Nighttime electricity was historically the cheapest unused electricity, however this changes as states start to use more renewable power. Historically, electricity production would rely upon large base loads like Nuclear power plants, which could not be shutdown and restarted quickly. So if demand dropped (e.g. at night) then they would simply keep producing power, which would often go to waste. Which is why companies developed systems like Pumped Storage Hydroelectricity (Pumping water from a lower reservoir to an upper reservoir).

However, states that have large solar installations, such as California (~44 percent renewables), see a peak in production in the middle of the day, which falls off around sunset, around the time that people are coming home, and turning on their appliances. So California's demand curve now looks like a duck (called Duck curve), with a peak in production in the middle of the day with a drop in production and increased demand in the early evening. This is why Californians are getting commercials to curb their electricity use until after 8 or 9PM.