⚡

Electrical Calculator

Calculate electrical parameters, power consumption, and circuit analysis

Electrical Calculator: Ohm's Law and Power Calculator

Table of Contents - Electrical

How to Use This Calculator - Electrical

Select your calculation type: Power (find power from voltage, current, or resistance), Energy (calculate energy consumption), Cost (estimate electricity costs), or Frequency (for AC circuit calculations like resonance and reactance).

For Power calculations, enter any two known values:

  • Voltage (V) in volts
  • Current (I) in amps
  • Resistance (R) in ohms
  • The calculator solves for the remaining values plus power

For Energy calculations, enter power and time to find energy consumption.

For Cost calculations, enter power and time to estimate electricity costs based on rate per kWh.

For Frequency calculations, enter inductance and capacitance to find resonant frequency, or frequency and component values for reactance.

Click "Calculate" to see results. The output shows all calculated values, the formula used, and additional derived information like energy consumption over time.

The Core Principle: Ohm's Law and Power Relationships

Electrical circuits obey fundamental relationships that allow calculating unknown values from known ones.

Ohm's Law: V = I × R Voltage equals current times resistance. If you know any two, you can find the third.

Power equations:

  • P = V × I (power = voltage × current)
  • P = I² × R (power = current squared × resistance)
  • P = V² / R (power = voltage squared / resistance)

These equations are interchangeable—use whichever form matches your known values.

Energy: E = P × t Energy equals power times time. A 100W bulb running for 10 hours uses 1,000 Wh (1 kWh).

For AC circuits, reactance and impedance replace simple resistance. Capacitors and inductors resist current flow in frequency-dependent ways, and power factor describes how much of the apparent power does useful work.

How to Calculate Electrical Values Manually

Finding power from voltage and current: P = V × I

Example: 12V battery powering a device drawing 2A P = 12 × 2 = 24 watts

Finding power from voltage and resistance: P = V² / R

Example: 120V across a 60Ω heating element P = 120² / 60 = 14,400 / 60 = 240 watts

Finding power from current and resistance: P = I² × R

Example: 3A through a 10Ω resistor P = 3² × 10 = 9 × 10 = 90 watts

Finding current from voltage and resistance (Ohm's Law): I = V / R

Example: 9V across a 1000Ω resistor I = 9 / 1000 = 0.009A = 9mA

Finding resistance for a desired current: R = V / I

Example: Need 20mA from a 5V supply R = 5 / 0.020 = 250Ω

Energy calculation: E (kWh) = P (W) × t (hours) / 1000

Example: 1500W space heater for 8 hours E = 1500 × 8 / 1000 = 12 kWh

Real-World Applications

LED resistor calculation. An LED needs a current-limiting resistor. With 5V supply, 2V LED forward voltage, and 20mA desired current: R = (5-2) / 0.020 = 150Ω.

Power supply sizing. Your project draws 2A at 12V. That's 24W. Choose a power supply rated for at least 24W, preferably with 20-50% margin (30W or more).

Wire sizing. Higher current requires thicker wire to avoid excessive heating. Calculate current, then consult wire gauge tables for safe ampacity.

Electricity cost estimation. A 3000W dryer running 5 hours weekly: 3 × 5 × 52 weeks = 780 kWh/year. At $0.15/kWh, that's $117/year.

Fuse selection. Calculate maximum expected current, add safety margin, select fuse. A circuit drawing 8A maximum should have a 10A fuse—close enough to protect, not so close it nuisance-trips.

Scenarios People Actually Run Into

The LED that burns out instantly. You connected an LED directly to 5V without a resistor. LEDs have very low resistance when forward-biased—current skyrockets and the LED dies. Always use a current-limiting resistor.

The undersized power supply. Your 5V/2A supply powers a project that draws 2.5A peak. The supply either shuts down, overheats, or produces unstable voltage. Always allow headroom.

The mysterious voltage drop. Your 12V LED strip is dim at the far end. Long wire runs have resistance; current through that resistance drops voltage (V = IR). Use thicker wire or inject power at multiple points.

The blown fuse puzzle. Your 15A circuit keeps blowing fuses. Add up all connected loads—you're probably exceeding 15A, especially when multiple devices start simultaneously.

The hot wire warning. A wire gets noticeably warm during operation. This means significant power dissipation in the wire (P = I²R). Either reduce current, use thicker wire, or investigate—warm wires can become fire hazards.

Trade-Offs and Decisions People Underestimate

Voltage versus current for power transmission. P = V × I, so for the same power, higher voltage means lower current. Lower current means less I²R loss in wires. This is why power lines use high voltage.

Component power ratings. A 1/4W resistor can dissipate 0.25W continuously without overheating. Exceed this and it fails. Always calculate power through components and select appropriate ratings.

Efficiency losses. Real circuits aren't 100% efficient. A 50W amplifier might draw 75W from the wall. The difference (25W) becomes heat. Factor efficiency into power supply sizing.

Peak versus average current. Motors draw several times their running current at startup. Size fuses and wiring for peak current, not just average.

AC versus DC calculations. DC calculations are straightforward. AC adds phase relationships, power factor, and the distinction between real and apparent power. Same formulas, more complexity.

Common Mistakes and How to Recover

Confusing resistance and impedance. Resistance is for DC or purely resistive AC loads. Impedance includes capacitive and inductive effects and varies with frequency. Use resistance for DC, impedance for AC.

Ignoring wire resistance. Long wire runs have measurable resistance. A 100-foot run of 18 AWG wire is about 0.6Ω. At 10A, that drops 6V and dissipates 60W—significant for a 12V system.

Forgetting power ratings. Calculating the right resistor value doesn't help if it can't handle the power. A 100Ω resistor at 10V dissipates 1W—a 1/4W resistor would overheat immediately.

Mixing up series and parallel formulas. Series: add resistances, current is same everywhere, voltages divide. Parallel: voltages are same, currents add, resistances combine as reciprocals.

Overloading circuits. Adding devices without calculating total current can overload circuits, trip breakers, or cause fires. Always know your circuit's capacity and what's drawing from it.

Related Topics

Series and parallel circuits. Components in series share current; voltage divides. Components in parallel share voltage; current divides. Total resistance differs in each configuration.

AC power factor. In AC circuits with capacitors or inductors, current and voltage are out of phase. Power factor (0-1) measures how much of the apparent power does useful work.

Capacitive and inductive reactance. Capacitors resist DC but pass AC (reactance decreases with frequency). Inductors pass DC but resist AC (reactance increases with frequency).

Three-phase power. Industrial power uses three AC phases 120° apart. Total power is √3 times single-phase for the same voltage and current per phase.

Safety standards. Electrical work must meet codes (NEC in the US, BS 7671 in the UK) that specify wire sizes, protection devices, and installation practices for safety.

How This Calculator Works

Power calculations:

From V and I:

P = V × I
R = V / I

From V and R:

P = V² / R
I = V / R

From I and R:

P = I² × R
V = I × R

Energy calculation:

E (Wh) = P × t (hours)
E (kWh) = E (Wh) / 1000

Cost calculation:

Cost = E (kWh) × Rate ($/kWh)

Resonant frequency (LC circuits):

f = 1 / (2π√(LC))

Inductive reactance:

XL = 2πfL

Capacitive reactance:

XC = 1 / (2πfC)

All calculations happen locally in your browser.

FAQs

What's the difference between power and energy?

Power is the rate of energy use (watts). Energy is total use over time (watt-hours or kilowatt-hours). A 100W bulb uses 100W of power; running 10 hours, it consumes 1000Wh (1 kWh) of energy.

Why do I need Ohm's Law?

It's the fundamental relationship between voltage, current, and resistance. Any electrical calculation eventually uses Ohm's Law directly or through derived formulas.

What's the difference between AC and DC calculations?

DC calculations use simple resistance. AC calculations must account for reactance (from capacitors and inductors), impedance (total opposition to current), and phase relationships.

How do I calculate the resistor for an LED?

R = (Supply voltage - LED forward voltage) / Desired current. Example: (5V - 2V) / 0.020A = 150Ω. Then verify power: P = I²R = 0.020² × 150 = 0.06W, so a 1/8W resistor works.

Why does my wire get warm?

Current through wire resistance dissipates power as heat: P = I²R. Higher current or longer/thinner wire (more resistance) means more heat. Warm wires may need upgrading to thicker gauge.

What's power factor and why does it matter?

In AC circuits with inductors or capacitors, current and voltage peak at different times. Power factor (0-1) measures useful power as a fraction of apparent power. Low power factor means higher current for the same useful power.

How do I size a fuse or circuit breaker?

Calculate maximum expected current, add a safety margin (10-25%), and select the next standard size up. The fuse should blow before wires overheat but not trip during normal operation.

What causes voltage drop in long wire runs?

Wire has resistance. Current through that resistance drops voltage (V = IR). Long runs or thin wire have more resistance, more drop. Thicker wire or shorter runs reduce voltage drop.

What's the difference between watts and volt-amps?

For DC and purely resistive AC loads, they're the same. For AC loads with inductors or capacitors, volt-amps (VA) is the apparent power, while watts is the real power. The ratio is the power factor.

How do I calculate power for three-phase systems?

For balanced three-phase: P = √3 × V × I × power factor. The √3 (approximately 1.732) accounts for the phase relationships between the three voltages.

What's the relationship between horsepower and watts?

One horsepower equals approximately 746 watts. A 1 HP motor draws about 746W of mechanical power, but electrical input is higher due to motor efficiency (typically 80-95%).

How do I calculate battery capacity needs?

Battery capacity (Wh or Ah) = Power draw × Required runtime. A 50W device running for 4 hours needs 200Wh of battery capacity. Add 20-30% margin for inefficiency and battery longevity.

What safety margins should I use for electrical projects?

Use 80% of rated capacity as practical maximum. A 15A circuit should run no more than 12A continuously. This protects against overheating and provides margin for startup surges.

How does temperature affect electrical components?

Higher temperatures increase resistance in conductors and reduce capacity ratings. Many electrical ratings assume 25°C ambient. In hot environments, derate components or use larger sizes.