Photo Illustration courtesy DandeChiaro

New Billcast feature allows each building to explore relationship between occupancy and energy use

If you’ve ever been in a crowded conference room, you know that each occupant produces heat in a building. Something on the order of 70 Watts per person. Add a laptop, monitor, phone and lighting and it’s clear that occupants drive load in buildings. It follows that when tenants leave a building, energy use should drop. Right?

As usual, it’s more complicated than that. A variety of factors determine the actual relationship between occupancy and energy use:

  • Type of tenant: 20 years ago, a tenant was a tenant. Now small changes in occupancy can drive large changes in energy use, in part it is because occupancy only maps roughly to energy use. For example, a small tenant with heavy computing needs might drive far more energy use than a larger but more conventional tenant.
  • HVAC system: a building’s physical plant reflects years of buildouts and changes. Even the best engineer can’t block off a space that it is shared with an active tenant. Buildings with central air handlers and chillers in particular show limited energy savings from small occupancy reductions.
  • Management practices: property managers vary greatly in how they treat unoccupied space. In a frothy real estate market, some property managers choose to keep space conditioned to make the best possible impression. On the other hand, if the same building is riding out a weak market, it probably skips the conditioning of empty space.

Easily track occupancy-related energy use in Billcast

Current practices in forecasting and energy tracking vary widely. Many buildings create budgets using a formula that divides energy use into fixed and variable components. Others ignore these factors. Almost no building can produce data that shows a reliable historical relationship between occupancy and energy use.

Until now.

Working with our existing customers, we’ve added occupancy tracking to Billcast. Simply enter your monthly occupancy figures — actual or forecast — and we’ll track it against energy use. Below is an example that shows reduced energy use starting in April 2013 as occupancy falls.

What makes occupancy tracking in Billcast different than just using a spreadsheet? For starters, we normalize your energy data to remove weather effects. In general, the large temperature fluctuations from summer to winter or from one year to the next are enough to swamp any effect from tenant changes. By isolating non-weather dependent load, we reveal the true relationship between occupancy and energy use.

In the example chart, weather-dependent energy use is shown in light blue and non-weather dependent operational use is in dark blue. The orange line showing monthly occupancy tracks well to energy use, but only when weather-dependent load is removed. This is critical — a property manager looking at raw bills would not see the relationship (or savings) from reduced occupancy.


Weather-normalizing energy use reveals the relationship between occupancy and energy use

If you’ve entered square footage for your building, Billcast will now compute and track weather-normalized energy expenses, demand and usage per occupied square foot, providing a fair basis of comparison across time periods or buildings.

For customers, tracking occupancy on a quantitative basis is one more step in understanding your building. With Billcast, you get a clean signal from the noise so you can continue to lower your energy expenses.

About Tom Arnold

Tom Arnold is co-founder and CEO of Gridium. Prior to Gridium, Tom Arnold was the Vice President of Energy Efficiency at EnerNOC, and cofounder at TerraPass. Tom has an MBA from the Wharton School of Business at the University of Pennsylvania and a BA in Economics from Dartmouth College. When he isn't thinking about the future of buildings, he enjoys riding his bike and chasing after his two daughters.

You may also be interested in...

Are You on the Wrong Utility Rate? (Hint: Probably.)

Attention building owners & operators in California: do you know what energy rate each of your buildings is on? And, more importantly, are you sure those rates are the most-cost effective for each building, given their unique operations?