Transformer Size Calculator

If your site takes an HV (11 kV or higher) supply, you own the transformer, you maintain the transformer, and you pay the energy losses across it for the life of the contract. Sizing matters in two directions. Undersized, and you cap demand artificially, run hot, shorten insulation life, and risk a forced outage at the worst possible moment. Oversized, and you carry copper and iron losses every hour of every year for a load that never materialises. Either way the cost compounds across a fifteen to twenty year asset life, and the wrong call shows up on every monthly invoice.

Transformer sizing also drives the conversation with the DNO at connection time. The kVA you contract sets your Maximum Import Capacity, which then sets the available demand charges on your DUoS bill and the headroom you carry for future expansion. Sites planning a major electrification project (heat pumps replacing gas boilers, EV charging fleets, new production lines) need to walk the transformer sizing forward by five to ten years, not just the day one load. The cost difference between a one-shot uplift and two sequential uplifts can run into six figures across an I&C estate.

Purely Energy works on the procurement side of every HV-fed client we manage, but the transformer is where engineering and contract economics meet. Embedded transformer condition monitoring, regular oil sampling, and a documented load factor inform the next contract renewal, the next MIC review, and the case for embedded generation or storage. Get the kVA sizing right at the design stage and the rest of the asset's working life is significantly cheaper. Use this calculator as the first cut, then take the numbers to a competent HV designer for the formal selection.