Start with a physical denominator
Choose a product measure that makes operational sense, such as tonnes of a specified grade. Keep the boundary fixed: purchased electricity alone cannot be compared with electricity plus process heat without a consistent conversion. A switch to outsourced processing may reduce metered consumption while leaving the energy requirement of the finished product unchanged. Site energy also answers a different question from energy including upstream conversion losses. Label which boundary is being studied before comparing sites.
Build a sequential spending bridge
Imagine a plant making 1,000 tonnes, using 2 MWh per tonne at €80 per MWh: the bill is €160,000. Next period it produces 900 tonnes, uses 1.9 MWh per tonne and pays €100 per MWh. Spending reaches €171,000. Holding the old intensity and price constant, lower output saves €16,000. The intensity change then saves €7,200 at the old price; repricing the remaining 1,710 MWh adds €34,200. A different sequence reallocates the interaction but leaves the total change unchanged.
Interrogate the possible saving
Even the fall in MWh per tonne may reflect a shift towards easier grades, milder weather or a shorter warm-up schedule. Ask for product-level throughput and operating hours before crediting equipment upgrades. Reconcile commodity charges to fixed network fees and contract hedges separately. Those items can change accounting expense or cash payments without altering the energy absorbed by one tonne of finished output.
Practical check
Show total MWh, physical production and effective unit price side by side; label unallocated tariff and hedge effects separately.