I have seen completely two different answers to the below question on this website. A small machine shop, with 30 hp of

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I have seen completely two different answers to the below question on this website.
A small machine shop, with 30 hp of connected load, purchases electricity at the following monthly rates (assume any demand charge is included in this schedule) per hp of connected load:
First 50 kWh at 12.6_c per kWh
Next 50 kWh at 10.6_c per kWh
Next 150 kWh at 9.0_c per kWh
Over 250 kWh at 7.7_c per kWh
The shop uses 2800 kWh per month.
a) Calculate the monthly bill for this shop. What are the marginal and average costs per kilowatt-hour?
b) A contract for additional business would require more operating hours per day. This will use an extra 1200 kWh per month. What is the “cost” of this additional energy? What is this per kilowatt-hour?
c) New machines would reduce the labor time required on certain operations. These will increase the connected load by 10 hp, but since they will operate only on certain special jobs, will add only 100 kWh per month. In a study to determine the economy of installing these new machines, what be considered as the cost of this energy? what is this per kilowatt-hour?