How much power the installation actually needs, and what size supply, panel and transformer will carry it.
Everything downstream depends on getting this right. Size the demand too high and you pay for a service and a board you will never use; size it too low and the whole design has to be revisited when the load grows.
The trap at this stage is adding up nameplate ratings. Real installations never run everything at once, which is why every code has diversity or demand factors — and why a house with 40 kW of connected load does not need a 400 A service.
Start here. Connected load with the demand factors applied, to NEC Article 220.
The demand figure sets the service conductor and the main protective device.
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A different question from "what service do I need" — this one is about spare capacity in what is already installed.
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Three-phase changes the arithmetic everywhere downstream, and an unbalanced load carries current in the neutral.
Where the installation has its own supply transformer rather than a direct connection.
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BS 7671 and IEC use different diversity assumptions from the NEC. Not yet covered — the load calculator is NEC-based today.
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Size a house service the way an electrician does: NEC Article 220 standard calculation, with the demand factors that stop a 40 kW house needing a 400 A service.
3ØkW, kVA and kVAr from line voltage and current, with star and delta phase values and what power factor correction would save you.
2 calculators in this category. Every one is free to use on ElectroCalculators, with no account required, and every result cites the standard it came from.