Energy Saving Calculation for Gas and Electric Usage
Energy Saving Calculation for Gas and Electric Usage
(OP)
I am the Resource Advisor for 17 facilities, I monitor the utility usage for these buildings. I have the gas and electric usage for the last three years for each individual building. I wanted to calcuate the savings for the last 18 months since I've implented some saving techniques. I don't think just comparing months will due, because of cooling and heating day degree difference. If the month of June was an average of 82 degrees for 2003, and a average of 95 degrees for 2004, just comparing the two will not work. Does anyone know how to calculate heating and cooling days into a converstion for energy savings?





RE: Energy Saving Calculation for Gas and Electric Usage
http://www.vesma.com/ddd/whatthey.htm
RE: Energy Saving Calculation for Gas and Electric Usage
RE: Energy Saving Calculation for Gas and Electric Usage
Example April 2005 had 216 HDD(Heating Degree Days) and 41 CDD (Cooling Degree Days), whereas April 2006 had 134 HDD and 55 CDD.
For Building 1, my consumption for the month of April 2005 is 67,440 and my consumption for the month of April 2006 is 53,880.
What numbers do I use in this calculation to compare April 2005 with April 2006?
RE: Energy Saving Calculation for Gas and Electric Usage
but for example if the kWh listed was just cooling energy, your normalised answers would be:
April 2005 67440/41 = 1645 kWh/CDD
April 2006 53880/55 = 979 kWh/CDD
This is obviously not accurate, as your heating costs more than your cooling, but it shows the final method. I'm going to come back to this later - you can solve your relative heating and cooling cost simultaneously to estimate the proportion of each.
RE: Energy Saving Calculation for Gas and Electric Usage
OK, after some wildly assumptive maths:
If we assume that kWh is in direct proportion to the heating and cooling degree days, we have two april data sets:
216H + 41C = 67440
134H + 55C = 53880
giving H = 239 kWh/HDD 37.6%
and C = 397 kWh/CDD 62.4% our proportions.
I pause to correct my previous comment in that cooling uses more energy than heating in this scenario. Cost not involved here.
So the percentages applied to the kWh should roughly give us the proportional breakdown of the total energy:
April 2005
Heating 37.6% x 67440 = 25357
Cooling 62.4% x 67440 = 42083
April 2006
Heating 37.6% x 53880 = 20259
Cooling 62.4% x 53880 = 33621
Getting back to the original purpose of finding the normalised energy use:
April 2005
25357/216 = 117 kWh/HDD
42083/41 = 1026 kWh/CDD
April 2006
20259/134 = 151 kWh/HDD
33621/55 = 611 kWh/CDD
So comparing the two years figures, it looks like the heating energy went up a bit, while the cooling energy consumption seems to have dropped significantly.
Try this on a few other months and see if the proportionality is the same over the different months. I'd be interested to hear if this method looks valid.
Metering the heating energy separate from the cooling energy is a better way forward.
RE: Energy Saving Calculation for Gas and Electric Usage
RE: Energy Saving Calculation for Gas and Electric Usage
This is the proportion that each of the heating and cooling provide to the total kWh value.
RE: Energy Saving Calculation for Gas and Electric Usage
Did you check this against any of the other months to see if the proportions held?
RE: Energy Saving Calculation for Gas and Electric Usage
Clever approach, although the use of the same proportionality factors for 2005 and 2006 may not be appropriate in this case since there were energy savings measures taken between 2005 and 2006 which may have changed the proportionality of energy used for heating and cooling.
You are correct in saying that sub-metering is the best way to get a handle on the affect of the energy savings measures. I guess the next best way would be to calculate the degree-day factor for each utilization appliance (or group of appliances; or master meter) during months that do not contain mixed usage. This would be done by calculating the heating consumption per HDD for January 2005 and January 2006. Similarly the cooling degree-day factor can be found by calculating the consumption per CDD for August 2005 and 2006. The assumption here is that the energy consumption per degree-day is a characteristic of the particular appliance (or group of appliances; or master meter), which is a pretty good assumption, especially if the appliance operation is on-off and thermostat settings are reasonably constant. Using this method, the April 2005 heating consumption would be found by multiplying the April 2005 HDDs by the 2005 HDD factor. The April 2005 cooling consumption would be found by multiplying the April 2005 CDDs by the 2005 CDD factor. Similar results can be obtained for 2006.
At this point, you should be able to generate a monthly estimate of heating and cooling loads and you should be able to do a year over year comparison of your monthly energy savings.
RE: Energy Saving Calculation for Gas and Electric Usage
I'm not sure I follow your method given that the heating and cooling were bundled together.
Where you say...
"This would be done by calculating the heating consumption per HDD for January 2005 and January 2006"
...Do you actually mean total energy divided by HDD or CDD?
And won't the HDD/CDD factors suffer the same proportionality problem after improvements?
RE: Energy Saving Calculation for Gas and Electric Usage
To illustrate the method, assume a house that is heated by an electric space heater and cooled by an electric air conditioner, both of which are connected to the same electric meter.
The way to seperate out the appliance useage is to observe the meter during a period of time in which only one appliance is used and observe the number of DD for the same period. In my part of the world, the month of January never requires any air conditioning. So, if you take the meter reading for January and divide it by the cumulative DDs for January the resulting quotient would be the heating unit's DD factor (kWh/HDD).
The same could be said for the cooling load. In my part of the world, the month of August never requires any space heating so an air conditioning DD factor can be determined by using the procedure outlined above for August to get the air conditioning unit's DD factor (kWh/CDD).
Now, everything being equal (thermostat setting, appliance efficiency, building insulation, occupants etc.), the DD factors will remain reasonably constant and you should be able to use them to estimate the mixed heating and cooling consumption for any other month by using the relation:
Total Mixed Load = (HDD x kWh/HDD) + (CDD x kWh/CDD)
Note, if you undertake energy savings measures, you will need to determine new DD factors to account for the changes.
"And won't the HDD/CDD factors suffer the same proportionality problem after improvements?"
The HDD/CDD ratio is not used in the calculation.
RE: Energy Saving Calculation for Gas and Electric Usage
I perhaps falsely assumed that mdalle couldn't split out the heating and cooling in any month - due to, say, process cooling in winter, or hot water generation in summer etc.