Energy bills are engineered to resist arithmetic. A unit rate in pence, a standing charge in pence-per-day, usage in kWh you never chose, an “estimated” flag doing quiet work, and a headline discount computed against a tariff you weren’t on. Most households pay them the way they read terms and conditions — scroll, sigh, accept.

But underneath, an energy bill is two lines of arithmetic, and two lines of arithmetic is a spreadsheet:

annual cost = standing_charge_p × 365 / 100
            + unit_rate_p × kwh_per_year / 100

That’s the whole machine, per fuel. Build it with named inputs and everything about energy pricing becomes checkable.

Step one: your real usage number

Everything depends on one input most people never look up: your annual kWh (electricity and gas separately). It’s on any bill or in your supplier’s app — real, measured, yours. Failing that, UK “typical household” figures (~2,700 kWh electricity, ~11,500 kWh gas) start the model, but the whole point is replacing folklore with your measured number — comparisons priced on someone else’s usage are how “save £300!” headlines happen to people who then save £40.

While you’re in the app: submit real readings (or confirm the smart meter is actually sending them). An E on the bill means the numbers are fiction that gets trued-up later — usually in winter, the expensive month.

Step two: the tariff comparison table

One Table, one row per tariff — current, the price cap default, any fix you’re quoted: standing charge, unit rate, exit fees, and the model’s output column:

=([@StandP] * 365 + [@UnitP] * kwh) / 100

Now every tariff is one comparable £/year against your usage — the renewal-ritual move, applied to the utility where the marketing is thickest. The table instantly exposes the trade the headline hides: a low unit rate with a high standing charge suits big users; a low standing charge suits the frugal — and which one you are is exactly what kwh decides. (A Data Table sweep over the kwh input shows the crossover usage where the ranking flips — one grid, the entire “which tariff” question.)

Tariff A · cheap standing, dear units Tariff B · dear standing, cheap units crossover kWh your usage → B wins, for YOU
Neither tariff is "cheaper" — there's a crossover, and which side your red line falls on is the only comparison that counts.

Step three: price the appliances

The same unit rate answers the household’s daily arguments, because kWh = kW × hours:

=power_kw * hours * unit_rate / 100

A 2 kW heater through an 8-hour evening: ~£4.50 a night, £130 a month — suddenly a real number in the budget rather than a vibe. A 10 W lamp left on all week: 4p — not worth the nagging. The model’s quiet service is ranking: a sorted little table of your actual appliances usually shows heat (space, water, tumble-dryer) as 80% of the story and standby LEDs as noise — so effort goes where the money is, not where the guilt is.

Two closing honesty notes. Winter is not summer: usage is seasonal, so monthly direct debits smooth an annual reality — your sinking-fund instinct already handles the shape, and the model’s annual figure is the true anchor when a “your payments are increasing” email arrives mid-year. And unit rates change with the price cap: date the rows, keep the old ones — the history-is-leverage rule applies to the utility everyone claims is uncomparable.

Standing charge, unit rate, your kWh. Three named cells, and the most opaque bill in the drawer becomes the easiest one to check.