Two kinds of current
Electric current comes in two forms, and almost every power-quality question — which UPS to buy, why a breaker trips, why an inverter is rated in VA — comes back to the difference between them.
Direct current (DC) is a one-directional flow of electric charge: the direction never changes. Batteries, solar cells and fuel cells produce DC, and DC appliances must be connected with the correct polarity. Electronics, LEDs, motors in electric vehicles and the cells of every battery run on DC internally.
Alternating current (AC) reverses direction cyclically. The utility supply to homes and businesses is a sine wave at 50 Hz in Thailand, Europe and most of Asia, and 60 Hz in the Americas and parts of Japan. Audio and radio signals are AC too. AC won the grid because transformers can step it up for long-distance transmission and down again for use, with low losses.
Every solar and storage system lives at the boundary between the two: solar modules and batteries are DC, the loads and the grid are AC, and the inverter is the device that converts between them.
Power consumption: DC is simple, AC is not
For a DC appliance, power is the product of the voltage it works at and the current it draws:
DC power (W) = voltage (V) × load current (A)
A 12 V pump drawing 5 A consumes 60 W. Done.
For an AC appliance the current changes direction 100 times a second, so the instantaneous voltage and current are always moving. To get a single meaningful number, both are expressed as RMS (root mean square) values — the equivalent steady DC value that would deliver the same heating power. A "230 V" mains supply is 230 V RMS; its peak is about 325 V.
Even with RMS values, AC power depends on *what kind of load* is connected.
Linear and non-linear loads
Linear loads draw a current that follows the voltage waveform — a sine wave in, a sine wave out, in step. They consume real power only, and their power factor is 1. Incandescent lamps, resistive heaters and kettles are linear.
Non-linear loads draw current in pulses rather than a sine wave. The pulses contain harmonic currents — multiples of the 50 Hz fundamental — on top of the original current, and the power factor is less than 1. Computers and servers, LED and fluorescent lighting with electronic drivers, variable-speed motor drives, chargers, televisions and almost everything with a switch-mode power supply is non-linear.
Measured side by side at the same voltage and the same rated power, an incandescent lamp draws a clean sine-wave current at power factor 1.0, while a desktop PC draws sharp current spikes at a power factor around 0.5. The PC's current peaks are far higher than its average — which is why wiring, breakers and UPS must be sized for the current, not the watts.
What are watts and VA?
- Watt (W) is the unit of real power — the rate at which energy is actually used or produced.
- Volt-ampere (VA) is the unit of apparent power — RMS voltage multiplied by RMS current, the power the supply must be able to deliver regardless of how much of it does useful work.
Apparent power (VA) = voltage (V RMS) × current (A RMS)
Real power (W) = apparent power (VA) × power factor
Power factor (pf) is the ratio of real power to apparent power, a number between 0 and 1. Linear loads have pf = 1; non-linear loads have pf < 1. The difference between VA and W is current that flows in the wires and heats them without doing work.
A measured example
A desktop PC with a 17-inch monitor, measured with a power meter:
| Measurement | Value |
|---|---|
| Voltage (RMS) | 229.3 V |
| Current (RMS) | 1.10 A |
| Apparent power | 229.3 × 1.10 = 252 VA |
| Real power (power meter reading) | 132 W |
| Power factor | 132 ÷ 252 = 0.52 |
Why a UPS is rated in VA
A UPS, an inverter or a generator must supply the load's full apparent current, not only the watts, and its inverter stage heats up according to the current it carries. So the VA rating sets the limit. A "1,000 VA" UPS with a 0.8 rated power factor can deliver 800 W of real power, but never more than 1,000 VA — and a load of non-linear equipment adding up to 700 W at pf 0.6 is 1,167 VA, more than the UPS can carry.
The rule when sizing:
- Add up the load in VA — take each device's nameplate VA, or its watts divided by its power factor (use 0.6-0.7 for older IT equipment if the nameplate gives only watts).
- Add 20-30% margin for growth and for the inrush of starting equipment.
- Check both the VA and the W rating of the UPS against the totals.
Why non-linear loads matter for power quality
Harmonic currents from non-linear loads do more than reduce the power factor. They distort the voltage waveform for everyone on the same circuit, overheat neutral conductors and transformers, trip breakers below their rated current, and can disturb other electronics. Sites with many computers, LED lights and motor drives — offices, data rooms, factories — see all of these. The remedies are correctly sized cabling and transformers, power-factor correction, and a double-conversion (true on-line) UPS that regenerates a clean sine wave for the critical load regardless of what is connected upstream. LEONICS has designed power-quality equipment for exactly these conditions since 1991; the UPS range and AVR range cover loads from a single desk to a whole plant.
Frequently asked questions
What is the difference between AC and DC current?
DC flows in one direction at a steady value — the current of batteries, solar cells and electronics. AC reverses direction cyclically as a sine wave, 50 or 60 times a second — the current of the utility grid. Inverters convert DC to AC; rectifiers and chargers convert AC to DC.What is the difference between watts and VA?
Watts measure real power, the energy actually used. VA (volt-amperes) measure apparent power, RMS voltage times RMS current — the power the supply must carry. Watts equal VA times the power factor, so for a linear load they are the same and for a computer the VA can be twice the watts.What is power factor?
The ratio of real power to apparent power, between 0 and 1. A power factor of 1 means all the current does useful work; 0.5 means half the current is circulating without doing work. Non-linear electronic loads and inductive motors have power factors below 1.Why is a UPS rated in VA and not watts?
Because its inverter must carry the load's full current, including the part that does no work. A UPS lists both a VA rating and a W rating; the load must stay within both.What is RMS voltage?
The root-mean-square value of an AC waveform — the equivalent steady DC voltage that would deliver the same power into a resistor. Mains voltage is quoted as RMS: 230 V RMS has a peak of about 325 V.How do I measure the real power of an AC appliance?
With a power meter that samples voltage and current together and computes watts directly. Multiplying a voltmeter reading by an ammeter reading gives VA, not watts, unless the load is purely resistive.Summary
DC flows one way and is easy to calculate; AC reverses and needs RMS values and a power factor. Linear loads use all the current they draw; non-linear electronic loads draw far more apparent current than their real power, which is why watts and VA differ, why power factor matters, and why UPS, inverters and generators are rated in VA. Size protection equipment for the apparent power of the load, with margin, and let a true on-line UPS clean up what the loads do to the waveform.
See the LEONICS UPS range, automatic voltage regulators, or the guide to hospital backup power for a worked application.