU.  If we were using SI units (metric) it would be the Mega Joule (MJ).  Since most HVAC contractors understand things in terms of BTUs, it is easiest to use this unit.

A BTU is defined as amount of heat required to raise the temperature of one pound of liquid water by one degree Fahrenheit.   That is close enough for our purposes.

Therefore, the formula to calculate energy use is:

BTUneeded= 8.34 x Gallons x (desired°F-supply°F) x Standby

Where:

  • BTUneeded = BTUs needed to heat the water for one day
  • 8.34 = Weight in pounds of one gallon of water
  • Gallons = Gallons of hot water used in one day
  • desired°F= Desired temperature of the hot water
  • supply°F= Cold water supply temperature
  • Standby= Standby loss of the heating appliance

A typical family of four heating their hot water with electric or oil would expect to use:

BTUneeded = 8.34 x 80 x (115°F-53°F) x 1.10 = 45,503 BTU/day

A typical family of four heating their hot water with gas or propane would expect to use

BTUneeded = 8.34 x 80 x (115°F-53°F) x 1.40 = 57,913 BTU/day

To get an idea of cost, BTUs need to be converted to energy units that are used for electricity, oil, and gas.

  • Electricity has 3412 BTU per kWh.  Therefore 45,503 ÷ 3412 = 13.3 kWh.  Going rate per kWh is about $0.16.  13.3 kWh x $0.16 = $2.13 per day or $778.83 per year
  • Heating oil has 138,700 BTU per gallon.  Therefore 45,403 ÷ 138,700 = 0.33 gallons.  Going rate per gallon $2.459.  0.33 gallons  x $2.459 = $0.81 per day or $269.19 per year.
  • Propane has 93,000 BTU per gallon.  Therefore 57,913 ÷ 93,000 = 0.62 gallons.  Going rate per gallon $2.428.  0.62 gallons  x $2.428 = $1.51 per day or $549.46 per year.
  • Natural gas has 102,000 BTU per CCF.  Therefore 57,913 ÷ 102,000 = 0.56 CCF.  Going rate per CCF is $1.633.  0.56 CCF x $1.633 = $0.93 per day or $338.42 per year.

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05 May 09 | Solar Hot Water, solar thermal
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