EV Fleet Charging Peak-Demand & Transformer Sizer
Estimate EV fleet charging peak demand, an indicative transformer kVA and a user-entered monthly demand charge. The model compares charger simultaneity with the average power needed to deliver the fleet's daily energy within its charging window.
Result
Estimated transformer capacity
Estimated coincident site peak
Fleet energy per day
Average fleet charging power needed
Charging power at entered simultaneity
Minimum simultaneity for the energy window
Charging-window feasibility
Estimated monthly demand charge
How to use this tool
- Enter fleet size, daily kWh per EV and the hours available for charging.
- Enter charger nameplate kW and the simultaneous charging factor expected from your unmanaged or managed schedule.
- Add the existing coincident site peak, power factor, transformer design margin and utility demand charge if known.
- Check window feasibility, estimated peak kW, transformer kVA and monthly demand-charge estimate; compare these with an electrical site study.
Assumptions
- Existing site peak and EV load are assumed coincident; actual load shapes, phase balance, power quality and local network constraints are not modeled.
- Simultaneous charging factor is a user estimate. If the resulting available charging power is below the average power needed for the window, the schedule is flagged as infeasible and the modeled peak uses the minimum average power needed as a planning floor.
- Power factor and design margin are user inputs, not universal electrical-code requirements. This is not a transformer selection, electrical design or connection approval.
- Monthly demand charge assumes the modeled site peak is billable under the utility tariff; ratchets, time bands, seasonal billing, fixed charges and export credits are excluded.
Technical details: what the fields mean
- EVs in fleet
vehicleCount - Number of EVs served by this site or charging group.
- Daily energy per EV
dailyEnergyPerVehicleKwh - Average electrical energy that must be delivered to each EV per day, in kWh.
- Charging window
chargingWindowHours - Hours in which the fleet's daily energy must be delivered.
- Charger nameplate power
chargerPowerKw - Nameplate AC charging power per charger in kW; use the site configuration's applicable rating.
- Simultaneous charging factor
simultaneousChargingPercent - Share of the fleet's total charger nameplate power expected to be active at the same time. Managed charging can lower this value only if the energy-window check remains feasible.
- Existing coincident site peak
existingSitePeakKw - Existing site demand at the time the EV charging peak is assumed to occur; the model adds EV load coincidentally.
- Power factor
powerFactorPercent - Assumed site power factor as a percentage, used to convert kW to kVA.
- Transformer design margin
transformerDesignMarginPercent - Additional capacity margin applied after converting coincident kW to kVA. This is a planning input, not a code-mandated value.
- Demand charge
demandChargeGbpPerKwMonth - Utility demand charge in GBP per billed kW per month. The estimate assumes the modeled coincident site peak is the billable demand.
Methodology
Fleet daily energy = EV count × daily energy per EV. Average fleet charging power required = daily energy ÷ charging-window hours. Available charging power at the entered simultaneity = EV count × charger kW × simultaneity factor. The modeled fleet peak is the greater of available charging power and the average power required to meet the energy window; infeasible simultaneity is flagged. Site peak = existing coincident site peak + modeled fleet peak. Required transformer kVA = site peak kW ÷ power factor × (1 + design margin). Monthly demand charge = site peak kW × entered charge rate.
Worked example
30 EV fleet with an 8-hour overnight charging window
Inputs: EVs in fleet: 30 EVs, Daily energy per EV: 45 kWh/EV/day, Charging window: 8 hours, Charger nameplate power: 11 kW/charger, Simultaneous charging factor: 60%, Existing coincident site peak: 120 kW, Power factor: 95%, Transformer design margin: 20%, Demand charge: 25 GBP/kW/month
Result: The fleet needs 1,350 kWh/day and 168.8 kW average charging power. At 60% simultaneity, chargers provide 198 kW, so the window is feasible. Estimated site peak is 318 kW; with 95% power factor and 20% margin, transformer capacity is about 401.7 kVA. Estimated demand charge is £7,950/month.
Practical guidance
- Use interval-metered site peaks and a real fleet charging schedule; a single simultaneity percentage is a simplified load-shape model.
- Confirm transformer capacity, protection, cabling, voltage drop, phase balance and utility connection requirements with a qualified electrical designer and the network operator.
- Check the actual tariff's demand-charge measurement window and ratchet rules before treating the monthly charge as a bill forecast.
Common mistakes
- Sizing from charger nameplate power alone without checking whether the charging window can deliver daily fleet energy.
- Adding existing site peak and EV load as if they can never coincide, or assuming they always coincide without checking interval data.
- Treating the estimated transformer kVA or demand charge as an approved equipment rating or utility quote.
FAQs
Why can the charging window be infeasible?
The entered simultaneous charger capacity may be lower than the average kW needed to deliver all EVs' daily energy within the available hours.
Does this choose a standard transformer size?
No. It estimates required kVA. Standard sizes, redundancy, harmonics, protection and local utility rules require an electrical design review.
Last reviewed: 2026-10-18
An early planning estimate for fleet charging load. It does not model charger taper, EV arrival/departure distributions, phase imbalance, harmonics, voltage drop, local grid studies, transformer standards or utility billing details.
This tool provides an indicative estimate, based on the assumptions stated above.
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