Electrical, Solar & HVAC

Array sizing, battery banks, charge controllers, inverters and generators.

• Generator Size Calculator Size a generator from running watts and starting surge, with the fuel it will burn per hour.
• Inverter Size Calculator Size an inverter from your running load and the surge of the largest motor, with the DC current it will draw.
• Off-Grid Battery Bank Calculator Battery bank size for an off-grid system — usable capacity, days of autonomy, depth of discharge and temperature derating.
• Solar Charge Controller Calculator Size an MPPT or PWM charge controller — output current, safety margin, and the maximum array the controller can take.
• Solar Panel Calculator How much solar you need — array size in kW, how many panels, and what it will generate over a year.
• Solar String Sizing Calculator How many panels can go in series — with the cold-weather voltage rise that decides the real limit.

Ohm’s law, power, energy, running costs and three-phase supplies.

• Cable Power Loss Calculator Power wasted as heat in a cable run, the efficiency it costs you, and what that energy is worth over a year.
• Electricity Cost Calculator What an appliance costs to run — per day, month and year — from its wattage, hours of use and your unit rate.
• kWh Calculator — Energy Use and Cost Converts watts and hours into kilowatt-hours, and works out what that energy costs over a day, a month and a year.
• Motor Power and Efficiency Calculator Electrical input power, current draw and losses for a motor from its shaft output, efficiency and power factor.
• Ohm's Law Calculator Enter any two of voltage, current, resistance and power, and get the other two — with the formula used shown.
• Power Factor Calculator Power factor, real, apparent and reactive power for a single-phase supply, plus the correction capacitance needed.
• Three-Phase Power Calculator Real, apparent and reactive power for a balanced three-phase load, with line and phase voltages.
• Transformer Sizing Calculator (kVA) The kVA rating a transformer needs for a given load, with growth headroom and the current on each winding.
• Transformer Turns Ratio Calculator Turns ratio from primary and secondary voltages, with the secondary winding turns and the current relationship.
• Voltage Drop Calculator Voltage lost along a cable run — from conductor area, length and current, for copper or aluminium.
• Wire Gauge Calculator — Size for Voltage Drop Conductor size needed to hold a cable run within a voltage drop limit, in mm² and AWG. Voltage drop only — not an ampacity or code check.

Capacity, runtime and charge time for battery banks and UPS systems.

• Battery Capacity Calculator The amp-hours and watt-hours a battery bank needs to run a given load for a given time, allowing for depth of discharge and inverter losses.
• Battery Charge Time Calculator How long a battery takes to charge from a given state of charge, at a given charger current.
• Battery Runtime Calculator How long a battery will run a load, accounting for usable depth of discharge and inverter efficiency.
• UPS Runtime and Sizing Calculator How long a UPS will hold up a load, and the VA rating and battery capacity needed for a target runtime.
• Watt-Hours to Amp-Hours Calculator Convert between watt-hours and amp-hours at a given battery voltage — the conversion every battery spec sheet assumes you can do.

Resistors, capacitors, inductors, dividers and LED current limiting.

• Current Divider Calculator How a current splits between two parallel resistors, with the voltage across the pair.
• LED Resistor Calculator The series resistor an LED needs — value, nearest standard size, power rating and what the resistor wastes.
• Op-Amp Gain Calculator Voltage gain of an inverting or non-inverting op-amp stage, in volts per volt and decibels, with a clipping check.
• Resistor Colour Code Calculator Reads a 4, 5 or 6-band resistor colour code into a resistance value, with its tolerance band and the range it guarantees.
• Series & Parallel Calculator Combine resistors, capacitors or inductors in series or parallel — enter any number of values and get the equivalent.
• Transistor Bias Calculator Operating point of a voltage-divider biased BJT — base voltage, emitter current, collector voltage and whether it is in the active region.
• Voltage Divider Calculator Output voltage of a two-resistor divider, with the current it draws and a warning when the load will disturb it.
• Zener Diode Calculator Series resistor for a Zener shunt regulator, with the worst-case power dissipated in the diode and the resistor.

Time constants, reactance, impedance, resonance, decibels and antenna lengths.

• Antenna Length Calculator Physical length for a dipole, vertical, loop or 5/8-wave antenna at any frequency, including the end-effect correction.
• Decibel Calculator (dB) Converts a gain or loss between a plain ratio and decibels, for power or for voltage.
• Impedance Calculator Magnitude and phase angle of a series RLC impedance, with the resistive and reactive parts separated.
• PCB Trace Width Calculator Minimum copper trace width for a given current and temperature rise, using the IPC-2221 formula, plus the resulting resistance and voltage drop.
• RC Circuit Calculator — Time Constant and Cutoff Time constant, charge and discharge times, and the cutoff frequency of a resistor–capacitor circuit.
• Reactance Calculator Inductive and capacitive reactance at a given frequency, and the frequency where they are equal.
• Resonant Frequency Calculator (LC) The resonant frequency of an inductor and capacitor, with the period and angular frequency.
• RL Circuit Calculator Time constant, current rise and fall, and cutoff frequency for a resistor–inductor circuit.
• RLC Circuit Calculator — Q Factor and Bandwidth Resonant frequency, Q factor, bandwidth and damping for a series RLC circuit.
• Signal-to-Noise Ratio Calculator Signal-to-noise ratio in decibels, with the effective number of bits and the Shannon channel capacity.
• Wavelength Calculator Converts between frequency and wavelength for radio waves, light and any other wave.

Air-conditioner sizing in BTU and tons, ventilation rates and duct sizes.

• BTU Calculator — Air Conditioner Sizing Works out the cooling capacity a room needs in BTU, tons and kW, and the nearest air-conditioner size to buy.
• CFM Calculator — Airflow and Air Changes Airflow in CFM, m³/h and L/s — from room volume and air changes per hour, or from cooling capacity.
• Duct Size Calculator Round duct diameter and the rectangular equivalents for a given airflow and air velocity.

Heat loss, furnace and heat pump sizing, and radiators at real flow temperatures.

• Furnace Size Calculator Turns a heat loss figure into the furnace input rating to buy, allowing for efficiency and a sizing margin.
• Heat Pump Size Calculator Sizes a heat pump in tons against the heating and cooling loads, and works out the backup heat needed at design temperature.
• Heating BTU Calculator How much heat a room or a house needs on the coldest day, in BTU per hour and kilowatts.
• Radiator Size Calculator Converts a room’s heat requirement into the radiator output to specify, corrected for your actual flow temperature.