Watt Happens Next

What you build: the Bill Predictor

One house, its two meters and one month of bills, worked into a model that calls the next bill before it arrives. Every chapter hands it one number.

• Your use, measured: two meter reads become kWh a day (9.6 kWh of electricity and 93.3 kWh of gas a day for the book's house); the meter's flashing light gives the watts right now; a plug-meter audit finds where they go.
• Your house, measured: a month of gas against the weather gives its heat loss (300 W per °C for the book's house); one cold night's log gives how fast it cools (a time constant of 36 hours) and how much heat its walls hold.
• The call: next month's bill with a band, 308.95 ± 22.58 for the book's house, from your model and the month's normal weather.
• The audit: every line on the bill rebuilt from your own reads and tariff. The book's bill came in at 343.11 on an estimated gas read 282 kWh too high; corrected to 320.55, the call was 3.6% away.
• The decisions: an overnight setback, a loft top-up, an old fridge, standby, a heat pump against gas, each worked in your own numbers with its payback. Twelve more household decisions are worked out at the end.

These are worked examples at an illustrative tariff from November 2026, with a column for your own numbers, not promises.

Who it is for

• Adults who never got calculus and want it taught from the basics through something that pays them back the same month: every idea is used on your own meters the week you learn it.
• Households that want to stop guessing at their bills: know what the next one will be, and check every line when it comes.
• A gift for a teen of 13 or older, with an adult alongside for the meter cupboard.
• Homeschool families who want calculus with a real number at the end of every step.

Not for: exam-syllabus coverage or proofs, or anyone who wants the answers without the arithmetic. It is never a substitute for a gas engineer, an electrician, an installer or the regulator where the book says so.

The game

Call It: every chapter ends with a number to write down before you measure it, then Check It: the gap scores 3, 2, 1 or 0 points. Spot the Slip: every chapter hides one planted mistake of the kind real bills and real people make (an estimated read taken as actual, kW written for kWh) for you to find. The Bill Score: when the bill arrives, your call within 5% of it is Gold, within 10% Silver, within 20% with the audit sheet complete Bronze; reproducing every line earns the Audit level. A Quest Map tracks the eleven stations, and a Trophy page holds the season scoreboard.

Honest limits

No saving is promised: your house, your habits, your tariff and the weather decide it, and published savings figures are quoted as their sources', with their conditions. The book's house, its meters and its bill were worked out on paper from published figures, not measured by us: read your own meters and log your own night. Tariffs are illustrative and change. It is not professional advice: never open a meter, a consumer unit (US: panel) or a gas appliance; cold has costs, so never set back the heating where pipes can freeze, where there is damp or mold, or where someone vulnerable lives; use a plug meter rated above the appliance; heat pumps, solar, batteries and EV chargers are licensed or certified installers' work. For a bill dispute, follow the regulator's route and keep paying the part that is not in dispute.

The book was written in collaboration with AI tools and reviewed by the author. Its numbers are checked by a verification program, and its figures are drawn by the same program, but mistakes may remain: see corrections & errata.