Wattage Calculator — P=VI Solver, PC PSU Sizer & kWh Cost
Four wattage tools in one page: solve the power equation for any unknown (P, V, I, or R), size the right PC power supply with proper headroom, check power factor (W vs VA vs VAR), and estimate what an appliance costs to run. Everything runs in your browser — no data leaves your device.
Enter any two known values — the solver rearranges P = V × I, V = I × R, P = V² ÷ R, and P = I² × R to find the unknown, then derives the fourth value too.
Result
Recommended PSU
Planning estimate, not a measurement. TDP/TBP are thermal design figures; transient spikes can briefly exceed them. Always buy a quality unit from a reputable brand — wattage alone does not guarantee reliability.
Power factor PF = P ÷ (V × I). Apparent power S = V × I (VA). Reactive power Q = √(S² − P²) (VAR).
Result
Estimated cost
Estimate only — real consumption varies with thermostat cycling, standby draw, and voltage. kWh = watts × hours × days ÷ 1000; cost = kWh × tariff.
The formulas behind the calculator
P = V × I
The core wattage formula. 460 W = 230 V × 2 A. Rearranged: V = P ÷ I, I = P ÷ V.
P = V² ÷ R = I² × R
For resistive loads. 115 Ω = 230² ÷ 460. Handy when you know resistance instead of current.
PF = P ÷ (V × I)
Power factor: real power over apparent power. A 1500 W motor at 230 V drawing 8.15 A has PF ≈ 0.80.
PSU = (CPU + GPU + 150) × 1.25
PC sizing rule: 125 + 200 + 150 = 475 W, × 1.25 headroom = 593.75 W → buy the 650 W tier.
E = P × t, Cost = E × tariff
Energy and money. 1500 W × 2 h = 3 kWh; at Rp1.444,7/kWh that costs Rp4.334,10.
S² = P² + Q²
The power triangle: apparent (VA), real (W), and reactive (VAR) power always satisfy this relation.
Standard PSU tiers
| Tier | Typical builds it covers (at 25% headroom) |
|---|---|
| 450 W | Office PCs, APUs, low-power builds (base ≤ 360 W) |
| 550 W | Entry gaming PCs, e.g. 65 W CPU + 170 W GPU (base ≤ 440 W) |
| 650 W | Mid-range gaming, e.g. 125 W CPU + 200 W GPU (base ≤ 520 W) |
| 750 W | Upper mid-range, e.g. 125 W CPU + 320 W GPU (base ≤ 600 W) |
| 850 W | High-end, e.g. 170 W CPU + 350 W GPU (base ≤ 680 W) |
| 1000 W | Flagship builds, e.g. 170 W CPU + 450 W GPU (base ≤ 800 W) |
Scope boundary
What this tool covers: wattage-centered math — solving P=VI for any unknown (P, V, I, R), power factor and the W/VA/VAR triangle, PC power-supply sizing with standard tiers, and quick single-appliance kWh + electricity-cost estimates.
What it does not cover: bulk energy-unit conversions (megawatt-hours, joules, BTU), which are not covered here. This page also does not replace an electrician: sizing breakers, wiring, and mains work must follow local electrical codes.
Frequently asked questions
What is the wattage formula?
Wattage (electrical power) is calculated with P = V × I — power in watts equals voltage in volts multiplied by current in amps. For resistive loads you can also use P = V² ÷ R or P = I² × R. The P=VI solver above rearranges these formulas automatically: pick the value you want to find and enter the two you know.
How do I calculate wattage from volts and amps?
Multiply them. Example: a device drawing 2 amps at 230 volts uses 230 × 2 = 460 watts. If you know watts and one of the other values, divide instead — amps = watts ÷ volts, volts = watts ÷ amps.
How many watts does my PC need?
Add your CPU TDP and GPU TBP, add 150 W for the rest of the system (motherboard, RAM, drives, fans), then add 20–30% headroom. Example: a 125 W CPU plus a 200 W GPU gives 125 + 200 + 150 = 475 W, and 475 × 1.25 = 593.75 W, so a 650 W PSU is the right tier. The PC PSU sizer above does this math and rounds up to a standard PSU tier.
What PSU wattage tier should I buy?
Buy the smallest standard tier that covers your target: 450, 550, 650, 750, 850, or 1000 W. If your target lands between tiers, always round up — a PSU running at 50–70% load is quieter, cooler, and more efficient than one running near its limit. If your target exceeds 1000 W, look at high-wattage units above 1000 W.
Why does the calculator add 150 W for the rest of the system?
The CPU and GPU are the big consumers, but the motherboard, RAM, storage drives, fans, and USB devices also draw power. 150 W is a conservative combined allowance that keeps the estimate safe for a typical gaming PC. Very minimal builds may draw less; heavily accessorized builds may draw more.
Why add 20–30% headroom to a PSU calculation?
Three reasons: transient power spikes from modern GPUs can briefly exceed the rated TBP; PSUs are most efficient around 50–70% load; and headroom leaves room for future upgrades and capacitor aging. The default 25% in the calculator above follows the same rule used by major PSU manufacturers.
How do I calculate kWh from watts?
Multiply watts by hours of use, then divide by 1000. A 1500 W appliance running for 2 hours uses 1500 × 2 ÷ 1000 = 3 kWh. Multiply by your tariff to get the cost: at Rp1.444,7 per kWh, 3 kWh costs Rp4.334,10.
How much does it cost to run a 1500 W appliance for 2 hours in Indonesia?
1500 W × 2 hours = 3 kWh. At the Rp1.444,7/kWh tariff, that is 3 × 1444.7 = Rp4.334,10. The appliance mode above uses the same math with your own wattage, hours, and tariff.
What is power factor and how is it calculated?
Power factor (PF) is the ratio of real power (watts, the power that does work) to apparent power (volt-amps, the power that flows): PF = P ÷ (V × I). It ranges from 0 to 1. Resistive loads like heaters have PF ≈ 1.0; motors and cheap power supplies are typically 0.6–0.9, meaning they draw more current than their wattage suggests.
What is the difference between watts, VA, and VAR?
Watts (W) is real power — what does work and what you pay for. Volt-amps (VA) is apparent power — volts times amps, what flows through the wires. Volt-amps reactive (VAR) is reactive power — energy oscillating between source and load. They relate as S² = P² + Q², where S is apparent, P is real, and Q is reactive power.
Can I find volts or amps if I only know watts?
You need one more value besides watts. With watts and amps: volts = watts ÷ amps. With watts and volts: amps = watts ÷ volts. With watts and resistance: volts = √(watts × resistance) and amps = √(watts ÷ resistance). The solver above covers all six combinations.
How do I find resistance from power and voltage?
Use R = V² ÷ P. Example: a 460 W load at 230 V has resistance 230² ÷ 460 = 115 ohms. From power and current instead: R = P ÷ I². Select “R” in the solver and enter the two values you know.
Is the PSU recommendation exact or just an estimate?
It is a planning estimate, not a measurement. CPU TDP and GPU TBP are thermal design figures, not exact power draw, and transient spikes can exceed them briefly. The 150 W system allowance and 20–30% headroom are what make the estimate safe in practice. For a final build, check reviews that measure total system draw at the wall.
Why is my measured wattage different from the calculation?
Common causes: the device draws less than its nameplate rating at idle or partial load; voltage sags under load change the result; cheap meters misread non-sinusoidal current from switching power supplies; and for PCs, the PSU's own efficiency loss (typically 10–15%) sits between the wall measurement and the components. The formulas are exact — the inputs are what vary.
Free tool by NRTechStudio