It doesn't matter how much it costs to store water. It costs money to heat the water, and that's what you have to pay attention to. This is much harder to work around as you don't have much choice when you use a hot water. Hot waters don't just store cold water in the tank to be heated when electricity prices are low. Instead, it'll start heating after you shower, for example.
I am in an apartment: I have a small water heater - I easily run out of water taking a shower if I stick around too long. There are two people in the house, and if we both want to shower in one day, the hot water . That way, I have hot water to do dishes and someone can take a shower in evening. I'm happy the water heater for the washer is separate, but again, it heats when the temperature dips and doesn't keep the water cold until electricity is cheap.
Even if the hot water heater waited to heat until prices were low, you are going to have water that is not-as-hot if you are the second person taking a shower. This is simply because as the tank empties, cold water is added to the tank. All subsequent hot water will be cooler.
I'll add that some folks sweat at night and must take a shower in the morning (I do when hormones act up. Night sweats are no joke).
I'm in an apartment: I cannot update the hot water heater. It literally cannot be done for me and millions of others because we cannot replace such things. You'd have to mandate landlords do improvements, and even then, it'll be years before it is all upgraded.
I'd argue that the real answer is to go with tankless heaters in this case: It isn't that you'll be spending money when the electricity is cheap, but you'd use less overall for the actual heating of water.