A watt hours calculator should require power and elapsed time together. For this original helper, each row assumes the supplied power stays constant over its stated interval. Several rows represent separate segments whose energies can be summed.
The examples are fictional mathematical records. They are not measured consumption, battery runtime, device ratings or an energy forecast. The local HTML example provides a segment ledger; verify its controls in the intended host before relying on a result.
Normalize duration before multiplying
OpenStax states the energy-power-time relation E=Pt and connects kilowatt-hours to joules. Use that relation without copying its device examples, historical electricity prices or lighting claims. OpenStax: Electric Power and Energy
Power input is W only. Duration choices are h, min and s. Normalize to hours: h stays unchanged, min divides by 60, and s divides by 3,600. Then row energy in Wh equals supplied W multiplied by normalized hours.
The result record should carry both the original duration and normalized hours. Someone reviewing 30 W and 90 min should see 90/60=1.5 h before 30×1.5=45 Wh.
Use the declared output conversions kWh=Wh/1,000 and J=Wh×3,600. NIST's selected energy factors also identify Wh-to-J and kWh-to-J relationships. NIST: Energy Conversion Factors
Compare the minute and second records
| Original independent row | Normalized time | Row energy |
|---|---|---|
| 30 W, 90 min | 1.5 h | 45 Wh; 0.045 kWh; 162,000 J |
| 30 W, 5,400 s | 1.5 h | 45 Wh; 0.045 kWh; 162,000 J |
| 30 W, 1 h | 1 h | 30 Wh |
| 10 W, 2 h | 2 h | 20 Wh |
The first two rows are equivalent independent fixtures; do not add them as though they describe two different intervals in the same record. The last two form a separate two-segment example with total 30+20=50 Wh.
For that segment ledger, total duration is three hours. An unweighted average of 30 and 10 W is 20 W, which multiplied by three hours would give 60 Wh and disagree with the segment sum. If an average is discussed, the time-weighted value is 50/3 W, not the simple mean. This helper needs only the row-energy sum.
Keep the segments explicit so the calculation does not hide their different durations. A piecewise-constant assumption is not a measurement of continuously changing power.
Sum one bounded ledger
Accept one through ten rows, each with a stable row ID, power, duration and duration-unit selector. Add Row creates a blank required row. Remove Row removes the selected row and its prior contribution; keep at least one row so an empty ledger cannot masquerade as a zero-energy record.
Each valid row has an unrounded normalized duration and row energy. Total Wh is the sum of those row energies. Derive total kWh and J from that unrounded sum, rather than summing rounded displayed rows or converting already formatted text.
Do not silently drop an invalid row from the total. If one required value is missing or invalid, the ledger has no current total. Show which row needs correction, with any diagnostic row output clearly separated from a complete aggregate.
The bounded interface does not verify whether user-supplied intervals overlap or belong to the same activity. Its total means the sum of the declared rows. Retain the assumption so the result is not mislabeled as independently verified consumption.
Make zero a supplied value
For each manual power and duration, accept finite nonnegative dot-decimal values up to 1,000,000 with at most six fractional digits. Trim surrounding whitespace while retaining raw text. Reject signs, commas, exponent notation, nonfinite values, missing units and unsupported IDs.
Zero duration or zero power is valid and gives zero contribution. An empty duration is missing, not a zero-length interval. A row with zero power still requires its duration and unit; the helper cannot guess an omitted value merely because the product might be zero.
These limits are interface bounds, not physical operating limits. A numerical result cannot establish that a rated device draws the entered constant power or that an energy store can deliver it for the stated time.
Invalidate totals when the rows change
Use native Add Row, Remove Row, Calculate and Reset controls with labeled fields and useful focus placement. Every field edit, unit change, added row or removed row clears the old total and calculation record until the next successful Calculate.
Retain raw power/duration, normalized hours, per-row formulas, row energies, total derivation and the supplied-constant-power assumption. Format visible values to at most six decimal places where needed, marking rounded values approximate. Small positive internal results are not exact zero.
Reset returns one blank row with no selected duration unit, no results or record and cleared feedback. It does not retain the first example as though the reader supplied it. Keep the fictional fixtures in the separate worked explanation.
No cost, emissions, charge in Ah, battery-runtime guarantee or meter import is included. The helper also does not solve an unknown duration from an unspecified record. Both W and elapsed time are needed for every row.
Copy this prompt to specify the native ledger
Create an original self-contained HTML/CSS/JavaScript constant-power energy ledger, complete native source, text worked record and manual test matrix. Cite https://openstax.org/books/college-physics/pages/20-4-electric-power-and-energy for E=Pt and https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b9 for Wh/J factors. No copied device/price/lighting examples, forecasts, costs/emissions, batteries/Ah/runtime guarantees, electrical design, measurement import, network or storage.
Accept 1–10 rows with stable IDs, supplied power in W and duration in h/min/s. Per-row constant power is a declared mathematical assumption, not measured consumption or a nominal device rating. Normalize h unchanged, min/60, s/3600. Wh=P_W×t_h. Sum unrounded row energies; total kWh=Wh/1000 and J=Wh×3600. Do not sum rounded rows or invent absent durations.
Independent equivalence fixtures: 30 W×90 min→1.5 h→45 Wh=0.045 kWh=162000 J; 30 W×5400 s gives the same. Separate segment ledger 30 W×1 h plus 10 W×2 h gives 50 Wh. Do not add the equivalence fixtures to this ledger. Simple mean 20 W×3 h gives 60 Wh and is not the correct segment result; optional time-weighted average is 50/3 W. No measured time-varying integration claim.
Native Add/Remove Row, Calculate and Reset; keep 1–10 rows. New row is blank and required. An invalid row clears aggregate rather than being silently excluded. Every edit/unit/add/remove invalidates old total/record; current diagnostic rows cannot imply a complete total. Reset returns one blank row, unselected duration unit and no result/feedback/record. Maintain useful focus after row removal/addition.
Each typed quantity accepts digits with optional dot, at most six fractional digits and finite nonnegative magnitude ≤1000000. Trim outside whitespace but retain raw text. Reject signs, commas, exponents, nonfinite, blank or unknown units. Zero power/duration gives zero contribution, but other required fields remain necessary; missing duration never becomes 0. Bounds are interface choices.
Record row IDs/raw units, normalized hours, row formulas/energies, unrounded total, displayed Wh/kWh/J and assumption. Display at most six decimals with approximation context; tiny positives are not exact zero. Sum declared rows only, without claiming independent overlap/interval validation. Test both time-equivalence fixtures, two-segment sum, zero versus blank, every duration unit, invalid row/units/grammar/range, ten-row limit, stale edits/add/remove, Reset and keyboard focus. Check actual article width and 360 px without overflow. Report performed arithmetic separately from proposed runtime tests; no built helper, verified host or accessibility claim. Native source/ledger remains authoritative; optional NOS explanation promises no saved records, export or automatic updates.
Keep the elapsed-time record with the energy total
Maintain the native file and separate equivalence/segment fixtures together. The 45 Wh record is useful because its 90-minute input remains visible; the 50 Wh total is useful because both segment durations remain attached to their power values.
NOS supports HTML saving and rendering. A prepared HTML explanation can show those duration normalizations and segment totals beside the maintained source. Actual hosted row controls, calculations, invalid-total clearing and Reset need verification before claiming a working ledger.
Explore NOS with the interval review. The reader should retain an energy total whose power assumptions and elapsed times can both be inspected.