How do I calculate watts from volts and amps?

For steady DC, multiply voltage by current: watts = volts × amps. For example, 12 V × 2 A = 24 W.

How do I calculate amps from watts and volts?

Divide power by voltage: amps = watts ÷ volts. For example, a 100 W DC load on 24 V draws about 4.17 A.

Can watts be converted to volts without amps or resistance?

No. Watts alone do not determine voltage. You also need current, resistance, or another circuit constraint to solve voltage.

Does DC power use power factor?

Simple steady-state DC power does not use AC power factor. AC real power calculations need RMS values and power factor.

What is the difference between watts and watt-hours?

Watts measure the rate of power at an instant. Watt-hours measure energy over time, equal to watts multiplied by hours.

How much resistor wattage margin should I use?

A common practical choice is to use a resistor rated at least two times the calculated dissipation, with more margin for heat, enclosed spaces, or reliability-critical circuits.

Which two values do I need?

Any two of V, I, R, P are sufficient for an ideal Ohm’s-law DC circuit. The calculator uses V=IR and P=VI=I^2R=V^2/R to compute the other two.

Is everything private?

Yes. All calculations run locally in your browser and are not uploaded.

Watts / Volts / Amps / Ohms DC Power Calculator

DC watts, volts, amps, and ohms calculator

This DC power calculator solves the relationship between voltage (V), current (I), resistance (R), and power (P). Use it to calculate watts from volts and amps, amps from watts and volts, ohms from volts and amps, or any other two-value Ohm’s law combination in a direct current circuit.

DC power in plain language

Direct current (DC) means electricity flows in one steady direction, like in batteries, solar panels, and USB chargers. Voltage is the electrical “push,” current is how much charge is moving, and resistance is how much the circuit resists that flow. Power is the rate of energy use and is measured in watts. In simple DC circuits, these values are tied together by two core relationships: the power equation and Ohm’s law.

Formula table by value to find

The basic DC power formula is $$ P = V \times I $$ and Ohm’s law is $$ V = I \times R $$. Combining them gives each common output formula:

Find	Use these formulas	When you know
Resistance (R)	$$ R = \frac{V}{I} $$ $$ R = \frac{V^2}{P} $$ $$ R = \frac{P}{I^2} $$	Voltage/current, voltage/power, or power/current
Current (I)	$$ I = \frac{V}{R} $$ $$ I = \frac{P}{V} $$ $$ I = \sqrt{\frac{P}{R}} $$	Voltage/resistance, power/voltage, or power/resistance
Voltage (V)	$$ V = I R $$ $$ V = \frac{P}{I} $$ $$ V = \sqrt{P R} $$	Current/resistance, power/current, or power/resistance
Power (P)	$$ P = V I $$ $$ P = \frac{V^2}{R} $$ $$ P = I^2 R $$	Voltage/current, voltage/resistance, or current/resistance

How to use the calculator

Choose what to solve: power, voltage, current, resistance, or advanced any-two mode.
Enter two known values, such as volts and amps or watts and volts.
Pick the correct units, including micro, milli, base, kilo, mega, or giga choices where available.
Click Calculate to solve the missing DC values.
Review the substituted formula steps and confirm the assumptions match your real-world setup.
Use Copy result or Copy link when you need to save or share the calculation.

Worked examples

12 V battery at 2 A

Battery sizing: $$ P = V \times I = 12 \times 2 = 24\ \text{W} $$ A 12 V accessory drawing 2 A needs about 24 W while it is running.

5 V USB at 3 A

USB devices: $$ P = 5 \times 3 = 15\ \text{W} $$ A 5 V, 3 A USB supply can deliver up to 15 W before conversion losses.

9 V across 1 kΩ

LED and resistor checks: $$ P = \frac{V^2}{R} = \frac{9^2}{1000} = 0.081\ \text{W} $$ The resistor dissipates 81 mW, so a 0.25 W resistor has comfortable margin in many open-air builds.

100 W load on 24 V

Solar and control systems: $$ I = \frac{P}{V} = \frac{100}{24} = 4.17\ \text{A} $$ A 100 W DC load on a 24 V bus draws about 4.17 A.

Common DC reference charts

These quick charts cover common 5 V USB, 12 V battery/automotive, 24 V control, and 48 V solar/telecom systems.

Amps for common wattages

DC system	10 W	25 W	50 W	100 W
5 V USB	2 A	5 A	10 A	20 A
12 V battery	0.83 A	2.08 A	4.17 A	8.33 A
24 V control	0.42 A	1.04 A	2.08 A	4.17 A
48 V solar/telecom	0.21 A	0.52 A	1.04 A	2.08 A

Watts for common currents

DC system	0.5 A	1 A	2 A	5 A
5 V USB	2.5 W	5 W	10 W	25 W
12 V battery	6 W	12 W	24 W	60 W
24 V control	12 W	24 W	48 W	120 W
48 V solar/telecom	24 W	48 W	96 W	240 W

Units and practical tips

Power is measured in watts (W), while small electronics may use microwatts (µW) or milliwatts (mW), and larger DC systems may use kilowatts (kW) or megawatts (MW). Current is in amps (A), voltage in volts (V), and resistance in ohms (Ω). If you input microamps, milliamps, kilohms, megaohms, or other supported units, the calculator converts everything to standard units internally.

Tip: Always double-check units. Mixing volts with milliamps or ohms without converting can lead to incorrect results.

Assumptions and limits

This calculator assumes steady-state DC and ideal Ohm’s-law behavior. AC real power calculations need RMS voltage/current and power factor. Non-ohmic loads such as LEDs, motors, batteries, and switching supplies can change behavior with temperature, speed, state of charge, control electronics, or startup surge, so simple resistance math may not describe every operating condition.

Safety note: High-current DC systems can overheat wires, connectors, fuses, and batteries. Mains-related or high-energy electrical work should be designed and checked by a qualified person using appropriate safety standards.