Explain how you know that the amount of electricity used is not independent

I am not particular sure under what context you are asking, but I will do the best I can. This is how electricity gets to your home:
1. Electricity is created at some form of generator or power plant.
2. Electricity is then converted to a very high voltage and transmitted.
3. The electricity is then distributed or doled via power lines to businesses, homes, etcetera
4. We then use the electricity at our homes.
I hope I was helpful in some way. You might want to check out energy.gov

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