After hours energy use – a great place to start

28 Jun 2015 by Peter

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One of the biggest challenges of any commercial building energy efficiency program can be deciding where to start. This can be tough even if you only have one property, but for managers of large portfolios it can be particularly tricky. Whilst buildings generally share some similarities in their infrastructure – walls, roof, HVAC, lighting, plugs and people – there are often significant differences related to size, occupancy, hours of operation, age and function.

Of course, it’s these differences that can make it hard to come up with a portfolio wide approach to energy efficiency.

Data is your friend

Here at Greensense we spend a lot of time dreaming up ways to use data to help drive efficiency projects. If analysed and presented correctly data can be hugely powerful, both in terms of helping identify where you might begin on your efficiency quest, as well as providing the means to track and quantify performance improvements over time.

Aside: given the constantly changing externalities to building performance such as occupancy or weather, accurately monitoring performance improvements (or degradation) can present some significant challenges in its own right.

After hours energy use

So what does all this data tell us when it comes to figuring out where to begin your efficiency efforts?

One thing: Out-of-hours energy use; and here’s why.

To begin with, as the graphic below shows, most buildings are empty for the majority (~70%) of the time over the course of a year. As obvious as this sounds, it’s a very important consideration you’re looking at improving commercial building energy efficiency.

occupancy v energy

From a sustainability perspective, in the same way that a busy and productive building will inevitably use energy, there’s nothing wrong with having an empty building, just so long as you’re not consuming any resources while it’s sitting idle. Unfortunately, that is never the case, far from it in fact.

Most buildings have relatively high levels of “base load”, meaning they continue to consume significant amounts of power even when they’re empty. We typically see over 50% of the total electricity used over a year going into empty buildings. That’s from our own dataset of more than 1,000 commercial buildings.

Understanding the cost

As always in discussions around building (in)efficiency, when it comes to costs we need to consider both the environmental and the financial implications. The environmental cost is pretty easy to work out. In Australia, each kilowatt-hour of electricity poured into an empty building generates around 0.9kg** of greenhouse gas emissions. Of course you might be subsidising your wasted electricity through some onsite renewables, most likely solar, but remember that the majority of this out-of-hours consumption is going to occur at night when your PV can’t help.

The financial cost is a little trickier to calculate, as it will depend on the tariff for each building.

In order to explore the cost question a little further, and at the same time to illustrate the value in understanding your building’s energy profile, we’ve put together a graphic to illustrate how significant out-of-hours consumption can be to the financial bottom line.

As an aside, it also demonstrates the value of benchmarking and comparison as tools to help you put the performance of your buildings in context.

In the graphic below we compare the average weekly energy consumption for three local government administration buildings. The buildings are located within a few kilometers of each other in a major Australian city, are of very similar size, have very similar operating hours and perform the same function. In many ways they are typical examples of commercial office buildings.

profile comparison councils

Despite the similarities, the amount of energy going into these buildings during out-of-hours periods varies hugely, with correspondingly significant implications in terms of operating costs. Even the “best” performing building here is spending over $100,000 a year powering an empty building!

What the graphic also shows clearly is that, while the benchmarking is useful in putting the after-hours performance of each building in context, the 7-day profile is also crucial in understanding when that energy is being used.

So where is all that energy going?

Exactly where all that out-of-hours electricity is going will vary a bit from building to building but, even without sub-metering data, it’s easy enough to make some informed guesses.

In a typical office building there are three main energy sinks: HVAC (50%), lighting (25%) and plug loads (25%)***.  HVAC is normally pretty well centralised in most larger offices, although you might still have a few split-system units around the place, and lighting control is increasingly centralised too. So, while they may be playing a part in the story, it will most likely be plug loads that are the real culprit and must inevitably form the focus of any out-of-hours performance improvements. More on that in the next article.

It just makes sense

Sometimes it’s important to be pragmatic when you’re looking to roll out environmental programs, and improving commercial building energy efficiency is no different.

There will likely be a number of (often competing) project options, whether it’s covering the building in solar panels or turning up the set point on the A/C. While many of these projects will have merit, a number will present significant obstacles such as upfront cost (solar panels) or push back from building occupants (A/C tweaks).

By starting your energy saving quest by focusing on out-of-hours use though, you remove most of these barriers. First and foremost, nobody can deny that pouring energy into an empty building is a dumb and wasteful thing to do. Secondly, the steps taken to reduce this waste are often relatively uncomplicated. Why? Because the building is empty for a start. It’s much easier to affect change in an empty building, as you don’t have to worry about upsetting the occupants. Coupled with that, the savings you’ll find are often from simple, operational tweaks such as shutting down printers and PCs at night.

Wrap up

For those aspiring to improve commercial building energy efficiency, getting to grips with out-of-hours consumption is an obvious, and often rewarding, place to start.

By focusing on one particular element of building performance, it helps bring direction and clarity to your efficiency program. All you need to get going is some decent quality interval data and the ability to analyse it in a way that helps you determine how much of your precious and increasingly expensive energy is going into empty buildings. I promise, the results will be startling and should provide all the incentive you need to start tracking down those savings.


*A 365 day year is composed of 8760 hours. Of those 365 days, only around 250, or 2500 hours (assuming a 10 hour operating day), are working days. The rest are weekends and public holiday. This means that the majority of commercial buildings are actually only operating for about 28% of the year. For the other 72% of the time they’re sat empty.
** Based on the 2012 NGA factors
*** 2006 review of LEED-NC v2 energy modeling

For more tips on reducing Out-of-Hours energy use, download this free ebook:

Ebook cover

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