Electricity Calculator
A suite of tools for electrical estimations.
Understanding Voltage, Current & Power
At the heart of all electrical calculations are three fundamental concepts: Voltage, Current, and Power. Think of electricity flowing through a wire like water flowing through a hose.
- Voltage (V), measured in Volts, is like the water pressure. It's the potential difference that pushes the electric charge (current) through a circuit.
- Current (I), measured in Amperes (Amps), is like the flow rate of the water. It's the rate at which electric charge flows past a point in a circuit.
- Resistance (R), measured in Ohms (Ω), is like the narrowness of the hose. It opposes the flow of current.
- Power (P), measured in Watts (W), is the rate at which electrical energy is transferred or consumed. It's the product of voltage and current (
P = V × I).
These four quantities are related by Ohm's Law (V = I × R) and the basic power formula (P = V × I). Our calculator's first tab allows you to input any two of these values to find the other two, demonstrating these relationships in action.
How to Calculate Energy Use & Cost
Understanding your electricity bill starts with knowing the difference between power and energy.
- Power (kW) is how fast you use energy. A 1000-watt microwave has a higher power rating than a 100-watt light bulb.
- Energy (kWh) is the total amount of power used over a period of time. Your utility company bills you for the kilowatt-hours (kWh) you consume.
The formula is simple: Energy (kWh) = Power (kW) × Time (hours).
For example, if you run a 1,500-watt (1.5 kW) heater for 3 hours, you've consumed 1.5 kW × 3 h = 4.5 kWh of energy. To find the cost, you multiply this by your electricity rate. If your rate is $0.15 per kWh, the cost would be 4.5 kWh × $0.15/kWh = $0.675. Our Energy & Cost estimator automates this process for any appliance and time period.
Single-Phase vs. Three-Phase Power
Electrical power is typically delivered in two forms: single-phase and three-phase.
Single-Phase Power
This is the standard power supply for most homes and small businesses. It uses two wires (a phase wire and a neutral wire) to deliver power. It's sufficient for lighting, computers, and most household appliances. The formula for current is: I = P / (V × pf).
Three-Phase Power
Used in commercial, industrial, and large residential settings, three-phase power is more efficient for running large motors and heavy machinery. It provides more constant power delivery using three or four wires. Because the three phases are offset from each other, the power delivery is smoother and can carry more load with smaller, less expensive wiring. The formula for current involves the square root of 3 (approximately 1.732): I = P / (√3 × V × pf).
Voltage Drop and Cable Considerations
When electricity flows through a cable, a small amount of voltage is lost due to the cable's inherent resistance. This is known as voltage drop. Excessive voltage drop can cause equipment to malfunction, lights to dim, and motors to overheat. It represents wasted energy.
Several factors influence voltage drop:
- Cable Length: Longer cables have more resistance and thus a higher voltage drop.
- Current: Higher current flowing through the cable results in a greater voltage drop.
- Conductor Size (CSA/AWG): Thicker cables (larger cross-sectional area) have lower resistance and less voltage drop.
- Conductor Material: Copper is a better conductor (lower resistivity) than aluminum, resulting in less voltage drop for the same size and length.
Electrical codes typically specify a maximum allowable voltage drop, often around 3-5%, to ensure safety and efficiency. Our calculator provides an approximate voltage drop, but it's crucial to understand this is an estimate. Always use ampacity tables from your local electrical code and consult a professional for proper cable sizing for any installation.
Batteries and Runtime Calculations
For off-grid systems, backup power (UPS), or portable devices, understanding battery runtime is essential.
- Capacity (Ah): Ampere-hours (Ah) measure a battery's charge capacity. A 100Ah battery can theoretically supply 10 amps for 10 hours, or 1 amp for 100 hours.
- Voltage (V): The nominal voltage of the battery (e.g., 12V, 24V, 48V).
- Energy (Wh): The total energy a battery can store is its Watt-hours (Wh), calculated as
Wh = Ah × V. A 100Ah 12V battery holds 1200 Wh of energy. - Load (W): The power consumed by the devices connected to the battery.
To estimate runtime, you divide the total energy by the load, accounting for inefficiencies. If you're using an inverter to convert DC battery power to AC, the inverter itself consumes some power. This is its efficiency rating. The formula is: Runtime (hours) = (Battery Energy (Wh) × Inverter Efficiency) / Load Power (W). Our estimator helps you quickly determine how long your batteries can power your equipment.
Frequently Asked Questions
- 1. What is Power Factor (pf)?
- Power Factor is a measure of how efficiently electrical power is being used. A pf of 1.0 (or 100%) is ideal, meaning all power is doing useful work. A lower pf indicates some power is being wasted as reactive power, common in circuits with motors. Utilities sometimes penalize large consumers for low power factors.
- 2. Why is three-phase voltage often 400V or 415V?
- In a typical three-phase "wye" system, the voltage between any phase and neutral is 230V (line-to-neutral). The voltage between any two phases (line-to-line) is higher by a factor of √3. So, 230V × √3 ≈ 400V.
- 3. Can I use this calculator for DC circuits?
- Yes. For DC circuits, the power factor is always 1. The Ohm's Law, Watts↔Amps (use single-phase), and Battery Runtime calculators work perfectly for DC. Just set the power factor to 1.
- 4. What does CSA and AWG mean for cables?
- CSA stands for Cross-Sectional Area, typically measured in square millimeters (mm²), and is a common international standard. AWG stands for American Wire Gauge, a standard used primarily in North America. For AWG, a smaller number indicates a thicker wire.
- 5. What's the difference between real, reactive, and apparent power?
- Real Power (P, watts) does the actual work. Reactive Power (Q, var) sustains magnetic fields in inductive loads. Apparent Power (S, VA) is the vector sum of real and reactive power; it's the total power the grid must supply.
- 6. Why is my calculated current different from my device's label?
- Device labels often state the maximum or startup current, which can be higher than the normal running current. Also, the actual voltage supplied to your property can vary slightly, which will affect the current drawn.
- 7. How accurate are the voltage drop calculations?
- They are estimates. The actual resistance of a cable can vary with temperature. The formulas used are simplified and do not account for all factors like conductor reactance, which becomes more significant in large cables and AC circuits. They are suitable for initial planning but not for final design.
- 8. Is a higher inverter efficiency always better?
- Yes. A higher efficiency means less of your precious battery energy is wasted as heat inside the inverter, giving you longer runtime for your devices.
