Free tools
Electrical & energy calculators
66 engineering and procurement tools, all free to use. Amps to kW, kVA sizing, EV charging costs, cable ampacity, carbon footprint and the rest — built by Purely Energy for the UK businesses we manage 2,000+ sites for.
Unit conversions
Swap between the units a building services engineer uses (amps, volts, kVA) and the units a finance director sees on the bill (kW, kWh, £/MWh).
3-Phase Power Calculator
Calculate real power, apparent power, reactive power, and line current for balanced three-phase AC systems.
Open calculatorAh to kWh / Wh Battery Converter
Convert battery capacity between amp-hours (Ah), watt-hours (Wh), and kilowatt-hours (kWh) at any voltage.
Open calculatorAmps to kVA Calculator
Convert current in amps to apparent power in kilovolt-amps (kVA) for DC, single-phase and three-phase AC.
Open calculatorAmps to kW Calculator
Convert electrical current in amps (A) to real power in kilowatts (kW) for DC, single-phase AC, and three-phase AC.
Open calculatorAmps to VA Calculator
Convert current in amps to apparent power in volt-amps (VA).
Open calculatorAmps to Volts Calculator
Calculate voltage from current using Ohm's law (V = I × R) or power formula (V = P / I).
Open calculatorAmps to Watts Calculator
Convert current in amps to power in watts for DC, single-phase and three-phase AC circuits.
Open calculatoreV to Volts Calculator
Convert electron-volts (eV) to voltage (V).
Open calculatorHorsepower to kW Converter
Convert between horsepower (HP) and kilowatts (kW) for motors and engines. Supports mechanical, electrical, and metric HP.
Open calculatorJoules to Volts Calculator
Convert energy in joules to voltage using electric charge.
Open calculatorJoules to Watts Calculator
Convert energy in joules to power in watts over a time period.
Open calculatorkVA to Amps Calculator
Convert apparent power in kilovolt-amps (kVA) to current in amps for single-phase and three-phase AC.
Open calculatorkVA to kW Calculator
Convert apparent power in kVA to real power in kW using a power factor.
Open calculatorkVA to VA Calculator
Convert kilovolt-amps (kVA) to volt-amps (VA).
Open calculatorkVA to Watts Calculator
Convert apparent power in kVA to real power in watts using a power factor.
Open calculatorkW to Amps Calculator
Convert real power in kilowatts (kW) to current in amps for DC, single-phase and three-phase AC.
Open calculatorkW to kVA Calculator
Convert real power in kilowatts (kW) to apparent power in kilovolt-amps (kVA).
Open calculatorkW to kWh Calculator
Convert power in kilowatts (kW) and time to energy in kilowatt-hours (kWh).
Open calculatorkW to VA Calculator
Convert real power in kilowatts to apparent power in volt-amps using power factor.
Open calculatorkW to Volts Calculator
Convert power in kilowatts to voltage in volts using current and power factor.
Open calculatorkWh to kW Calculator
Convert energy in kilowatt-hours (kWh) to average power in kilowatts (kW).
Open calculatorkWh to Watts Calculator
Convert energy in kilowatt-hours (kWh) to average power in watts (W).
Open calculatormAh to Wh Calculator
Convert battery capacity from milliamp-hours (mAh) to watt-hours (Wh).
Open calculatorPower Calculator
Calculate electrical power using voltage, current, resistance, or energy and time.
Open calculatorPower Factor Calculator
Calculate power factor, apparent power, reactive power, and correction capacitor capacitance.
Open calculatorReactive Power & kVAR Calculator
Calculate reactive power (kVAR), power factor angle, and the capacitor size needed for power factor correction.
Open calculatorVA to Amps Calculator
Convert apparent power in volt-amps (VA) to current in amps for single and three-phase circuits.
Open calculatorVA to kVA Calculator
Convert volt-amps (VA) to kilovolt-amps (kVA). Simple divide by 1000.
Open calculatorVA to kW Calculator
Convert apparent power in volt-amps (VA) to real power in kilowatts (kW).
Open calculatorVA to Watts Calculator
Convert apparent power in VA to real power in watts using a power factor.
Open calculatorVolts to Amps Calculator
Calculate current in amps from voltage using Ohm's Law (I=V/R) or power formula (I=P/V).
Open calculatorVolts to eV Calculator
Convert voltage in volts to energy in electron-volts (eV) using charge.
Open calculatorVolts to Joules Calculator
Convert voltage in volts to energy in joules using electric charge.
Open calculatorVolts to kW Calculator
Convert voltage (V) to power in kilowatts (kW) using current and power factor.
Open calculatorVolts to Watts Calculator
Convert voltage in volts to power in watts using current or resistance.
Open calculatorWatts / Volts / Amps / Ohms Calculator
DC power calculator — enter any 2 of 4 values to find the rest. Uses all Ohm's Law and power formulas.
Open calculatorWatts to Amps Calculator
Convert power in watts to current in amps for DC and AC circuits.
Open calculatorWatts to Joules Calculator
Convert power in watts to energy in joules over a time period.
Open calculatorWatts to kVA Calculator
Convert real power in watts to apparent power in kilovolt-amps (kVA) using power factor.
Open calculatorWatts to kWh Calculator
Convert power in watts and time to energy in kilowatt-hours (kWh).
Open calculatorWatts to VA Calculator
Convert real power in watts to apparent power in volt-amps (VA) using power factor.
Open calculatorWatts to Volts Calculator
Convert power in watts to voltage in volts using current.
Open calculatorWh to mAh Calculator
Convert watt-hours (Wh) to milliamp-hours (mAh) at a given voltage.
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Cost and bills
Estimate the running cost of a load, an appliance fleet, an EV charging plan, or a lighting payback before signing the capex.
Appliance Running Cost Calculator
Calculate the annual running cost of any household appliance based on power rating, usage, and your electricity tariff.
Open calculatorElectricity Bill Calculator
Estimate your electricity bill by entering appliance power, usage hours, days, and your energy tariff rate.
Open calculatorEnergy Consumption Calculator
Calculate total energy consumption in kWh for an appliance over a set time period.
Open calculatorEnergy Cost Calculator
Calculate the cost of electricity consumption given energy in kWh and tariff rate.
Open calculatorEV Charging Cost Calculator
Calculate the cost to charge an electric vehicle at home or public charge points, and cost per mile.
Open calculatorLED vs Halogen Energy & Payback Calculator
Compare LED and halogen/incandescent lighting energy costs and calculate the payback period for switching to LED.
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Sizing and capacity
Specify cables, breakers, transformers and generators with the right headroom — neither under-sized and tripping, nor over-sized and burning Availability Charges.
Cable Ampacity Calculator
Find the current-carrying capacity (ampacity) of copper or aluminium cables by cross-sectional area. Based on typical installation in air (30°C).
Open calculatorCircuit Breaker Size Calculator
Calculate the correct circuit breaker (MCB) rating for a circuit based on load current, cable size, and NEC/BS7671 guidelines.
Open calculatorGenerator Size Calculator
Calculate the kVA generator size needed to power your appliances or site loads, including motor starting surge.
Open calculatorHeat Pump COP & Running Cost Calculator
Calculate heat pump COP, heat output, electricity consumption, and annual running cost compared to a gas boiler.
Open calculatorSolar Battery Storage Sizing Calculator
Calculate the battery capacity (kWh) needed to store solar energy for a UK home, including overnight usage and EV/heat pump loads.
Open calculatorSolar Panel System Sizing Calculator
Calculate how many solar panels you need, the system size in kW, and estimated daily/annual generation for a UK home.
Open calculatorTransformer Size Calculator
Calculate the required kVA rating for a transformer to supply a given electrical load.
Open calculatorWire Gauge Calculator
Calculate wire diameter, cross-sectional area, and resistance per unit length from AWG gauge number.
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Carbon and sustainability
Convert kWh into tCO2e with the right grid intensity factor for the reporting year, so SECR and CSRD numbers stand up to audit.
Electronics reference
Bench-level tools for in-house engineering, control panels, battery and storage system design.
Capacitor Energy & RC Time Calculator
Calculate the energy stored in a capacitor, charge, and RC circuit time constant.
Open calculatorLED Resistor Calculator
Calculate the correct dropping resistor value for an LED circuit given supply voltage, LED forward voltage, and desired current.
Open calculatorOhm's Law Calculator
Calculate voltage, current, resistance, or power. Enter any 2 of 4 values.
Open calculatorPeak & RMS Voltage Calculator
Convert between peak (Vpeak), peak-to-peak (Vpp), and RMS voltage for AC sinusoidal waveforms.
Open calculatorResistor Colour Code Calculator
Decode 4-band and 5-band resistor colour codes to find resistance value and tolerance.
Open calculatorSeries & Parallel Resistor Calculator
Calculate total resistance for resistors connected in series or parallel. Supports up to 8 resistors.
Open calculatorVoltage Divider Calculator
Calculate output voltage, input voltage, or resistor values in a resistor voltage divider circuit.
Open calculatorVoltage Drop Calculator
Calculate the voltage drop across a wire given current, conductor material, length, and cross-section.
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