How do you calculate power usage? (Utility costs)

[kjm4422]

Power calculations

Dear Sir or Madam: I was hoping you could help me. I am responsible for developing the bill we charge commercial office tenants for what we call “overtime utilities”. Which is an estimate of how much power they use if they work nights or weekends. Generally utilities are included as a part of the rent but only for Monday through Friday for 10 hours a day. After that, you have to pay.

Back years ago this was never much of an issue but now everyone is keeping a very close eye on the money and with the cost of electricity going up so much in some markets it has come under notice.

90% of the costs are electrical and 90% of that is for air conditioning. We always used to add up the horsepower of the motors we had to run to deliver the ventilation. We would then come up with some run factor (usually 90%). Add up the number of hours they wanted us to run it and then make this “formula”

(motor load) = 25hp x .746kw = 18.65 kw
times the run factor 18.65 x .9 = 16.78 kw
times the number of hours they want to run it:
16.78 kw x 12 hrs = 201.42 kwh.

We would then charge them whatever the kwh charge is on the electric bill, 8 or 9 cents a kwh. We never charged them for the demand costs because we did know how to figure it out.

The only other major item we charged them for was running the chiller. We would calculate # tons (size of chiller) x 12,000 = BTUs 175 tons x 12,000 = 2,100,000 BTUs

Then to find kW we would use the conversion factor of 3,412 BTUs = 1kw so;
2,100,000 / 3,412 = 615kw times the number of hours and a run factor if we thought the machine wouldn’t run fully loaded. Then it would be 615 kW x 12 = 7380 kWh. So for this particular bill we are looking at a bill of 7581 kWh x .08 = $606.00 to run the HVAC system in a commercial office building. It doesn’t seem unreasonable.

Now comes a chorus of engineers and each one telling us a different way to find the costs for these utility charges. One more complicated than the next. What I am hoping to learn from you is some direction or “tips” in this matter. Just remember these are Property Managers and they are not going to figure out the power factor or the square root of three. It would need to be defensible and easy. Any ideas? Thanks. k

 

[jbartos]

Suggestion: The cost of electricity related to cost of rented space per square foot is normally a small ratio. The above mentioned figure of 615kW probably cools very large square footage considering that 10000BTU or 2.9kW can cool about 700 sq feet apartment. This aspect and overall rent collected is not also mentioned in the original posting.

 

[busbar]

One could do a temporary time-of-use load-profile study with portable instruments, or install permanent time-of-use energy metering to monitor the subject loads. Recurring costs for obtaining meter data should be included in the project scope of work.

In some areas where transfer of funds are involved, the security of metering components and data retrieval may have to meet the standards levied on electric utilities.

 

[kjm4422]

Sorry. I should of mentioned the fact that we often need to provide HVAC to a large area of the building to serve just one space. Usually half or a third of the building at least. thanks. km

 

[peterb]

Here's a suggestion for you - read the energy meter at the start and end of each "overtime" period. This will give you the actual kWH used to provide the service, which should be a reasonable basis for the charge.
Of course, if you have more than one tenant working overtime at any given time then you will have to come up with a formula for sharing out the costs - perhaps based on the area served for each?

 

[Rocketir]

KISS
I total agree with peterb. It sounds the most logical.

 

[jghrist]

A more defensible solution would be to meter the common A/C load and the circuits to the individual tenants' offices.

The A/C would operate regardless of whether or not the tenant worked nights and weekends. If the thermostat is normally setback during nights and weekends, there would still be some usage. If it is not setback, then the increased usage is only because of extra heat load of people and equipment operating. Charging the entire A/C cost does not seem reasonable.

 

[peebee]

Most commercial spaces in the Chicago area here put in landlord-owned chilled water metering for off-hours HVAC use. Electricity is usually billed directly to the tenants by the utiltiy company, separate utility meters are provided for each tenant space. Landlord electricity expenses required to provide off-hours HVAC are usually not charged to the tenant (unless some adder is included in the chilled water metering).

There are very few exceptions in this market area.