Why the clock matters on some bills and not others
On a time-of-use (TOU) rate plan, the price of a kWh depends on when you use it. The same load of laundry or the same EV charging session can cost much more in the peak than it does after midnight; how much more depends on the plan, and each utility page on this site shows the exact price of every period. On a flat or tiered plan, it costs the same whenever you run it.
Many people don’t know which kind of plan they are on, or where its price boundaries fall. This calculator reads your utility’s actual tariff and shows the answer as a clock.
How this calculator works
You choose your utility and rate plan, then set these:
- The load: pick a preset (an EV top-up of 40 kWh at 7.2 kW, a clothes dryer load, a dishwasher cycle, an electric water heater’s daily reheat, or a pool pump run), or enter the energy per run in kWh. Appliance presets use the power and run time from the electricity cost calculator’s appliance table.
- Run length in hours: how long the appliance draws that energy.
- Runs per month.
- Day type: weekday or weekend.
- Season: July or January, since many tariffs have different periods and prices across the year.
The calculator builds the 24-hour price clock for your plan on that day type in that month, showing each hour’s price per kWh, with any tier set by a typical home’s use that month. It then works out the cost of the same run started at every hour of the day. It spreads the energy evenly across the run length, so a run that crosses a period boundary is charged partly at one rate and partly at the next. Finally, it highlights the cheapest start time and the most expensive one. “Saved per year” compares the best and worst start in every month of the year, for weekdays and weekends, at your number of runs a month.
A worked example
Take a hypothetical TOU plan with a peak rate of $0.45/kWh from 4 to 9 p.m., a super off-peak rate of $0.15/kWh from midnight to 6 a.m., and an off-peak rate of $0.30/kWh the rest of the day. Assume the plan uses these periods every day. An EV needs 40 kWh, and a Level 2 charger at 7.2 kW takes 40 ÷ 7.2 ≈ 5.6 hours; the calculator’s EV preset rounds that to 5.5 hours, or 40 ÷ 5.5 ≈ 7.27 kWh an hour.
Started at midnight, the whole session falls in super off-peak and costs 40 × $0.15 = $6.00. Started at 4 p.m., five hours run through peak and half an hour at off-peak: 36.4 kWh × $0.45 + 3.6 kWh × $0.30 ≈ $17.45. That is $11.45 a session; at 12 sessions a month, the preset’s figure, it is 12 × 12 × $11.45 ≈ $1,650 a year between always charging at the best hour and always at the worst.
How tariffs define TOU periods
From the billing side, where we work, a TOU plan is essentially a lookup table. Every hour of every day maps to a named period, and each period has a price. The tariff document defines the table with a few rules:
- Hour ranges for each period, such as “on-peak: 4:00 p.m. to 9:00 p.m.”
- Day types, usually weekdays versus weekends and holidays.
- Seasons, with the months each one covers.
- Prices for each period, which may also vary by season.
The OpenEI Utility Rate Database (URDB) publishes many of these schedules in a structured form, as the period for each of 24 hours on weekdays and weekends in each of the 12 months. The calculator reads the current URDB version of each plan. Your utility’s tariff book is the authoritative source; each utility page on this site shows the effective date of every plan and links its tariff document. Start and end times vary a lot between utilities, and even between plans at one utility.
Super off-peak windows
Some plans, especially ones aimed at EV owners, add a super off-peak period with a much lower price. It’s often overnight, and in some solar-heavy regions it falls in the middle of the day, when the grid has surplus solar output. If your plan has one, it is almost always the right window for large, flexible loads such as EV charging or water heating.
Weekends and holidays
Many tariffs treat weekends as off-peak all day, or shorten the peak window. Many also list specific holidays that are billed like weekends. That is why the calculator asks for the day type. It treats holidays as ordinary weekdays, so on a holiday, check your tariff. A load that should wait until night on a Tuesday may be fine at 5 p.m. on a Saturday.
Seasonal differences
At some utilities summer peaks are priced highest because air conditioning drives grid demand. Some also have a winter peak, sometimes including a morning window. Seasons follow each utility’s own months. A schedule that works in July may not be optimal in January, so check both; the yearly saving already covers all 12 months.
Using delay-start features
You don’t need to stay up until midnight to use cheap hours. Many modern dishwashers, washers and dryers have a delay-start button. Many EVs and Level 2 chargers let you set a charging schedule or a “departure time” that fills the battery during the cheapest hours. Heat pump water heaters and smart thermostats can often be scheduled as well. Set it once and it keeps saving.
Match schedules to your plan’s periods, and recheck them when daylight saving time starts or ends and when your utility revises its tariff.
When timing doesn’t change your bill
On a flat-rate plan, every kWh costs the same, so the calculator will show the same cost for every start hour. On a tiered plan, the price depends on how much you use in the billing period, not when you use it. Shifting a load to 2 a.m. doesn’t move you between tiers.
Timing can still matter to the grid. Evening peaks are when utilities rely on their most expensive and often least efficient generation, so moving flexible loads away from them helps, and prepares you if your utility moves you to a TOU plan.
A note on demand charges
A few residential plans include a demand charge: a fee per kW based on your highest power draw during a window, often the peak period. On those plans, running the dryer, the oven and the EV charger at the same time can cost more than running them one after another, even if total kWh is unchanged. If your plan has a demand charge, stagger large loads as well as shifting them. This site leaves plans with demand charges out, because their cost depends on your peak kW.
Safety first
Cheap overnight hours are not worth a fire risk. Don’t run a clothes dryer unattended overnight or while you’re away from home, and clean the lint filter before every load. The same caution applies to any appliance whose manufacturer advises against unattended operation. Dishwashers and EV chargers are generally designed for scheduled operation, but follow the manual, keep smoke alarms working, and have a qualified electrician install any EV charging circuit.
Frequently asked questions
How do I know if I’m on a time-of-use plan?
Your bill usually names the rate schedule and, on TOU plans, breaks usage into periods such as peak and off-peak. You can also select your utility in the calculator and look at the plans listed. If the price clock is a single flat color, timing won’t affect your bill.
What is the cheapest time to charge an EV?
On many TOU plans, it’s overnight, particularly during a super off-peak window if your plan has one. In some regions with a lot of solar, midday rates are lowest. The calculator finds the exact start time for your plan, day type and month (July or January).
Does the calculator account for runs that cross into a new period?
Yes. It spreads the run’s energy evenly across its length and prices each portion at the rate for the hour it falls in. Real appliances draw power unevenly, so treat the result as a close estimate.
Is it worth shifting small appliances?
The savings per run are small. The dishwasher preset uses about 0.3 kWh a cycle (330 W for about 55 minutes), so even a 30-cent gap between peak and off-peak saves about 9 cents a run. But delay-start costs nothing once set. Focus on the big loads: EV charging, water heating, dryers and pool pumps.