Volt-amps vs watts: what is a volt-amp anyway?
You may have noticed, when shopping around for an inverter or inverter/charger, that the power rating is given in volt-amps (VA) and not watts (W).
When discussing power, we’ve typically used the unit of watts. This gives us the rate a system consumes energy. Naturally, it’s important to know the wattage of different appliances when, for example, we’re trying to work out what size inverter to buy.
However, there is an important distinction between watts and volt-amps. In this article, we’ll explore what on earth a volt-amp is and why people use them. Plus, we’ll explain how knowing about a volt-amp can help you to design your electrical system.
Just interested in what a volt-amp rating means for sizing your inverter? Head down to the ‘How to work out what inverter you need’ section.
What is the difference between watts and volt-amps?
Real power is power that does work or is lost to heat. It’s measured in watts (W) and is a measure of the rate of energy consumption. One watt of power is equal to one joule of energy per second. It’s useful to know real power when working out how much energy your electrical system consumes.
Apparent power is the total power moving through a circuit, including some that isn’t consumed. We’ll explain why this happens in a bit. It’s measured in volt-amperes (VA) or ‘volt-amps’ and is equal to the RMS voltage times the RMS current. RMS stands for root mean squared. The RMS current and voltage of an AC supply is the current and voltage that would give a DC supply the same power. Knowing the apparent power of an appliance allows us to find the RMS current by dividing by the RMS voltage. This is useful when sizing cables, fuses and breakers in our electrical systems.
Why is real power different to apparent power?
Many appliances in AC circuits contain inductors and capacitors. These passive components are able to store electrical energy. Inductors store energy in a magnetic field, whereas capacitors store it in an electric field. They absorb energy when they charge and return it to the power source when they discharge.
In a circuit with an ideal capacitor or inductor, there is a net-zero energy transfer. This means that all the energy they absorb returns to the supply. In contrast, energy consuming components, such as motors, do work to convert electrical energy to mechanical and heat energy, for example. The power, drawn by both capacitors and inductors, is called reactive power.
Your campervan’s AC circuit will contain appliances with capacitors, inductors and many energy consuming components. These appliances will draw both real power, which will be used up, and reactive power, which will bounce to and fro between them and the inverter. We call the combination of real and reactive power the apparent power.
As you can see, real power is consumed by components such as a light bulb. Conversely, reactive power bounces to and fro between a capacitor or inductor and its power source.
A useful analogy
A common analogy for these different types of power is a pint of beer. Let’s assume that the head doesn’t collapse and is left at the bottom of your glass once you’ve finished. We can think of the beer as the real power, the bit you’re going to drink. The reactive power is the head on top, that for argument’s sake we don’t drink (yeah…right!). Finally, the apparent power is the full pint, beer and head together. Although we only drink the beer, the glass needs to be big enough to hold both. We can apply this to thinking about an AC circuit. Although the system only consumes the real power, we need to size our cables, fuses and breakers to handle the apparent power.
A pint of beer is a great way to visualise different types of power. It's good practice to always keep one in your fridge, just in case a mate asks you to describe apparent power to them…
Is this all getting a bit complicated? Nohma can help! Our engineers will design, provide an electrical system bespoke to your needs. Plus, it doesn’t cost any more than the components themselves.
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How do you measure watts?
As you know by now, one watt of power is equal to one joule of energy used over one second:
Power (W) = Energy (J) / Time (s)
In a DC circuit, both voltage and current are constant. This means that power will also be constant. Therefore, you can multiply them to get power in watts:
Real Power DC (W) = Voltage DC (V) x Current DC (A)
In an AC circuit, however, voltage and current are constantly changing. Therefore, power is also constantly changing. So, the real power would be different depending on at which point in the alternating cycle you measured. To get an average value for real power then, you have to measure the instantaneous current and voltage at many points. Next, you need to multiply them together, and take an average.
These graphs show how voltage varies relates to time in DC and AC power. The same relationship exists between current and time.
You wouldn’t be able to do this with a traditional multimeter, you would instead require a wattmeter. Whilst these aren’t overly expensive, they’re only really useful if you’re trying to work out how much energy a certain appliance in your electrical system is using.
How do I measure volt-amps?
In DC circuits, apparent power and real power are the same. So, we can use the same equation to find apparent power.
Apparent Power DC (VA) = Voltage DC (V) x Current DC (A)
For AC circuits, apparent power is found by multiplying the RMS voltage and RMS current. These can both be found using a standard multimeter.
Apparent Power AC (VA) = RMS Voltage AC (V) x RMS Current AC (A)
Volt-amps were created as a way to find the RMS current draw of a component so that you can size connected cables, fuses and breakers. If you know the apparent power of an appliance, you can divide by the RMS voltage (230VRMS in the UK) to find the RMS current. Subsequently, you can size connecting components to handle this current.
How do I find the total power in a circuit?
To calculate the total real power in both DC and AC circuits, you simply add the power draw of each load. Conveniently, this works for series, parallel and series-parallel circuits. Simply add the real power of each load in your circuit to find the total power.
Total Power (W) = Load 1 Power (W) + Load 2 Power (W) + … + Load ‘x’ Power (W)
For DC circuits, seeing as real and apparent power are equal, you find apparent power in the same way as real power. For AC circuits, it’s not really possible to calculate the total apparent power. This is because current through the different loads could be out of phase from each other. The graphs show how the phase between the power in two loads affects their total power. Here, the red and black lines are the power of two loads, and the blue line is the total power.
As you can see, the maximum power occurs in an AC circuit occurs when the power of each load is in phase.
As you can see, when the current is in phase (lined up) between each load, the total power is the sum of the maximum powers. When the current is 180 degrees out of phase, you can see that the total power is the difference between the maximum current and voltage. This shows us that in real life, we can’t find a total apparent power, but we can find a maximum for it. This is useful when sizing fuses and cables, as it allows us to work out the maximum current they may carry.
Max Apparent Power (VA) = Load 1 Power (VA) + Load 2 Power (VA) + … + Load ‘x’ Power (VA)
What is a power factor?
You often see a metric known as ‘power factor’ given on an appliance’s datasheet. This is a measure of an appliance’s efficiency. It’s given as a number between zero and one, and is the ratio between its real and apparent power.
Power factor, PF = Real power (W) / Apparent power (VA)
A power factor of one is ideal, as it means that the power is 100% real. This is beneficial, as you could power it with thinner cable than in a less efficient device. Moreover, we lose some reactive power in the circuit as heat energy due to a voltage drop over the cables. So, a larger power factor means less wasted energy. Appliances frequently have added inductors or capacitors to help improve the power factor. A power factor of zero would occur with a pure capacitor circuit or a pure inductor circuit, where the power is 100% reactive.
Visualising power as a pint of beer, we can see that a power factor of one is ideal.
Relating back to our beer analogy, a pint with no head would have a power factor of one. Whereas, a pint that’s entirely foam would have a power factor of zero.
Why are some power ratings given in volt-amps?
We now know the difference between real and apparent power, but why are some power ratings given as VA instead of W? Inverters are an example of components that are often rated in VA. For example, all Victron inverters and inverter/chargers are rated in volt amps. Because we can connect them to any number of different appliances, we don’t know how much power will be real, and how much will be reactive.
Capacitors or inductors in the appliances we connect can make the voltage lead or follow the current. In other words, they make the voltage and current sine waves misalign. Only if they were in phase with each other would the power be all real power. In contrast, if they’re 90 degrees out of phase (seen in a perfect inductance or capacitance circuit), the power would be completely reactive. Therefore, the power rating in volt-amps is the maximum power the inverter could output if the rest of the circuit had a power factor of one.
How to work out what inverter you need
We now know why manufacturers give inverter power ratings in VA, but how do we use this information when choosing an inverter? Simply, you need to make sure that the apparent power rating is large enough to account for the power factor of your camper’s appliances. Power factors vary between appliances, but it’s unlikely that you will see one below 0.8 in a campervan electrical system. Therefore, it’s safe to assume that your electrical system’s average power factor won’t be over 0.8.
Appliances with motors, such as an electric drill or blender, are inductive loads and draw a high amount of reactive power to magnetise their coils. Commonly, the power factor of these components will sit at around 0.8. Another appliance with inductors is an induction hob. Unlike appliances with motors, their power factors are heavily compensated with capacitors, bringing them close to 1. Heating appliances on the other hand, such as hair straighteners, are entirely resistive, so their power factors are very nearly 1. Remember, the higher the power factor, the closer the VA and W ratings are to each other.
PF: 0.8
←
Power Factor
→
PF: 1
It’s impractical to work out the power factor of your circuit too accurately. Moreover, powering different appliances on your AC system will change this total power factor. Therefore, it’s best to assume a conservative average of 0.8. So, to work out what size inverter you need, you can divide your system’s total power requirements in watts, by 0.8 to find the power rating in VA your inverter needs to have.
Inverter power rating (VA) = power requirement (W) / 0.8
We hope this short article has helped demystify the differences between watts and volt-amps and has assisted you in choosing an inverter! If you don’t feel confident in picking the right inverter, let us help you. Nohma employs scientists and engineers who, alongside our clever algorithm, design thousands of bespoke electrical systems each year for a broad range of applications. We can design your electrical system and ship it to you for no extra cost than the components themselves!
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