The following text explains the makeup and value of the graphs produced by ERA in their monthly reports.  The example used is for one of our premier clients, John Muir Medical Center, and is real data that is included in ERA's monthly management reports to JMMC.  This information is presented with full knowledge and approval of JMMC.

In the presentation of this report, we have included links to the actual graphs and when "clicked" will open a new window displaying the graph or schematic.  You may then scroll, resize, maximize/minimize, move and close this new window in your browser.  For instance you could minimize the new (graph) window and read some text and then maximize it to view the actual graph.  These graphs will be displayed in Adobe Acrobat Reader format, and the reader can be obtained direct from Adobe.  Just click on the Logo below, download the reader from Adobe (version 3.0 or 4.0), exit your browser, install the reader and restart your browser.




Three special graphs are prepared as part of the regular energy accounting procedures each month. These graphs are intended to show long term trends in energy use at the Medical Center and the effects of building operating practices and other influences on energy use. These special graphs include a "long term trend analysis", "energy units saved" and "energy cost saved", and will each be discussed below, in-turn.


This line graph shows the energy used at the Medical Center since 1987. The data points plotted are each 12-month totals. That is, the data displayed, for example for January 1987, is the total consumption for the 12 months ending January 1987. This method of displaying the data is very helpful for analyzing such things as energy use which have seasonal variations. By plotting 12 month totals, seasonal variations in the data are removed (as each data point contains all seasons) and the effects of other influences are then revealed. However, since the data only shows long term effects, the data have a unique appearance when short-term changes take place. For example, when examining the square footage plot, it can be seen that even though Phase-3 was occupied in late 1989, it takes 12 data points to show the full effect of adding that space to the facility. Similarly, when 4 &, 5 West were occupied in early 1992, this change in square footage also took 12 months to be fully shown in the plot.

Examining the graph, a few conclusions can be drawn. 

First of all, looking at the energy use prior to late 1989, it can be seen that, without any overt efforts at energy conservation, both electric and gas use grew at rates of approximately 3% and 5% respectively per year. This is commonly referred to as "creep" and results from more and more medical equipment (both large and small, including ancillary equipment thereto) being added to the facility and the normal slight deterioration of the building’s heating and air conditioning equipment over time.

Secondly, while it is very clear when Phase-3 was occupied, the energy use for this facility lags behind somewhat as occupancy is not an instantaneous process. Not every department can move in on the same day and heating and air conditioning systems must be turned on for the whole building even though it is not yet fully occupied, and full "occupancy" is also not immediate in terms of patients, staff and medical and other equipment.

Finally, JMMC began an energy conservation program in late 1990, which just began to take effect during 1991 (during the period that Phase-3 was still in the process of coming to full "occupancy"). As full occupancy appears from the graphs to have been achieved around October 1992, this date was chosen as the "starting" point for monitoring the results of energy conservation efforts. At this point in the graphs, 2 additional lines have been plotted, which assume that the trends that existed from 1987 through 1990 would have continued had no overt conservation efforts been made. These plots are entitled "ELEC EXPECTED" and "GAS EXPECTED".


This double bar graph shows the energy used saved at the Medical Center since October 1992. The data points plotted are each 12-month totals (though of course the measurement didn’t start until October 1992 so the initial totals are quite small). By October 1993, the energy conservation efforts were producing energy savings amounting to approximately 750,000 kwh per year of electricity and 90,000 therms per year of natural gas.


This single bar graph shows the energy costs saved at the Medical Center since October 1992. The data points plotted are again each 12-month totals. As can be seen, by October 1993, the energy conservation efforts were producing energy costs savings amounting to approximately $100,000 per year.


Some confusion can arise in the business of energy conservation regarding the concepts of "savings" as opposed to "avoided cost". For example, if utility rates rise by 10% in a given year (electric rates went up some 14% in 1992, for example), reducing energy use by 10% during that year would produce a total cost for the year that is the same as the prior year – in other words, no apparent "savings" would have been produced. However, had no conservation actions been taken, total costs would have risen by 10%. Therefore, an additional cost of 10% was avoided - which is the same as a savings (even though the total cost stayed the same). Similarly at the Medical Center, in years where the conservation efforts more than offset the normal "creep" in energy use, a reduction in energy costs is actually achieved. However, even in years where the total cost is still unchanged, a cost avoidance of 3 to 5% is still being achieved.


In addition to the graphs described above, one additional graph is produced each month, but is not intended for presentation to management. This graph is the ENERGY PURCHASED graph. It is not intended to be given to management because the difference between this graph and the ENERGY USED graph is exceedingly subtle and technical and will almost certainly serve to confuse non-technical people (as well as some technical people). For the technical folks, however, we will explain the difference in the following.

Energy Purchased.

This graph displays the energy actually purchased from the various energy vendors, Pacific Gas & Electric and Enron Gas. This is the energy that passes through the electric and gas meters at the boundary of the building. Referring to the accompanying Typical Building Schematic, (view Schematic) this energy is that which is measured at points P-1 and P-2 where the electric and gas passes through the "Energy Purchased" boundary.

Energy Used.

In most buildings, the energy used is equal to the energy purchased and no further consideration of the subject is needed. However, in a building which is equipped with cogeneration, some further consideration is warranted.

Further consideration is warranted because the use of cogeneration is an energy production option, not a facilities operation option. By employing cogeneration a facility is choosing to act like a utility and produce their own energy. This can make sense because virtually all utility generating plants operate at about 30% overall efficiency, including nuclear plants! If a facility can make use of the waste heat from the generating process they can produce electricity at 75 to 85% overall efficiency and reduce the cost of their electrical and heating energy. While this is a good thing, often the allure of the technology of cogeneration can mask energy end-use inefficiencies, and even cause "false" end use loads to be perpetuated in the name of keeping the cogeneration equipment in full operation. In addition, even if this is not the case, employment of cogeneration can confuse and "muddy" energy consumption data making it difficult to know whether energy end-uses are efficient or inefficient. The data gets muddied because the cogeneration equipment is located downstream of the utility company meters and because the cogeneration equipment consumes natural gas in order to produce electrical energy. Finally, cogeneration equipment is not 100% reliable and may be unavailable for periods of time. The net result is that just looking at the utility bills no longer gives a clear picture of the efficiency of plant operations.

Therefore, to manage plant operations effectively, it is important to know not just how much energy was purchased, but also how much energy was consumed for end uses, such as lighting, heating, etc., not for producing power. In order to accomplish this, certain steps must be taken to adjust or correct the quantities of energy purchased quantities to show the quantities of energy consumed for end uses. The first step is to define a new boundary where the units of energy will be measured. This is done by moving the boundary so that the cogeneration equipment is now outside the boundary (as though it is part of the utility company’s power production and distribution system). Referring again to the accompanying Typical Building Schematic, this boundary is called the "Energy Used" boundary and the energy used is that which is measured at points U-1, U-2 and U-3 where the electric, gas and heat passes through the boundary.

Quantification of the energy used is straightforward, but not necessarily immediately obvious nor intuitive.

Electrical energy used, as can be observed from the schematic, is that which passes through point U-1 and is simply the sum of the electricity purchased from the utility plus the electricity produced by the cogeneration equipment.

Heating energy used, as can be observed from the schematic, is that which passes through points U-2 and U-3, as follows:

  • the energy passing through point U-3 is simply the gas purchased from the utility less the gas consumed by the cogeneration equipment
  • the energy passing through point U-2 is simply the heat produced by the cogeneration equipment

However, even though it is possible to physically measure or meter the energy passing these points, stopping at this level of detail is not accurate. First of all, the cogeneration equipment still wastes some of the energy it uses, just like the heating boiler. Measuring just the heat produced by the cogeneration equipment would be just as inaccurate as measuring the heat produced by the boiler instead of the gas consumed by the boiler. In addition, it would be good for the data we are assembling to be equivalent to how the plant would operate without the cogeneration equipment in existence – so we can make valid comparisons to past periods without cogeneration and in future periods should the cogeneration equipment not be available for short or long periods of time.

Accordingly, a more correct way of valuing the heating energy produced by the cogeneration equipment is to make it equivalent to that which it substitutes for and take into account the efficiency of the heating boiler – by dividing the quantity of heating energy produced by the cogeneration equipment by the boiler’s efficiency.

A further issue is the fact that heating energy (or ”btu") meters are notoriously unreliable and inaccurate. In addition, the fewer meters that need to be read on a regular basis and the fewer readings we need to deal with in our data gathering, manipulation and analysis, the easier our collective task will be and the more consistent and accurate the resulting data will be. This being so, it would be nice to just use the data from the cogeneration electric meter to establish the value for the equivalent boiler gas that would have been consumed had the cogeneration equipment not existed. Fortunately, this can be done fairly easily, if not obviously.

The cogeneration equipment uses gas to produce both electricity and useful heat. It does this in a very consistent fashion, there being a quite reliable relationship for each machine between the gas consumed and the electricity and heat produced, as follows:


Tecogen CM-60

Gas Used:

7.80 therms/hour

Power Out:

60 kw

Heat Out:

4.4 therms/hour

We can calculate from the above the values of (in therms per kilowatthour) the total gas used, the boiler gas which is displaced and the remainder of the gas which can be allocated to producing electricity, as follows:

Total Gas Used (Gas in)              =

 7.80 therms/hour / 60 kw                           =

.130 therms/kwhr

Boiler Gas Displaced (Heat out)  =

4.40 therms/hour / 60 kw / 80% boiler eff.   =

.092 therms/kwhr

Gas for Electricity (Elec out)        =

 .130 therms/kwhr- .092 therms/kwhr          =

.038 therms/Kwhr

 Finally, then we can convert or adjust our electric and gas bills from the utility company from units of energy purchased to units of energy used by just two actions, as follows:

  • First, add the electricity produced by the cogeneration equipment to the electricity purchased from the utility company
  • Second, we can take the gas purchased from the utility and either:

ˇ         subtract the gas consumed by the cogen equipment and simultaneously add the equivalent heat produced by the cogeneration equipment, or....

ˇ         subtract the gas allocated to producing electricity in the cogeneration equipment (since this is equal to the above difference between the gas consumed by the cogeneration equipment and the equivalent heat produced)

since the latter of these two options allows us to use a single meter (the cogeneration output electric meter) to perform all our adjustments, we will use this method.


It is of great value having an analysis which utilizes energy units used because we can ignore the effects of the cogeneration equipment when interpreting such data since the effects of the cogeneration equipment have been completely removed from this data through the adjustments described above.



All material CopyrightŠ1998, 1999 Energy Resource Associates, Inc.
All rights reserved.

Site design by Design