Quick answer: Use volts (V) for electrical pressure, amps (A) for current, ohms for resistance, watts (W) for real power, VA/kVA for apparent power, and Wh/kWh for energy over time. Convert prefixes before calculating:
1,500 W = 1.5 kW,20 mA = 0.020 A, and12 kWh = 12,000 Wh.
Electrical work depends on using the right unit for the right question. The number may be correct, but if the unit is wrong, the decision can still fail. This guide covers the electrical units and measurement habits that matter most in U.S. field work, equipment review, and calculation checks.
Core Electrical Units
Use this table as the first pass when a drawing, meter, nameplate, or calculator asks for a value.
| Quantity | Common symbol | Common unit | What it answers |
|---|---|---|---|
| Voltage | V | volt | How much electric potential is available |
| Current | I | ampere | How much current flows through the circuit |
| Resistance | R | ohm | How strongly a resistive path opposes current |
| Impedance | Z | ohm | AC opposition that includes resistance and reactance |
| Real power | P | watt or kilowatt | Useful power converted to work or heat |
| Apparent power | S | VA or kVA | Voltage-current capacity before power factor is applied |
| Reactive power | Q | VAR or kVAR | AC reactive exchange from inductive or capacitive behavior |
| Energy | E | Wh or kWh | Power accumulated over time |
| Frequency | f | hertz | Cycles per second; common U.S. utility frequency is 60 Hz |
Current
ampere (A)
- measures the rate of electric charge flow
- commonly shown as A, mA, or microamps depending on scale
Voltage
volt (V)
- measures electric potential difference
- common U.S. nominal system values include 120 V, 240 V, 208Y/120 V, and 480Y/277 V
Resistance
ohm
- measures opposition to direct current flow in a resistive path
- impedance uses the same unit in AC work, but it is not the same quantity as simple DC resistance
Power
watt (W)
- measures the rate of real energy transfer
Energy
watt-hour (Wh) and kilowatt-hour (kWh)
- measure accumulated energy over time
Frequency
hertz (Hz)
- measures cycles per second
- common U.S. utility frequency is 60 Hz
Formula Worksheet
The units connect through a few basic formulas. Convert values into matching units before you calculate.
| Task | Formula | Unit discipline |
|---|---|---|
| Ohm's Law voltage | V = I x R |
amps x ohms = volts |
| Ohm's Law current | I = V / R |
volts / ohms = amps |
| Ohm's Law resistance | R = V / I |
volts / amps = ohms |
| DC or resistive AC power | P = V x I |
volts x amps = watts when power factor is effectively 1 |
| Single-phase apparent power | S = V x A |
volts x amps = VA |
| Three-phase apparent power | S(kVA) = 1.732 x V_LL x A / 1000 |
use line-to-line voltage for a balanced three-phase load |
| Energy | kWh = kW x hours |
watts must be divided by 1,000 before using kWh |
| Cost screen | cost = kWh x rate |
use the billing rate in dollars per kWh |
In AC systems, also keep these terms separate:
- W for real power
- VA for apparent power
- VAR for reactive power
A motor or transformer may be sized from VA or kVA, while the utility bill may track real power and kWh differently.
Helpful calculators:
- Electrical Unit Converter
- Ohm's Law Calculator
- Power Calculator
- Energy Calculator
- Electricity Cost Calculator
- Electrical Formulas Calculator
Prefixes and Notation
Prefix mistakes are one of the fastest ways to create bad results.
Common large prefixes:
- k = kilo = 1,000
- M = mega = 1,000,000
- G = giga = 1,000,000,000
Common small prefixes:
- m = milli = 0.001
- micro = 0.000001
- n = nano = 0.000000001
Examples:
- 1,500 W = 1.5 kW
- 0.02 A = 20 mA
- 0.000001 F = 1 microfarad
- 13,800 V = 13.8 kV
Case matters:
- mA is not MA
- kW is not KW
- MVA is not mVA
Common Conversion Checks
| Starting value | Converted value | Why it matters |
|---|---|---|
| 1,500 W | 1.5 kW | Use kW before calculating kWh |
| 12 kWh | 12,000 Wh | Use Wh when comparing smaller stored-energy values |
| 20 mA | 0.020 A | Use amps in Ohm's Law and power formulas |
| 480,000 VA | 480 kVA | Use kVA for transformer and service-capacity review |
| 0.75 kW | 750 W | Use watts for simple P = V x I checks |
| 13,800 V | 13.8 kV | Keep distribution voltage scale clear |
U.S. Field Conventions
Electrical quantities usually stay in SI-derived units, but U.S. field work also uses customary size conventions.
Conductor Size
- AWG is common for smaller conductors
- kcmil is common for larger conductors
Those are conductor-size designations, not direct statements of ampacity.
Raceway and Box Dimensions
- conduit commonly uses trade sizes such as 1/2 inch, 3/4 inch, and 2 inch
- box volume is commonly reviewed in cubic inches
- run length is often tracked in feet
This mixed-unit environment is normal in U.S. work. The key is to convert intentionally and never assume a physical size unit and an electrical unit are interchangeable.
Reading and Converting Values Correctly
Example 1: Power vs Energy
- Heater load: 1.5 kW
- Runtime: 8 hours
- Energy use: 1.5 x 8 = 12 kWh
The load is 1.5 kW. The energy used is 12 kWh. Those are not the same quantity.
Example 2: Nameplate Watts, kWh, and Cost
A portable load has this nameplate and use pattern:
- Nameplate: 120 V, 8 A
- Load type: mostly resistive
- Runtime: 4 hours per day
- Energy rate: $0.16/kWh
Power:
120 x 8 = 960 W = 0.96 kW
Daily energy:
0.96 x 4 = 3.84 kWh/day
Daily cost screen:
3.84 x 0.16 = $0.61/day
The same nameplate gives voltage, current, watts, kWh, and cost only after the units stay in the right order.
Example 3: Watts vs VA
A small UPS output is listed as:
- Voltage: 120 V
- Current capacity: 5 A
- Assumed power factor for the connected load: 0.80
Apparent power:
120 x 5 = 600 VA
Estimated real power:
600 x 0.80 = 480 W
Use VA for capacity screening and watts for energy or heat-related review. Treating both values as the same can overstate the useful real power.
Example 4: Motor Output in Horsepower
Horsepower is still common on U.S. motor nameplates.
- 5 hp x 746 W/hp = 3,730 W
- Output power = 3.73 kW
That is output power. Electrical input depends on efficiency and, in AC systems, operating conditions.
Example 5: Transformer Current from kVA
For a balanced three-phase transformer:
- Rating: 75 kVA
- Voltage: 480 V
- Current = 75,000 / (1.732 x 480) = about 90.2 A
This is a good example of why kVA and amperes matter together in equipment review.
Example 6: Prefix Error Check
A control drawing lists a sensor signal as 20 mA.
- Correct conversion: 20 mA = 0.020 A
- Incorrect reading: 20 MA = 20,000,000 A
The letter case changes the result by a factor of one billion. Prefix review is not cosmetic; it protects the calculation.
Measurement Practice
Use RMS Values in AC Work
Most AC equipment nameplates, meters, and common calculations are based on RMS quantities.
Separate Nominal from Measured Voltage
A system may be referred to as 120 V nominal, but field measurements can vary. Nominal system class and actual measured voltage are related, but they are not the same number.
Keep Precision Reasonable
Do not pretend field work is more precise than the instrument or installation supports.
- 18.03 A may not be meaningfully different from 18.0 A in practical review
- conductor sizing and protective-device decisions usually depend more on system context than on false decimal precision
Common Mistakes
- Confusing kW with kWh.
- Treating AWG as if it were an ampacity table by itself.
- Using watts and volt-amps as if they are always equal.
- Mixing milli and mega because of incorrect capitalization.
- Assuming resistance and impedance are interchangeable in AC work.
Which Calculator to Use
Use the unit guide to decide what the number means, then move into the calculator that matches the task.
| If the task is... | Use this unit first | Then use |
|---|---|---|
| Convert mV, kV, mA, kW, kWh, ohms, microfarads, or hertz | the input and output units | Electrical Unit Converter |
| Solve for voltage, current, or resistance | V, A, and ohms | Ohm's Law Calculator |
| Solve watts from voltage and current | W or kW | Power Calculator |
| Convert watts and runtime into kWh | kW and hours | Energy Calculator |
| Estimate bill impact | kWh and $/kWh | Electricity Cost Calculator |
| Review multiple formulas together | formula variables and units | Electrical Formulas Calculator |
Practical Checkpoints
Before using a number in design review or troubleshooting, confirm:
- what quantity is being measured
- what unit the meter or nameplate is actually using
- whether the value is nominal, measured, or calculated
- whether the quantity is real power, apparent power, energy, or current
- whether any U.S. field convention such as AWG, kcmil, feet, or trade size also needs to be interpreted
Frequently Asked Questions
What is the practical difference between kW and kWh?
kW is power at a given moment. kWh is energy accumulated over time. If a 2 kW load runs for 5 hours, it uses 10 kWh.
Why does uppercase and lowercase matter in electrical prefixes?
Because case changes the value completely. m means milli, k means kilo, and M means mega. Writing 20 mA instead of 20 MA changes the quantity by a factor of one billion.
Why does conductor size not by itself tell me ampacity?
Because ampacity depends on more than size. Material, insulation rating, terminal temperature limits, conductor count, ambient conditions, and installation method all affect the usable current.
What is the difference between watts and volt-amps?
Watts are real power. Volt-amps are apparent power. They are equal in resistive loads at unity power factor, but they differ in many AC systems with inductive or capacitive behavior.
What formulas should I check before converting electrical units?
Start with V = I x R, P = V x I for DC or resistive AC, S = V x A for single-phase apparent power, and kWh = kW x hours. Convert prefixes before using the formula.
When should I use AWG and when should I use kcmil?
AWG is used for smaller conductors. Larger conductors are commonly expressed in kcmil. Both are size designations and still need a separate ampacity review.
Summary
Good electrical calculations depend on unit discipline:
- Volts, amps, ohms, watts, and kWh answer different questions.
- Prefixes and case change the value materially.
- AWG and kcmil describe conductor size, not standalone ampacity.
- Watts and VA must stay separate in many AC applications.
- Nominal and measured values should never be mixed casually.
For quick work, start with the Electrical Unit Converter when the problem is only a scale change, then use the Ohm's Law Calculator, Power Calculator, or Energy Calculator when the unit conversion becomes a circuit or usage calculation.