EV load calculation
Electrical load calculations for EV charging installations — single charger to multi-megawatt depot scale.
EV load calculation is the electrical engineering that sizes the supply, distribution, and protection for a charging installation. Get it wrong and you either oversize (capex waste) or undersize (charger queuing, grid trips, retrofit pain).
We deliver load calculations for EV installations from single chargers to multi-megawatt depots, working to UK and US electrical codes.
Why load calculation needs more than arithmetic
Naive load calculation sums up nameplate ratings and asks for a connection at the total. Two problems:
1. Not all chargers run at maximum simultaneously. For most use cases — forecourts, retail, residential — diversity factor reduces realistic peak draw substantially. A 10-charger site at 150 kW each isn’t a 1.5 MW load in operational practice; it’s typically 600-900 kW peak depending on use case.
2. Operational realities matter. Booking systems, smart-charging algorithms, and tariff-driven demand-shifting all change the load profile. A load calc that ignores these specifies infrastructure for a worst case that never happens.
A well-built load calculation models the realistic operational profile, sizes for that, and includes future-proofing scenarios for the upgrades the site is likely to need.
What’s in a load calculation
A complete deliverable covers:
- Connected load schedule — every charger and ancillary load itemised with nameplate ratings
- Demand load calculation — applying diversity factors per charger group based on use case
- Operational profile model — utilisation patterns, charging session distribution, peak coincidence
- Single line diagrams — supply, distribution, protection, with calculated loads at each level
- Protection coordination — fuse, breaker, and relay coordination for safe operation under fault conditions
- Future-proofing scenarios — what happens if charger spec is upgraded (e.g. 150 kW to 350 kW) or if charger count expands
- Grid connection application supporting documentation — load profile, simultaneous demand evidence, connection size recommendation
Use cases and diversity assumptions
Different use cases carry different diversity factors. A few we typically work with:
| Use case | Typical diversity (10 charger site, 150 kW DC) |
|---|---|
| Motorway services / public DC fast | 0.75-0.95 (low diversity, peak coincides) |
| Retail forecourt | 0.55-0.75 |
| Workplace destination | 0.40-0.60 |
| Fleet depot (overnight charging) | 0.70-0.95 (scheduled, often coincident) |
| Residential MDU | 0.30-0.50 (low simultaneous draw) |
These are starting points. We refine per project based on the specific operational pattern.
Markets and codes
We work to:
- UK — BS 7671 (IET Wiring Regulations), Building Regulations Part S, IEC 61851 series for EV charging equipment, ENA G98/G99 for grid connection requirements
- US — NEC (National Electrical Code) Articles 625 (electric vehicle power transfer), state amendments, utility-specific interconnection requirements
- EU — IEC 61851, national amendments per market
Code compliance is checked at each step — from feeder sizing through protection coordination — not as a final review.
Inputs we need
A productive load calculation runs on:
- Charger configuration — types, counts, power ratings, manufacturer where known
- Use case — what kind of site this is (forecourt, depot, retail, residential)
- Operational profile — expected utilisation, hours of operation, fleet schedule if applicable
- Existing electrical infrastructure — current supply, switchgear, distribution
- Future expansion plans — if known, what’s the next phase
Output
Load calculations deliver as:
- Calculation report — methodology, assumptions, results
- Single line diagram in CAD or PDF
- Schedule of loads in structured format (Excel or PDF)
- Application supporting documents for grid connection where required
Common pitfalls in outsourced load calculation
Diversity factor copied from generic tables. Diversity factors vary substantially by use case and operational pattern. Generic-table values often don’t apply. We build diversity from realistic operational modelling.
No future-proofing scenarios. Sites get charger upgrades. Calculations that don’t model future scenarios leave the operator stranded when they decide to move from 50 kW to 150 kW chargers. We include explicit upgrade scenarios.
Protection coordination as an afterthought. A load calculation without proper protection coordination passes the connection sizing question but fails when a fault hits. We include protection coordination as part of the calculation, not a separate step.
Generic single line diagrams. SLDs that don’t reflect the actual switchgear and distribution arrangement get rejected by the design contractor. We produce SLDs against actual equipment, not idealised representations.
Typical timelines
- Single site, single phase — 1-2 weeks
- Multi-charger site (10-30 chargers) — 2-3 weeks
- Large depot or multi-megawatt site — 3-6 weeks
- Multi-site programme — rolling delivery, scaled per site
How we deliver
Load calculation work runs onshore-led with offshore production support. Senior electrical engineers handle code interpretation, diversity assumptions, and protection coordination. Calculation production and SLD drafting run offshore with onshore QA before deliverable release.
Talk to us about a load calculation
Tell us the site, the charger configuration, and the use case. We’ll scope and price within two business days for typical sites.
Typical deliverables
- Connected and demand load calculations
- Diversity factor analysis for multi-charger sites
- Future-proofing scenarios (higher kW chargers, expansion)
- Single line diagrams
- Documentation for grid connection applications
- Protection and safety calculations
Who buys this
EV operators, electrical contractors, fleet operators, property owners, and developers specifying the electrical infrastructure for new or upgraded charging sites.
Talk to us about delivery options
Tell us what you need delivered, what your timeline is, and what format the downstream team needs the output in. We'll come back with scope, price range, and proposed approach.