Recording cost of kWh

Hi,

I have designed a pumping station on Simulink and have set up kWh (depending on how many pumps are running) and cost per kWh (depending on the time of the day as supply is from the electrical grid). I currently am using a scope to view the costs which give a good indication however I would like to have the costs add up so that after 24hours I can gain a total cost value for the day. This will easily show which pumping system has the cheapest running costs.

How can I get my running costs to add up throughout the day?

Cheers

Martin