Maths: From P = VI to a Household Energy Bill

Substitution and evaluation turn abstract formulas into practical answers, and few contexts show this better than modelling household energy use. At its core, the topic asks you to take a relationship you already know, substitute the given values, and interpret what the result means in the real world. Here, electrical power follows P = VI, linking voltage and current to the watts an appliance draws. From there, the reasoning builds in layers: convert watts to kilowatts, multiply by total operating hours to find energy in kWh, then multiply by the tariff to estimate cost. This chain matters because it connects physics to finance. Each step depends on the last, so a single substitution error propagates through the whole model. The final comparison against a budget also introduces critical thinking about assumptions — fixed electricity rates, constant daily usage, and an ideal power factor. Recognising these limitations is as important as the arithmetic itself, since real-world costs rarely behave as neatly as a model predicts.