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Electrical calculators and tools
Start with the value you know. These tools keep real power, apparent power and power factor clear, with the assumptions beside the result.
Real power and current
Watts to amps calculator
Convert watts or kilowatts to amps, or work backward from current. Choose DC, single-phase AC or balanced three-phase AC, then enter voltage and power factor where needed.
Includes voltage comparisons, batch calculations and copyable results.
Apparent power and current
kVA to amps calculator
Find current from apparent power and voltage for single-phase or balanced three-phase AC. Select the correct line-to-line or line-to-neutral voltage reference.
Uses apparent power directly, so no extra power factor adjustment is needed.
Matching W and VA readings
Power factor calculator
Divide real power by apparent power for the same load and operating condition. Enter W or kW alongside VA or kVA to see the factor as a decimal and a percentage.
Shows the ratio to use in a watts-to-amps calculation and flags incompatible readings.
Motor output and efficiency
Horsepower to amps calculator
Estimate motor current from shaft horsepower or kW with explicit efficiency, voltage and power factor.
Shows real electrical input and motor losses for DC, single-phase or balanced three-phase supplies.
Three-phase voltage reference
Line-to-line vs line-to-neutral voltage
Identify the voltage endpoints before calculating current. Explore a balanced-wye voltage pair, nominal rounding and the limits of the √3 relationship.
Includes a conceptual diagram and a helper that requires an explicit system choice.
Input, output and ratings
Electrical nameplate guide
Read label values in context and calculate the watts of one DC output mode. Keep capability ratings separate from measured consumption.
Includes a worksheet for matching measured input/output and original label-reading examples.
Changing power over time
kWh calculator
Add sequential operating intervals to calculate electricity use from real input watts or kilowatts and hours or minutes.
Shows interval energy, total kWh and the time-weighted average power, including standby or off periods.
Nominal battery energy
Amp hours to watt hours calculator
Convert Ah or mAh with an explicit nominal voltage. Optionally account for the voltage, capacity and energy of an ideal bank of identical units.
Keeps per-unit ratings separate from bank totals, without guessing usable energy or runtime.
Energy and operating time
Battery runtime calculator
Estimate battery runtime from a known watt-hour rating, constant load, usable-energy percentage and output efficiency. See the assumptions behind the hours estimate.
Estimate runtime with explicit assumptions.
Resistive circuit arithmetic
Resistor power calculator
Calculate current and power dissipated in an ideal resistor from the voltage across it and its resistance. Applies to DC or RMS AC voltage across a purely resistive load.
Calculate resistor dissipation with explicit assumptions.
Explicit-resistance DC circuit
DC voltage drop calculator
Calculate DC voltage loss, load voltage and cable power loss from current, one-way cable length and explicit conductor resistance per 1,000 feet.
Estimate cable voltage loss with explicit assumptions.
Ideal DC resistor circuit
Voltage divider calculator
Calculate unloaded DC voltage-divider output, series current and resistor power from input voltage and two resistor values in kilohms.
Calculate divider output with explicit assumptions.
Simple DC LED string model
LED series resistor calculator
Find ideal current-limiting resistance and resistor dissipation for identical LEDs in one DC series string from supply voltage, forward voltage and target current.
Find ideal series resistance with explicit assumptions.
Constant-power charging estimate
Battery charge time calculator
Estimate battery energy to add, wall-side energy and idealized charging time from watt-hours, state-of-charge endpoints, charger input watts and stated overall efficiency.
Estimate charging hours with explicit assumptions.
Single-phase AC to DC estimate
Inverter battery amp draw calculator
Estimate battery-side DC amps for an AC load using stated inverter efficiency, idle watts and battery voltage; show single-phase AC amps with explicit power factor.
Compare AC and battery current with explicit assumptions.
Choose by the reading you have
Use watts for real electrical input power and VA or kVA for apparent power. Current calculations also need voltage and the applicable circuit type. These tools explain their assumptions and do not select breakers or wire sizes.
Every calculator is free in full and runs in your browser. If a result or explanation needs attention, email contact@wattstoamps.app.