Controlling Electrical Costs for a Subway

Subway_Electrical_Peak_Load_Duration_ChartSubway restaurants in Southern California spend $12,000 to $19,000 per year on electricity. Typically they fall in the medium sized business rate schedule, over 20 KW. In this rate schedule you are billed for the energy used KWh, and for the rate at which you use it, demand KW. Demand (KW) is analogous to the speedometer on a car, and the highest speed recorded all month is what you are charged for. A Subway usually is 20 KW to 35 KW during summer months, and earns them the GS-2 rate schedule. If the demand (KW) can be kept below 20 KW they can qualify for the GS-1, small business rate schedule and save $2,000 to $4,000 per year.

Demand Control is how you stay under 20 KW. Automatically stagger loads, so that high rates of use are not incurred. This sounds scary, though the reality is that a Subway is over 20 KW about 90 hours per year. That is one or two 15 minute intervals during a summer day. This means that an electrical water heater or air conditioner onSubway_Electrical_Demand_Frequencyly needs to be delayed for a few minutes. Not a huge inconvenience for $4,000 per year per store.

To perform an anlysis to determine the financial impact of a demand contrtol system we need the 15 minute interval data from the utility (this can usually be downloaded from their website). We can then calculate how often we would need to intervene, and what the savings will be. Subway's usually have a straight cash payback period of 1.5-3 years.

One of the metrics used to quantify savings potential is the Load Duration. In the example above from a Subway in the Inland Empire, they are above 20 KW only 2% of the time. The Demand Frequency vs Time of Day is also a good metric. In this example from a Subway also from the Inland Empire it shows that most of the time (red and green colors) they are below 20 KW.

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