Key takeaways
- Price wall energy delivered by the charger, not only the battery-energy estimate.
- Allocate kWh to the actual rate periods; a session can span multiple prices.
- Check fixed fees, demand charges, export rules, and schedule constraints before changing plans.
Time-of-use pricing can reduce EV charging cost when a household can shift energy into lower-priced hours. But a cheap off-peak number alone does not prove that a new tariff saves money. The plan may also change prices for air conditioning, water heating, cooking, or solar exports.
Session calculation
Charging cost = Σ(wall kWh in each rate period × that period’s price). Wall energy includes conversion and charging losses; battery percentage multiplied by nominal capacity is not the same measurement.
| Period | Wall energy | Rate | Cost |
|---|---|---|---|
| Off-peak | 38 kWh | $0.12 | $4.56 |
| On-peak | 7 kWh | $0.38 | $2.66 |
| Total | 45 kWh | Mixed | $7.22 |
Pricing all 45 kWh at the off-peak rate would understate this session by $1.82. Use charger logs, vehicle records, or utility interval data to allocate energy.
Cost per mile
Divide wall kWh by miles added over a meaningful period. If 45 kWh supports 135 miles, wall energy intensity is 0.333 kWh/mile. The illustrative $7.22 session costs 5.35 cents per mile. Seasonal temperature, speed, tires, cabin conditioning, battery conditioning, elevation, and charging behavior change the result.
Compare the whole home
Download at least 12 months of interval data if available. Reprice every interval under the current and proposed tariffs. Include customer charges, tier interactions, critical-peak events, minimum bills, demand charges, taxes where applicable, and solar import/export treatment. A plan saving $200 on EV charging can still be worse if it adds $300 to summer cooling.
Charging losses and slow charging
Losses occur in power electronics, wiring, the battery, and thermal management. Fixed vehicle overhead can make very low-power charging less efficient in some conditions, while faster charging requires compatible electrical capacity and equipment. Use measured wall energy. Do not assume one universal loss percentage.
Schedule design
- Define required departure time and target charge.
- Identify every low-price window, weekend rule, holiday rule, and seasonal calendar.
- Set vehicle or charger scheduling using one controlling system where possible.
- Test that charging actually starts and completes.
- Review after utility tariff or daylight-saving schedule changes.
Charging equipment is a sustained electrical load. Circuit sizing, permitting, receptacles, wiring, and installation should follow code and qualified guidance. Price optimization does not override safety.
Demand charges and managed charging
Some rates include a charge based on the highest short-period demand. Simultaneous EV charging, air conditioning, cooking, and water heating may create a costly peak even when energy is off-peak. Load management or lower charging power can help, but calculate whether the demand reduction exceeds equipment or service costs.
Solar charging
“Free solar charging” can ignore the value of exported electricity. If a kWh could have earned a $0.08 export credit, using it for the car has an $0.08 opportunity cost before losses. Under full retail net metering, timing may matter differently. Apply the actual tariff.
Frequently asked questions
Is 12 a.m. always off-peak?
No. Windows vary by utility, season, weekday, holiday, and plan. Use the current tariff.
Should I charge to 100% every night?
Follow vehicle guidance and choose a target appropriate for the needed trip, battery care, and emergency margin.
Can the charger app replace the utility bill?
It helps measure sessions, but the utility determines billing intervals, prices, charges, and taxes.
Sources and editorial notes
Utility Cost Lab prioritizes primary government agencies, national laboratories, utility documents, and recognized efficiency programs. Sources reviewed August 12, 2026.
Calculations are planning estimates. Actual bills depend on equipment, weather, occupancy, utility tariffs, taxes, fixed charges, and local conditions. Follow manufacturer instructions and use qualified professionals for electrical, fuel, plumbing, pool, or HVAC work.
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