Two bills wearing one envelope
Energy is kilowatt-hours and rises with cars. Demand is the highest kilowatt draw the meter saw in any interval that month and rises with coincidence. At the defaults the site uses 20,522 kWh in a month costing 2,667.83 dollars, and pays another 1,613.97 dollars for a 115.3 kW peak. That is 38 percent of the bill sitting in a column that does not care how many cars were washed.
The consequence is that the headline rate on the bill understates what a kilowatt-hour really costs. Here the tariff says 13 cents and the blended figure is 20.9 cents.
Per-car cost falls when you are busy, with nothing changing
Wash half as many cars and the demand charge does not move, so it lands on half as many tickets. At 7,500 cars a month the electricity works out at 57.09 cents a car; at 3,750 it is 78.61 and at 11,250 it is 49.92. Nothing on the site changed. This is why a per-car energy figure quoted without the volume behind it cannot be compared to anything, and benchmark tables between sites almost never carry the volume.
Horsepower is not kilowatts
A nameplate horsepower is shaft output. The electrical input is 0.7457 kW divided by the motor efficiency, so at 90 percent one horsepower draws 0.829 kW rather than 0.746. Across the 115 nameplate horsepower on this site that difference alone is 9.5 kW, which at the default demand charge is 133 dollars a month.
The peak is the number to argue with
The page defaults to assuming every load can run at once, which is a ceiling and not a reading. Blowers, pumps, the conveyor, the reclaim skid and the lot lighting rarely coincide exactly, and most utilities can give you interval data showing what the meter actually recorded. Entering a measured 95 kW instead of the 115.3 kW connected total takes 284 dollars a month off the bill in this model. That is a phone call rather than a project.
Questions people ask
How many kWh does a car wash use per car?
At the defaults on this page, 2.74 kWh, of which about a quarter is the blowers and pumps that run per car and the rest is standing load spread over the day. Your own figure depends on your motor ratings, your run times and your volume, which is why all three are fields rather than constants. The standing share in particular changes with how busy the site is.
What is a demand charge and why does it matter at a car wash?
It is a monthly charge on the highest kilowatt draw the meter recorded in any interval, separate from the charge on kilowatt-hours. It matters at a wash because the connected load is large and lumpy, and because it does not fall when the site is quiet. At the defaults it is 38 percent of the bill and it is the reason the blended cost of a kilowatt-hour is 20.9 cents against a 13 cent tariff rate.
How do I turn motor horsepower into kilowatts?
Multiply by 0.7457 and divide by the motor efficiency, because nameplate horsepower is shaft output and the meter reads electrical input. At 90 percent efficiency one horsepower is 0.829 kW. Getting this backwards understates a large site by around ten percent, which at a demand charge is real money every month.
Why does my electricity cost per car go up in a slow month?
Because the demand half of the bill does not fall with volume. The energy charge tracks cars almost exactly and the demand charge is set by one peak interval, so a quiet month spreads the same demand charge over fewer tickets. The table on the page walks volume up and down so you can see how much of the per-car figure is that effect rather than anything on the site.
Should I use the connected load or a measured peak?
A measured peak, if you can get one. The connected total assumes every motor runs simultaneously, which is a ceiling rather than a reading, and most utilities can supply the interval data that shows what the meter really recorded. The page has a selector for it because the difference is normally worth more per month than anything else on the form.