# True Power vs Apparent Power

60 views
0

I’m reading through my Solar Inverter/Charge controller manual and I see the Peak Power Rating is 10,000VA. I didn’t recognize the unit “VA”, but it seemed suspiciously close to 10,000 (V)olt (A)mps but I already know Watts=volts x Amps so why wouldn’t the manual just say Watts if that’s what they meant?

So I googled what unit is VA and learned it is indeed an Volt-Ampere. So how does this differ from watts? Further googling showed me that it’s the difference between Real Power and Apparent Power. So what’s that?

After trying to watch several YouTube videos, I just don’t get it. They’re far too technical for me and they all seem to go into circuit design which; I already have a tenuous grasp on electricity as it is.

In: 2 I don’t think I ever used the term apparent power, but see [my previous comment here](https://www.reddit.com/r/explainlikeimfive/comments/wtjb98/comment/il4h6jt/?utm_source=share&utm_medium=web2x&context=3) for a discussion of why Volts and Amps don’t always line up.

The key point for you is that the apparent power is how hard the inverter has to work, and is directly related to how much current is rushing in and out and so heating up all the components inside (which is why this is how the rating is stated). In contrast real power is how much useful stuff you see at the output, which will necessarily be less.

Edit: think of it like this. There is a steady stream of power coming in from the input. There is a steady stream of (real) power flowing from the output of the inverter to the load. But there is also some amount of (reactive) power sloshing backwards and forwards between the inverter and the load. This doesn’t have any net flow so it doesn’t do useful work, nor does it place any demand on the input. However the total amount of power moving about in the inverter is the sum* of the steady flow and the sloshing (this sum is called the apparent power), and it is this sum which leads to heating which is what limits the capability of the inverter.

*The real and reactive power are actually out of phase so the apparent power is the hypotenuse of a triangle rather than a simple sum. Oversimplified: Watts is how much power a device uses to do actual work (or how much actually usable power you get)

Volt-Amperes is how much power flows through cables.

For example you can have a device use 100 Watts, but it would be 110VA due to losses (e.g waste heat, AC to DC to AC conversion, etc. ) Unfortunately it is pretty technical.

AC power looks like a sine wave when you graph voltage against time. In AC circuits there are components that can affect that sine wave by shifting it left and right. (like inductors or capacitors).

You can also graph the current over time and you will see the same sinusoidal graph.

If we put both waves on the same graph and they completely over lap eachother to the point that you can only see a single wave, then the AC power here is considered completely in phase of eachother.

When they dont match, they are out of phase. Things that have inductance or capacitance can shift the voltage wave left and right. Electric motors are basically really long wires wrapped into windings much like a coil, these add a lot of inductance to a circuit and shift the voltage left out of phase with the current.

real power is the portion of voltage that is in phase with the current

reactive power is voltage that is out of phase with the current.

True power (this is the VA measurement) is the theoretical maximum power of a circuit. Thic can be achieved with a purely resistive load (no inductance or capacitance shifting the voltage left or right) Real power is the power that does work (or is being converted into heat). If your load is either capacitive or inductive then it will store power (in magnetic fields (inductive) or electric fields (capacitive)) during some parts of the cycle and release it in others.

The power company doesn’t like it when you use a lot of apparent power relative to real power. Even though part of the apparent power is not actually used (it’s just temporarily stored energy) it increases the required peak power of the power plant. Furthermore, there are greater losses due to resistance in the cabling. Lets say you attach a perfect capacitor to the power lines. Every cycle of AC, the capacitor will charge and discharge, but it won’t actually use any energy, that goes back to the power grid.

The current is real, so your inverter or the power lines have to actually handle it, even if it doesn’t do any work.