Download Andy Shapiro‘s FREE Water and Hot Water Usage Estimator!

Hot Water

This tool is used in Building Energy Analytics: Measuring and Understanding Building Energy Use course, approved for 10 AIA LU/HSW Credits , 15 Building Performance Institute (BPI) CEUs , 6 GBCI credits (CE Hours for LEED Professionals)

This spreadsheet calculator has a very simple calculator at the top right, based on typical usage by number of bedrooms. It also has a detailed calculator that allows you to build an estimate from the “ground up,” looking at each usage, how often it occurs, what temperature the usage is and how much water each usage represents. It also takes into account the cold water temperature, the hot water tank temperature and the usage temperature. The detailed estimator can also be used to estimate total water usage.  

The second tab of the spreadsheet makes a comparison I am often asked about: is it more cost effective to heat water with electric resistance, a heat pump water heater or solar hot water? The “compare” tab allows you to compare the total installed cost of these three, including photovoltaics (PV) to provide the amount of electricity required for each of the three. This allows you to look at the least cost approach to net zero domestic water heating. I encourage you to play with the input variables to see under what conditions each type is most cost-effective. Other variables not included here are lifetime of different water heaters, embodied energy of water heaters and PV, global warming potential of refrigerants and others. The answers might also inform your measurement of hot water energy for these three types of water heating.

An important part of understanding how buildings, or systems within them, are performing, is to compare your expectations to what you actually measure. Sometimes a discrepancy between these two can inform your expectations, and sometimes a discrepancy will point out an error in measurement or analysis! As you do more and more measurements and more and more estimates of usage, your “sense of smell” will improve — that is, you will get a feel for how big measured and predicted values should be, building understanding of how buildings actually work!

Determining domestic hot water usage in a residence can sometimes by done by looking at actual monthly energy usage from bills, and can also be done by measuring energy usage of the water heater directly or indirectly. You can set your expectations by using a very simple tool to estimate usage, based on typical usage for a number of bedrooms or number of people in the home.


Andy Shapiro

With a B.A. in engineering from Brown University, Andy Shapiro, through his company Energy Balance, Inc, has been providing energy consulting and design services to a wide variety of clients since 1988. He works with homeowners, architects, engineers and builders, as well as towns, landlords, non-profit housing organizations, and electric utilities. Services range from house-doctoring to sustainable building design and include research projects as well as utility program design and impact evaluation. He is the author of The Homeowners Complete Guide to Add-On Solar Greenhouses and Sunspaces (Rodale Press, 1985). He became VEEP’s energy scientist in 1994. Shapiro provides a level of technical expertise unusual in classrooms and along with Barhydt is helping to enhance science teaching in New England schools. Andy lives in Montpelier.

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