Typically, the “brownest” utility energy is produced during peak power consumption—usually during the daytime—when “peaker” power plants have to be brought online to supply the additional demand for electricity. These power plants are typically powered by diesel or other petroleum fuel.

At night, when the grid is not being drawn on as heavily, utility power tends to be greener. This can be especially true if utilities are tapping into wind power, which could be used to charge EVs.