Key facts
- A time-of-use timetable and the rule choosing a session’s price are different information.
- The hypothetical 30 kWh example costs 15.00 with a 0.50 lock or 13.00 with the stated interval allocation.
- Equal-duration intervals need not contain equal energy, so time-weighted price averaging can be wrong.
- Separate connected-time or overstay components can change the total even with a fixed energy price.
Whether an EV charging price changes during a session depends on which pricing rule applies. Some energy prices are fixed by when a session begins; other billing arrangements apply rates to separate intervals. Connected-time or overstay charges can also change even when the energy price stays fixed. “Off-peak starts at midnight” is not enough information to calculate the bill.
Find the rule for the payment method, station and plan you will actually use. This guide shows how to distinguish the rules and reconcile a session that crosses a price boundary. All prices below are invented currency-neutral examples, not current operator quotations.
Ask which event chooses the price
| Rule in the applicable tariff | How to calculate the energy component | Record needed |
|---|---|---|
| Price selected at plug-in or session start | Session energy × applicable locked price | The stated triggering event and the price applying then |
| Energy charged at each time interval’s price | Add energy in each interval × that interval’s price | Metered energy allocated to the price intervals |
| Rate changes after elapsed connection time | Apply the rate and quantity to each defined stage | Connection timestamps and the threshold rule |
| Additional overstay or other fee | Add the separately triggered component | The trigger, grace period and fee-bearing duration |
“Plug-in” and “authorisation” need not be interchangeable terms. A tariff should identify its own trigger. Keep the observed connection time and the session-start timestamp separately if they differ; do not assume an app-open time controls billing.
What official provider guidance actually establishes
Tesla example: the Tesla Supercharging support page says the applicable charging price is determined at plug-in and is not revised when pricing changes during that session. Its guidance also describes separate minute-billing tiers and additional fee conditions. Apply the documented rule to the relevant session; a fixed energy price does not imply a fixed final total.
ChargePoint example, U.S. support guidance: its pricing-policy explanation says station owners or roaming partners set pricing. It describes connected-time rates that may vary while connected and prices that change after an initial charging period. Those statements are a reason to read the full tariff, not assume every ChargePoint station uses the same clock or charging unit.
EVgo example, United States: its time-of-use page identifies plan, location and time as pricing factors and labels its illustrated time window as an example. Check the exact station in the app. A generic timetable is not a substitute for the applicable session-price rule.
Worked example: the same 30 kWh costs 15.00 or 13.00
Suppose a fictional session runs from 23:40 to 00:20. A fictional tariff lists 0.50 per kWh before midnight and 0.30 after midnight. Assume equal tax treatment, no other charges and the following metered delivery:
| Interval | Time | Energy | Price | Interval cost |
|---|---|---|---|---|
| Before midnight | 20 minutes | 20 kWh | 0.50/kWh | 10.00 |
| After midnight | 20 minutes | 10 kWh | 0.30/kWh | 3.00 |
Under a rule locking the 0.50 energy price at 23:40, all 30 kWh cost 30 × 0.50 = 15.00. Under a rule allocating energy to the two clock intervals, the energy component is 20 × 0.50 + 10 × 0.30 = 13.00. These are alternative hypothetical contracts, not two bills that the same contract can arbitrarily choose between.
If the whole 30 kWh instead qualified for 0.30 under a different start-time scenario, the energy component would be 9.00. That third result requires different qualifying circumstances; crossing midnight alone does not establish it.
Do not split kWh using minutes unless power was constant
The two intervals above last equally long but contain different energy. The first averages 60 kW and the second 30 kW at the billing point. Averaging the prices by time gives 0.40/kWh; multiplying that by 30 kWh produces 12.00, which is wrong for the interval-energy contract.
The correct effective energy price is 13.00 ÷ 30 ≈0.4333 per kWh, weighted by delivered energy. A total-energy reading plus start and end times does not reveal the split when charging power varies. Request the operator’s interval allocation or itemised explanation instead of inventing it from elapsed time.
Where billing itself is per minute, apply that tariff’s time rules rather than this energy-price example. Our per-minute versus per-kWh guide explains the different comparison.
Home smart charging may have a separate eligibility rule
Do not transfer a public-network price lock to your household electricity bill. The UK Intelligent Octopus Go FAQ distinguishes its managed charging from manually initiated unscheduled charging, which uses the normal home energy rate. Its managed-session benefits are subject to the product’s current conditions.
The practical consequence is to preserve how the session was scheduled as well as when energy flowed. A manual override can change the applicable treatment. A screenshot of a cheap scheduled slot is not proof that a later manual session qualified. The home EV electricity-bill guide covers supply, delivery and fixed components separately.
Would stopping and restarting actually save money?
Only consider the comparison after confirming the operator’s rules. In another invented situation, 10 kWh remain to be purchased, and a permitted new session would reduce the energy price from 0.50 to 0.30. The potential energy saving is 10 ×(0.50 − 0.30) = 2.00.
A new 1.50 session fee reduces that to 0.50 before any other cost or delay. A 3.00 fee makes the restart 1.00 more expensive. A changed quote, a lost discount or different fee condition can overturn the calculation. This is a cost comparison, not an instruction to reconnect at a particular network; follow the station’s normal session and queue rules.
Keep a record that can resolve a boundary dispute
- Offer: station and connector ID, selected account/payment route, complete tariff and any qualifying plan.
- Clock: date, time zone and the stated start-price or interval rule; note a daylight-saving transition if relevant.
- Session: connection, authorisation, energy-start and energy-stop times where supplied.
- Quantities: billed kWh and any interval allocation, connected minutes and fee-bearing minutes.
- Settlement: itemised final receipt, separately from a pending card authorisation.
Reconstruct the bill component by component. If the disputed part is the energy-price timing, ask which tariff version and boundary timestamps were applied. If it is an overstay component, ask which event triggered it. Avoid treating a pending bank hold as a completed second charge; the charging-card holds guide explains that distinction.
When two apps show different offers for the same equipment, first check the payment-route and roaming price guide. It answers who is billing; this guide answers when that chosen contract applies each price.
Frequently asked questions
Does charging across midnight automatically reduce the bill?
No. Midnight must be a relevant tariff boundary, and the session’s rule must apply the lower rate to qualifying energy or time.
Can the bill increase while the energy price stays fixed?
Yes. More energy, connected-time fees or another applicable component can increase the total without changing the energy unit price.
Is a screenshot of the price enough to check a split-rate session?
It identifies the offer but may not identify each interval’s energy. Keep the complete tariff and request the session’s billed quantities as well.
Method: Official sources checked on 12 November 2026. Provider examples are limited to the cited guidance; no worldwide price-lock rule is assumed. All worked scenarios are original illustrative examples, not first-hand vehicle tests.