A watts amps volts calculator should ask which quantity is unknown and require the other two. This original helper uses nonnegative scalar magnitudes under an explicitly steady DC assumption, with three equation modes and a readable substitution record.
The examples are fictional educational calculations. They specify a native HTML helper, not wiring advice, a hardware rating, an AC model or verified hosted controls.
Keep the assumption beside the fields
OpenStax derives electrical power as P=IV and shows that watts are joules per second. Adopt that relationship with this helper's explicit steady DC scope. Do not extend it to AC/RMS, power factor or three-phase calculations through an unlabeled default. OpenStax: Electric Power
Accept W for power, V for voltage and A for current. Keep units in field labels and in the output. This bounded tool has no mA, kW or other unit conversion, resistance field or duration input.
| Unknown selected | Required known inputs | Equation |
|---|---|---|
| Power, W | Voltage V and current A | P=VI |
| Current, A | Power W and voltage V | I=P/V, with V>0 |
| Voltage, V | Power W and current A | V=P/I, with I>0 |
Only the two applicable known fields appear as active inputs. Their meaning should not depend on remembering which mode was previously selected. Changing modes clears old fields and results before relabeling.
Check three views of one fictional record
Use an original relationship E01: 12 V, 2 A and 24 W under the declared steady DC assumption. Its three views are independent mode tests:
| Mode | Substitution | Result |
|---|---|---|
| Find W | 12 V × 2 A | 24 W |
| Find A | 24 W ÷ 12 V | 2 A |
| Find V | 24 W ÷ 2 A | 12 V |
The input pair, unknown and assumption remain visible in each record. A result of 2 without A is incomplete documentation; a result of 24 W without the source inputs cannot show whether those inputs were invented.
A separate fixture uses 10 W and 2 A to obtain 5 V. The original opening's 10 W alone remains unresolved. It could be consistent with different voltage/current pairs, so choosing an unknown does not supply the missing known value.
No record identifies an actual circuit or measured device. The helper calculates the relation the reader supplied, not a current limit or an efficiency estimate.
Treat zero according to the selected equation
In forward power mode, a zero voltage or zero current is allowed. For example, 12 V × 0 A gives 0 W. This is arithmetic under the stated scalar model, not permission to infer a real circuit condition.
In current mode, voltage must be positive. In voltage mode, current must be positive. A denominator of zero is invalid whether power is positive or zero; 0/0 is not a guessed zero answer.
Zero power with a positive denominator is valid: 0 W ÷ 12 V gives 0 A, and 0 W ÷ 2 A gives 0 V. Keep those cases separate from missing input and zero-denominator errors.
An empty field is not zero. Reject it before numeric coercion. Field feedback should name the missing or invalid known quantity, rather than show a previous answer under the new mode.
Build a record the next reader can inspect
Store the raw known fields and units, selected unknown, steady DC assumption, parsed values, substituted equation, internal result and displayed result. Use ordinary numeric arithmetic internally and format at most six decimal places where needed, marking rounding as approximate.
For E01 in voltage mode, the record reads: supplied 24 W and 2 A; find V; steady DC magnitudes; V=24/2=12 V. It does not claim those values were measured or the device is suitable for any use.
An original formula triangle may put P above V and I to support the three rearrangements, with all equations written beside it. The text equations remain the authority. Do not copy a source wheel or let a decorative diagram replace denominator validation.
This helper stops at P, V or I. It does not infer wiring, load ratings, resistor sizing, efficiency, energy consumption or runtime. Solving P from V and I supplies no duration; it cannot also report watt-hours.
Clear state before changing the question
For manually entered values, accept digits with an optional dot and up to six fractional digits, finite nonnegative magnitude no greater than 1,000,000. Trim outside whitespace while retaining raw input. Reject signs, commas, exponent notation, blank and unsupported units.
The bounds are local interface choices. They are not electrical operating limits or a claim that every input pair represents an actual setup.
Provide native unknown selector, two labeled known fields, Calculate and Reset. Every edit invalidates the old record/output. A failed calculation leaves clear feedback and no stale answer. Reset empties mode, fields, record and feedback; a new mode then establishes the next question explicitly.
Actual keyboard, screen-width, assistive-technology and hosted control behavior remain test requirements. Arithmetic review alone does not establish that the interface handles a mode transition correctly.
Copy this prompt to specify the native helper
Create an original self-contained HTML/CSS/JavaScript educational steady-DC PVI helper, complete native source, substitution record and manual test matrix. Cite https://openstax.org/books/physics/pages/19-4-electric-power for P=IV and watt J/s concepts only. No copied circuit/examples/formula wheel. No AC/RMS/power-factor/three-phase model, wiring/load ratings, resistor sizing, efficiency, hardware control, duration/energy calculation, network or storage.
Three explicit unknown modes: W from V,A using P=VI; A from W,V using I=P/V with V>0; V from W,A using V=P/I with I>0. Nonnegative scalar magnitudes under visibly declared steady DC assumption. Units W,V,A only. Require both known values; 10 W alone cannot determine V. Mode change clears fields/result before relabeling the two active inputs.
Fictional E01 fixtures: 12 V×2 A=24 W; 24 W/12 V=2 A; 24 W/2 A=12 V. Separate 10 W/2 A=5 V. Forward zero accepted, e.g. 12 V×0 A=0 W. Inverse 0 W with positive denominator gives 0; zero denominator including 0/0 invalid, never guessed. Blank is missing, not zero. No real-device or measurement inference.
Native unknown selector, labeled known fields, Calculate and Reset. Manual grammar digits with optional dot, up to six fractional digits, finite nonnegative magnitude ≤1000000; trim outer whitespace but retain raw text. Reject signs/commas/exponents/nonfinite/blank/missing or unsupported units. Local bounds are interface choices, not hardware limits.
Record raw inputs/units, unknown, assumption, parsed values, equation/substitution, internal and displayed result. Unrounded internal arithmetic, at most six displayed decimals with approximation context; tiny positive outputs are not exact zero. Original P-over-V-and-I triangle may illustrate rearrangements, but readable equations and guards remain authoritative. Edits clear stale output/record; failed calculation shows no old answer. Reset empties mode/fields/feedback/record.
Test all fixtures, each inverse denominator, forward zero versus inverse 0/0, blank/wrong units/grammar/range, missing known values, mode clearing, stale answers and Reset. Check keyboard, actual article width and 360 px without page overflow. Report performed arithmetic separately from proposed runtime tests; no built helper, hosted script or accessibility claim. Native source and DC assumption record remain authoritative; optional NOS explanation promises no export, saved records or automatic updates.
Keep E01's assumption with its substitution
Maintain the native source and three E01 mode records together. Their value is that the unknown, two supplied quantities and steady DC assumption can all be inspected beside the equation.
NOS can save and render HTML. A prepared HTML explanation can present that three-mode relationship beside the maintained source. Actual hosted mode changes, Calculate and Reset require verification before presenting the helper as a working product example.
Explore NOS with the substitution review. The reader should leave with a result from two stated inputs, rather than a guessed value hidden inside the calculation.