When should you charge an electric car at home?

An electric car is the largest single electrical load most households will ever add, and also the easiest to move. A commute of 40 miles takes roughly 12 kWh — more than a dryer, more than a dishwasher, comparable to a day of air conditioning. Unlike any of those, it can be scheduled with total indifference, because the car is parked either way.

Why the middle of the night, specifically

Three reasons, in descending order of importance:

1. The off-peak price is lowest then on nearly every tariff. Peak windows almost always sit in the late afternoon and evening, and the cheapest hours are between midnight and 6am.
2. You avoid the shoulder. Many plans have a mid-peak or partial-peak period running until 11pm or midnight. Plugging in at 8pm and letting the car start immediately often means paying mid-peak for the first hours.
3. You avoid coinciding with everything else. This matters on any plan with a demand charge, where the car running at the same time as the dryer and the oven sets your billed peak for the whole month.

There is a fourth reason that has nothing to do with your bill: overnight charging draws on a grid running mostly on baseload generation, and in most regions the average carbon intensity of electricity is lower at night than during the evening ramp.

How much is it worth?

The honest answer is that it depends on your utility by an order of magnitude, which is exactly why generic advice is not much use. Work it out like this:

That is an upper bound, since you will not charge every single day and you may not be charging entirely at peak today. But it makes the point: on a steep plan, charging habits are worth more than almost any efficiency measure you could buy.

For the extreme case, on Southern California Edison Co’s Time-of-use Tiered Domestic (NEM 2.0): TOU-D-A-CPP, each kilowatt-hour a day moved out of the expensive window is worth roughly $532 a year. A 12 kWh charge moved from the worst hour to the best would be four figures.

Plans built for cars

Many utilities file tariffs aimed specifically at EV owners, usually with a much cheaper overnight rate and a correspondingly harsher peak. These are the steepest in our index:

Some of these meter the car separately, which means the cheap rate applies only to the charger and the rest of the house stays on its existing plan. That removes the usual downside of an EV tariff — the punitive peak applying to your air conditioning too — at the cost of installing a second meter.

Setting it up so you do not have to think about it

• Schedule in the car, not the charger, if you have the choice. The car’s scheduler knows the battery state and will stop when full; a dumb timer on the charger will not.
• Set a departure time rather than a start time if your car supports it. Preconditioning then happens on grid power at the end of the cheap window rather than off the battery.
• Consider capping the amperage. Charging at 24 amps instead of 40 still adds well over 100 miles overnight and cuts several kilowatts off your household peak.
• Check the weekend schedule. Many plans price weekends off-peak all day, so the constraint you have carefully automated on weekdays may not apply on Saturday.