Subtract the same cumulative import register twice. Its change is energy for that interval; dividing by time gives an average, not peak power or a complete household balance.
Choose the same import register
Read only an already installed, safely accessible display or an authorized record. Do not open a meter enclosure, break seals, expose terminals or change settings. If access is unsafe or a display is unclear, ask the supplier for help.
Keep meter serial, exact register name, unit and any supplier-confirmed multiplier. Import and export are separate quantities. On multi-rate meters, retain each register separately; do not add a total register to its own rate subregisters. The linked supplier instructions concern UK models, not a universal button sequence or colour rule.
Octopus Energy — identifying import, export and rate registers; UK meter examples (English) ↗Retain two actual readings and the real time interval
Record both timestamps, timezone and full readable cumulative kWh. Identify estimated or missing values instead of treating them as measurements. After replacement, reset or rollover, split the record into documented intervals; a lower final reading is not made valid by removing its minus sign.
For the calculator, enter actual elapsed hours. A fictional 14 × 24 h interval is 336 h; calendar dates spanning a clock change need their actual elapsed duration. Keep the readable increment: three displayed result decimals do not prove sub-kWh measurement precision.
Use the average without inventing a peak or a total balance
Fictional 12486.500 − 12345.000 = 141.500 kWh over 336 h means 10.11 kWh per 24 h and 421.13 W mean import. It is not a continuous 421.13 W appliance load, a startup requirement or the supply’s peak. A short high-load event and a steady load can produce the same interval energy.
With PV or storage, gross grid import is not total household consumption. Do not subtract export and call the result gross import. Use the separate solar-balance guide only within its stated no-storage boundary. Compare occupancy, weather and useful loads alongside equal-duration or time-normalized intervals.
Keep the selected energy charge separate from the bill
The price is entered by the reader; the default is fictional. One 141.5 kWh interval at 0.200 currency/kWh gives 28.30 energy-only units. Convert cents or pence consistently before entry. This does not select a tariff or predict a supplier’s account balance.
When prices differ by register or time, calculate matched energy segments at their own rates. For example, 90 kWh × 0.20 + 51.5 kWh × 0.10 = 23.15, not 141.5 × 0.20 = 28.30. Use non-overlapping records; if the necessary breakdown is unavailable, this calculator cannot reconstruct it. Fixed charges and other billing items remain separate even with zero recorded import.
Octopus Energy — standing charges are separate from energy use; UK explanation (English) ↗Minimal interval record
| Retain | Example or purpose | Avoid |
|---|---|---|
| Meter and register | Serial, import total OR disjoint rate registers | Mixing import/export or double counting totals |
| Readings and units | Start/end cumulative kWh and readable increment | Money, instantaneous W or estimated bill totals |
| Time | Both timestamps and actual elapsed hours | An assumed month or unexamined clock change |
| Price basis | Entered energy-only price, currency and valid segment | Treating an energy charge as the whole bill |
Worked example
Fictional interval, not a consumption target
One unchanged import register rises from 12345.000 to 12486.500 kWh in exactly 336 h. The entered price 0.200 is illustrative.
E = 12486.500 − 12345.000 = 141.500 kWh Ed = 141.500 × 24 / 336 = 10.11 kWh/day Pavg = 141.500 × 1000 / 336 = 421.13 W C = 141.500 × 0.200 = 28.30
The energy and time support these averages, not peak demand. The price supports an energy-only amount, not a complete bill, tariff recommendation or savings guarantee.
What to have ready
- Same meter and cumulative import register
- kWh unit, multiplier and readable increment documented
- Two real readings, timestamps and elapsed hours
- Rate segments kept disjoint without total-register duplication
- PV/storage boundary and excluded bill items noted
Common mistakes
- Subtracting an export register from an import reading.
- Using instantaneous kW or a money screen as cumulative kWh.
- Treating mean imported W as peak demand.
- Adding total import and its own rate subregisters.
- Calling a selected-rate energy charge the complete bill.
Use and limitations. Sources include US public agencies. Principles are general; regulations and design values must be checked where you live.Worked examples use stated hypothetical inputs.