Sizu Riatto
Watt Happens Next · Chapter 3

Watts Right Now

Find out what is on when nothing seems to be. Count the meter's light for a minute and you have watts right now; add up a plug-meter audit and the gap to the meter is the load nobody has found yet.

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Opens on the book's house. Change any number to your own and press Calculate.

Watts right now: count the meter's flashing light

The plug-meter audit: kWh a day for each appliance (a week's kWh ÷ 7)

Watts × hours (to fill a row)

Enter the impulses per kWh and a count of flashes, then press Calculate.

The formula

  • Each flash is 1 ÷ (impulses per kWh) of a kWh: watts = flashes × 3,600,000 ÷ (impulses per kWh × seconds)
  • An appliance's watts = the count with it on − the count with it off
  • kWh a day = a week on the plug meter ÷ 7; or watts × hours ÷ 1,000; or cycles a week × kWh a cycle ÷ 7
  • The mystery = the meter's kWh a day − the audit's total; in watts, × 1,000 ÷ 24
  • Base load = (the all-night rows' kWh a day) × 1,000 ÷ 24 + the mystery: check it against the night's flashes
The book's worked example, Chapter 3. 3 flashes a minute at night on a 1,000 imp/kWh meter is 180 W; 53 with the kettle on is 3,180 W, so the kettle is about 3,000 W. The audit finds 9.01 kWh of the meter's 9.6 a day (93.8%); the mystery 0.59 kWh a day is about 25 W. Fridge, always-on and standby make 155 W; add the mystery and the base load comes back as about 180 W, the night's count.
Check it. Count the flashes for a full minute, twice. If the two counts differ by more than one, something is cycling (a fridge, a pump): count for three minutes and divide.

Honest limits. A plug meter must be rated above the appliance: never run a space heater through one, or through an extension lead, unless the maker allows it. US plug-in meters are for 120 V devices only, not 240 V dryers, central air or water heaters. Readings below about 1 W are less accurate. Count the flashes through the meter's window: never open the meter. 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, as seen in November 2026 (shaped like the GB price cap of October to December 2026, with US prices beside), shown without a currency sign; put in your own. No saving is promised. Everything stays in your browser: no sign-up, nothing stored.

These calculators go with Watt Happens Next (Make It Add Up, Book 3), where every number on this page is worked by hand.