Calculator Inputs
Formula Used
This tool converts resistance into electrical power using measured voltage or current. For a resistive load:
- P = V² / R when resistance and voltage are known.
- P = I² × R when resistance and current are known.
- I = V / R and V = I × R for the missing quantity.
Use RMS voltage/current for AC resistive heating, lamps, and similar loads.
How to Use This Calculator
- Choose the input method based on what you measured on site.
- Enter resistance in ohms, then provide voltage or current as required.
- Select an output unit if you need kW, horsepower, or BTU/hr.
- Press Calculate to view results above the form instantly.
- Use CSV or PDF exports for reports, audits, or handover notes.
Example Data Table
| Scenario | Resistance (Ω) | Voltage (V) | Current (A) | Power (W) |
|---|---|---|---|---|
| Example 1 | 10.000 | 120.000 | 12.000 | 1,440.00 |
| Example 2 | 25.000 | 230.000 | 9.2000 | 2,116.00 |
| Example 3 | 4.0000 | 40.000 | 10.000 | 400.000 |
| Example 4 | 15.000 | 75.000 | 5.0000 | 375.000 |
These examples assume a purely resistive load and steady measurements.
Professional Guide: Ohms to Watts on Construction Sites
1) Why this conversion matters
Construction teams often need quick power estimates for temporary heaters, curing blankets, drying fans, and lighting. Converting resistance (Ω) to power (W) helps predict circuit loading, generator demand, and heat output before equipment is deployed. It also supports realistic cost allocation by translating a measurement into a usable wattage value for reporting.
2) Core equations used
For resistive loads, the calculator applies standard relationships: P = V²/R when voltage is known, and P = I²×R when current is known. It also derives missing values with I = V/R and V = I×R. These formulas assume the load behaves primarily like a resistor.
3) Typical resistance ranges in the field
Portable heaters and heating mats may present low-to-moderate resistance, while long cable runs, small lamps, and specialized elements can vary widely. A small change in resistance can shift wattage noticeably, especially at higher voltages. Recording the resistance used for each setup improves repeatability across shifts.
4) Voltage-based sizing example
If a resistive device measures 12 Ω on a 230 V supply, then P = 230²/12 ≈ 4,408 W. That is about 4.41 kW, which can influence breaker choice and feeder sizing. The tool also reports current (I ≈ 19.17 A) for quick checks.
5) Current-based sizing example
When current is known, power follows P = I²×R. For a 6 A reading on a 15 Ω load, P = 6²×15 = 540 W, and voltage is V = 6×15 = 90 V. This is useful when clamp meters are available but voltage is uncertain.
6) AC, RMS, and cross-checking
For AC resistive loads, use RMS voltage and RMS current. In cross-check mode, the calculator computes power from both voltage and current paths and shows a mismatch percentage. Larger mismatches can indicate unstable supply, meter issues, loose connections, or a load that is not purely resistive.
7) Generator and energy planning
The calculator reports energy for one hour in kWh. For example, 4.4 kW running for one hour consumes about 4.4 kWh. This supports generator loading plans, fuel estimates, and scheduling when multiple trades share temporary power. Heat rate in BTU/hr is also included for drying and warming tasks.
8) Documentation and site safety
Exporting CSV and PDF results creates a simple audit trail for inspections, handover packs, and daily reports. Always apply lockout procedures, verify conductor ratings, and follow local electrical codes when measuring or adjusting temporary installations. Accurate inputs help reduce nuisance trips and overheating risks.
Use measured ohms to estimate watts and plan safely.
FAQs
1) What inputs do I need for the calculator?
You need resistance in ohms plus either voltage or current. If you provide both voltage and current, the tool cross-checks the result and reports a mismatch percentage.
2) Does this work for motors and inductive loads?
It is best for resistive loads. Motors and inductive equipment involve power factor and impedance, so watts will differ from simple resistance-based estimates.
3) Which value should I use for AC systems?
Use RMS voltage and RMS current. RMS readings represent effective heating and match the equations used for resistive loads on AC supplies.
4) Why does cross-check mismatch happen?
Mismatches can occur from fluctuating supply, meter accuracy limits, poor connections, or a load that is not purely resistive. Recheck probes, clamp position, and measurement ranges.
5) How can this help with breaker and cable sizing?
Once watts and current are known, you can compare against circuit ratings and apply code-required safety margins. This reduces overload risk and helps select suitable conductors and protection.
6) What does “Energy for 1 hour” mean?
It is the power converted to kWh for a one-hour run time. Multiply that kWh figure by expected hours to estimate daily energy use for planning and cost tracking.
7) What do the CSV and PDF exports include?
Exports include your inputs, computed voltage/current, power in watts and selected units, kWh for one hour, BTU/hr heat rate, resistance check, mismatch percent, and your notes.