Power, Energy & Battery Calculators
Volts, amps, and hours all meet in Power, Energy & Battery Calculators. Solve Ohm’s law, find watts on DC, single-phase, and three-phase supplies, and price the kWh. Then size a battery, check runtime, and sink the heat.
What Power, Energy & Battery Calculators Answer
Power and energy get mixed up constantly. Power is the rate a load draws, measured in watts. Energy is power over time, measured in kilowatt-hours. Your meter bills the second one. Power, Energy & Battery Calculators keep the two apart and convert between them.
The same split runs through battery work. Amp-hours describe stored energy, while amps describe the draw. Divide one by the other and you get runtime.
Ohm’s Law and Watts
The Ohm’s Law Calculator solves for voltage, current, resistance, or power from any two known values. The Electrical Power Calculator and Current Calculator work the same relationships from the load side.
Four forms cover almost everything. Voltage equals current times resistance. Power equals voltage times current. Power also equals current squared times resistance, or voltage squared divided by resistance.
The squared forms explain heat. Five amps through a 2 ohm conductor burns 50 watts in the wire itself. That is why a long run gets warm and why conductor size matters as much as breaker size.
Single-Phase and Three-Phase Power
AC adds power factor, and three-phase adds a constant. The Three-Phase Power Calculator handles balanced loads.
| Supply | Real power |
|---|---|
| DC | Volts × amps |
| Single-phase AC | Volts × amps × power factor |
| Three-phase AC | 1.732 × line volts × line amps × power factor |
The 1.732 is the square root of three. It comes from the 120° spacing between the three phases, and it uses line-to-line voltage with line current.
Take a 480 V three-phase motor drawing 40 A at a power factor of 0.85. Real power is 1.732 × 480 × 40 × 0.85, or about 28.3 kW. Drop the power factor to 0.70 and the same current delivers only 23.3 kW, while the conductors still carry all 40 A.
Industrial loads commonly run at a power factor of 0.8 to 0.9. Resistive loads like heaters sit at 1.0.
Energy and Running Cost
The Energy Consumption Calculator and Electrical Energy Calculator turn watts and hours into kilowatt-hours, then into money.
Energy is power times time. A 1,500 W space heater running 8 hours uses 12 kWh. The EIA put the US average residential rate near 18.8 cents per kWh in April 2026, which makes that day about $2.26, or roughly $68 a month if it runs daily.
Rates vary far more than most estimates assume. EIA figures for 2026 run from about 12 cents per kWh in North Dakota to over 40 cents in Hawaii. Enter your own rate from a recent bill rather than a national average.
RMS, Peak, and Peak-to-Peak
The RMS Voltage Calculator and Peak to Peak Voltage Calculator convert between the ways an AC waveform gets measured.
A multimeter reads RMS, the value that produces the same heating as an equal DC voltage. A scope shows peaks. For a sine wave, peak is 1.414 times RMS, and peak-to-peak is twice the peak.
So a 120 V outlet peaks near 170 V, with 339 V from the bottom of the waveform to the top. Component voltage ratings have to clear the peak, not the RMS reading.
Battery Sizing and Runtime
The Battery Size Calculator works backward from the load and the runtime you need. The Battery Life Calculator goes the other way, from a pack you already have to how long it lasts.
Nameplate capacity is never all usable. Chemistry sets how deep you can discharge.
| Chemistry | Usual depth of discharge |
|---|---|
| Flooded lead-acid or AGM | 50% |
| LiFePO4 | 80%, sometimes higher |
Going past 50% on lead-acid causes sulfation, which permanently cuts capacity. LiFePO4 tolerates deeper cycling, and its management system enforces the limit.
Work an example. A 12 V 100 Ah battery stores 1,200 Wh. In lead-acid at 50%, that is 600 Wh usable, so a 100 W load runs about 6 hours on paper. Add a typical 85% inverter efficiency and it is closer to 5 hours.
The same 100 Ah in LiFePO4 at 80% gives 960 Wh, or about 8 hours of that 100 W load after inverter losses. That gap is why lithium packs of the same rating last noticeably longer.
Discharge rate matters too. C-rate is the draw compared with capacity, so 50 A from a 100 Ah pack is 0.5C. High C-rates cut deliverable capacity and heat the cells.
Supplies and Cooling
The Flyback Switch Mode Regulator Calculator estimates output power, switching frequency, duty cycle, and the stress on the transformer and semiconductors in a flyback supply.
The Heat Sink Calculator checks whether the part survives that power. Junction temperature is ambient temperature plus power times the total thermal resistance from junction to air.
A device dissipating 25 W through a 2 °C/W path in a 25 °C room sits at about 75 °C. Compare that with the maximum junction temperature on the datasheet, and leave margin for a hot enclosure.
EV Energy and Range
The Speed Distance Time Calculator covers travel speed, distance, and duration, then estimates EV energy demand, motor power, battery use, and regenerative braking for a trip.
It pairs with the battery tools. One gives the energy a trip needs, the others tell you what pack delivers it.
Common Mistakes
Billing watts instead of kilowatt-hours. A 1,500 W heater costs nothing until you multiply by hours.
Leaving power factor at 1.0 for motors. It overstates real power and understates the current the conductors carry.
Using full nameplate amp-hours. Depth of discharge and inverter losses can cut usable energy by half.
Rating parts to RMS voltage. Capacitors and semiconductors see the peak, which is 1.414 times higher.
Power and Battery FAQs
How do I calculate three-phase power?
Multiply 1.732 by the line-to-line voltage, the line current, and the power factor. For 480 V at 40 A and a 0.85 power factor, that is about 28.3 kW. Use 1.732, the square root of three, only with line-to-line voltage. If you measure phase-to-neutral instead, the constant becomes 3.
How long will a 100 Ah battery run my load?
Start with stored energy: 12 V × 100 Ah is 1,200 Wh. Apply the depth of discharge, 50% for lead-acid or about 80% for LiFePO4. Then divide by the load and allow for inverter losses near 85%. A 100 W load gets roughly 5 hours from lead-acid and about 8 hours from LiFePO4.
What does it cost to run a 1,500 W heater?
Eight hours uses 12 kWh. At the April 2026 US average of about 18.8 cents per kWh, that is roughly $2.26 a day, or about $68 over a month of daily use. Your own rate matters more than the average, since state rates in 2026 ranged from about 12 cents to over 40 cents.
What is the difference between kW and kWh?
kW is the rate of draw at any moment. kWh is that rate sustained for an hour. A 2 kW load for 30 minutes uses 1 kWh. Utilities bill kWh, while breakers, conductors, and generators are sized on kW and amps.
Why is peak voltage higher than what my meter reads?
Meters report RMS, the effective heating value of the waveform. A sine wave peaks 1.414 times higher, so a 120 V circuit peaks near 170 V and swings 339 V peak-to-peak. Insulation and component ratings must cover the peak.
How deep can I discharge a battery?
Keep flooded and AGM lead-acid at 50% or shallower, because deeper cycles sulfate the plates and shorten life sharply. LiFePO4 handles about 80% routinely, and its cycle life falls off far more gently. The datasheet’s cycle-life curve is the real answer for any specific pack.