Inverter peak power and inrush current – what can your inverter handle?

When choosing an inverter for your campervan electrical system, you have likely noticed two power ratings. Manufacturers often give a surge, or an inverter peak power rating, alongside the continuous power rating. As you can probably guess, this surge rating gives the power an inverter can output over a short period of time. However, this time is rarely stated and so the peak power rating regularly causes confusion. Moreover, appliances rarely state their peak power draw, or for that matter, give an accurate figure for continuous power draw.

In this article, we take a look at what an inverter’s peak power really means and how long your inverter can output it. We also take a look at the peak power draw, or inrush current, of various common appliances to help you pick the right inverter to power everything you need.

Inverter peak power vs rated power

The rated, continuous or nominal power rating of an inverter is simply the power that it can output indefinitely without any problems. This will be the rating advertised on the inverter itself and is usually given in watts or volt-amps. To read more about what volt-amps are and why you need to know about them when choosing an inverter, our article on volt amps vs watts gives a full overview. 

In contrast to rated power, the peak, surge, or instantaneous power gives the maximum power that an inverter can output over a short period of time. More often than not, this is stated as double the rated power. Because of its short time period, you shouldn’t pick an inverter based on its peak power rating. Trying to power a product with a power draw greater than the continuous rating but less than the peak rating, will result in the inverter tripping. This use is not the peak rating’s purpose. Instead, peak power is built in to cover the short current spikes known as inrush current, which occur when starting appliances such as a blender or hair straighteners.

What causes an inverter overload?

If an inverter becomes overloaded, it will shut itself down as a protective measure. This doesn’t happen immediately, and inverters can deliver a degree of peak power. The time it takes for the inverter to shut down depends on how much it’s overloaded by.

An overload occurs in two situations. The first is simply a high current draw, and the second is a drop in output voltage. Ohm’s law tells us that power is equal to current times voltage. So, if the inverter keeps outputting 230V, a higher current draw will result in a higher power output. Victron’s inverters can hold 230V to around twice their continuous power rating, so an overload up to this point is a direct result of an increased current.

Above this current draw at 2x the power output, the inverter can’t convert any more AC power. So, using Ohm’s law again, we can see that an increased current draw will result in a decreased voltage output, to keep the power consistent. Here, the overload is still a result of an increased current draw, but the inverter notices the decreased voltage output and shuts down quickly.

How long can an inverter output peak power?

A Victron inverter or inverter charger can output its rated power indefinitely. So, a 500VA inverter could supply 500VA indefinitely, or 430W once you’ve accounted for the power factor. If you are keen to learn more about power factors and the difference between VA and W, our article can help you choose the correct apparent power rating of your inverter. 

Victron inverters can output 130% times their nominal power rating for up to 30 minutes before shutting down. But this depends on the ambient temperature as well as the batteries supplying the power. The shutdown occurs when the inverter, which heats up with the power overload, reaches a certain temperature. At 25°C ambient temperature, this takes around 30 minutes. A greater ambient temperature will mean a quicker shutdown. Similarly, a lower ambient temperature will mean a longer peak power output. 

For overloads ranging from 130% to 200%, Victron provides conflicting information. They state that their inverters can handle up to a 150% overload for 2 minutes, and an overload of 150% – 200% for 5 seconds. Contradictorily, they state elsewhere that they can handle up to 200% for up to 2 minutes. So, we’d recommend erring on the side of caution and following the first statement, as it’s more conservative. Regardless, overloads over 130% are extreme cases, and you shouldn’t count on them.

At an overload over 200%, the power output reaches its peak rating and the voltage output will drop. At this point, the inverter will try for 30 cycles (wavelengths) of the AC power to bring the voltage back to 230V, if it fails, it shuts itself down. This process takes around 0.5 seconds. 

What’s an inrush current?

Many common appliances draw a peak current when turned on. We call this peak power an inrush current, and it can be many times an appliance’s typical current draw. Appliances with capacitors, transformers, heaters and motors all experience an inrush current. These surges last from a few milliseconds to many seconds. Because manufacturers rarely state the peak power on an appliance, people often purchase a product to be powered by an inverter, only to find that the inrush current trips the inverter out. So, to help you make sure you choose an inverter that can power all of your essentials, we measured the inrush current of numerous common appliances. 

The graphics below show the size of the inrush currents in the appliances we tested. The first shows the height of the peak as a percentage of the typical power. Next, the second graphic shows the time the peaks lasted.

How did we measure inrush current?

Firstly, we modified an extension cord to separate the live wire from the earth and the neutral. Then, we attached a current clamp and connected this to an oscilloscope. A current clap contains a coil, which induces a current proportional to the current in a neighbouring wire. Then, when connected to an oscilloscope, it allows us to visualise the real-time current travelling into an appliance.

We then plugged various appliances into the extension lead and recorded the current as it peaked and returned to a constant level.

Peak power of common appliances

The results were surprising, with recorded peaks of up to 21 times the advertised power draw lasting up to a few minutes. Luckily, they weren’t all that bad, and most peaks only lasted a few milliseconds.

The table below shows the peak power draw from the inrush current for all the appliances we tested. Additionally, it shows their typical power draw and the nominal (advertised) power draw.

For appliances where the peak lasted for more than 0.5 seconds, we recorded the power draw at 0.5 seconds, 5 seconds and 120 seconds. We can then use these values to work out if an inverter can cope with the peak power. We’ll go through how to work this out shortly.

What appliances had the largest inrush current?

Hair straighteners

Appliances which heated up very fast were the worst culprits for inrush current. Of these, a pair of hair straighteners peaked the most and for the longest time. The graph below shows how this RMS current draw from the straighteners varied with time. The RMS current of an AC supply, is the current that would give a DC supply the same power. 

As you can see, the current peaked at 3.7A and took almost 160 seconds to move to a stable current draw. This peak current is equal to 850W, which is massive even before considering the fact that the manufacturer advertised the straighteners as 40W. Like other appliances with longer peaks (>~0.5s), it was hard to define where the peak ended, so we recorded the current at half a second, five seconds and two minutes after the peak’s start. These were the times that Victron inverters can handle different overloads. 

These results show us that theoretically a 500VA inverter could just power these hair straighteners. This is because the power consumption at these times fit within the overload limits outlined by Victron for the same timeframes. However, this is under ideal conditions with a cold temperature and nothing else being powered by the inverter. Furthermore, this is for these specific hair straighteners, we’ve had a pair of 120W straighteners trip out a 500W inverter!

If there’s one thing we’ve learned whilst writing this article, it’s that you can’t rely on a product’s power rating! If you’re looking to power hair straighteners in your van, you will need at least an 800W inverter, but this could be way more.

The heating elements in the hair straighteners caused this inrush current. When cold, the resistance throughout the elements is very low, and they draw a high current. Then, as they rapidly heat up, the resistance increases and the current draw drops. The obvious conclusion from this is that other appliances containing heating elements, such as irons, toasters, and kettles should experience a similar inrush current. Well… they don’t. These appliances showed no peak power surge at all. And, we don’t really know why! Our thoughts are that the larger heating elements in these appliances mean a lower resistance. This lower resistance is closer to the resistance elsewhere in the appliance, unlike in the hair straighteners. As a result, the heating element isn’t the resistance bottleneck, and so its changing resistance has a small effect on the current draw.

Mixers, blenders, and drills

The second-largest current surges were caused by appliances containing large motors such as blenders, mixers, and drills. The graph below shows the inrush current of the drill on startup. On the left is a graph of the RMS current against time and on the right is instantaneous current against time.

The inrush current when a motor starts is equal to something known as the stall current. This is the current drawn by a motor when it is not rotating. At this point, current can freely flow into the motor and is only restricted by the resistance of the motor’s coil wires. Then, as the motor energises and begins to rotate, the movement of its magnets across its wire coils induces a voltage known as a back EMF in the opposite direction to the applied voltage. In effect, this opposing voltage restricts the current flow into the motor. So, we observe the end of the current surge in the graph above. The rest of the motor containing appliances showed a similar relationship for the same reason.

Which appliances had little to no current surge?

Iron, kettle, coffee machine, and toaster

As mentioned previously, just because two appliances work similarly, that doesn’t mean that they will have a similar inrush current. We were surprised to find that whilst hair straighteners and a hair dryer showed a large inrush current, the rest of the appliances containing a heating element showed no peak. This was likely because the heating elements in these products were much larger than in the straighteners, and therefore their change in resistance affected the current draw less. 

The graph below shows the alternating current passing into the toaster as it is turned on. As you can see, there is no inrush current.

Radio, TV and phone chargers

The phone, drill and laptop chargers, as well as the radio and TV, showed short or no current surges. The power adaptors for all of these products work to convert 230V AC to steady DC power. Internally, they all contain capacitors to smooth out the DC output. These are energy storage devices which, unlike batteries, can discharge almost instantaneously. When empty, there will be almost zero resistance across a capacitor. When we plug something into one of these chargers, there is a surge of current as the capacitors begin to charge. As the capacitors charge, resistance quickly increases and the current draw decreases. The graph below shows how the current and voltage vary as a capacitor charges.

For the appliances where the size of the capacitor is larger compared to the appliance’s power output, a small inrush current was noticeable. Because capacitors charge so much faster than a motor speeds up, or hair straighteners heat up, the current surge is far shorter in these chargers than in our previous appliances. 

The graph below shows the inrush current as the radio was plugged in. Although the peak current was far greater than the typical current draw, it only lasted for a few cycles. This equates to 0.02 seconds for the main peak, and 0.04 second before it reached the typical current

Can I trust power ratings?

From the data found in this experiment, it’s clear that a product’s power rating is not at all representative of its peak power. At the extreme, the hair straighteners peaked at 2160% of the nominal current draw. Plenty of other appliances peaked by many times more than the nominal current. So, it’s safe to conclude that the advertised current draw can’t be used as an indication of peak power.

Luckily, as mentioned previously, most inverters can handle a peak many times their rated power for under half a second. Therefore, we only really need to worry about the peak power of appliances containing a motor and some containing heating elements, such as hair straighteners and hair dryers. Bear in mind that we only tested one example of each appliance. Just because our kettle didn’t peak, it doesn’t mean that yours won’t! The data we collected should give you an idea of what size inverter you need to power appliances with longer current surges, but please only use them as guidelines. For example, you would need an inverter at least 12.5x the power rating of our hair straighteners to power them. However, this number likely varies vastly between different hair straighteners.

As for the typical power consumption of the various appliances, the nominal current was somewhat more representative, at least as an upper limit. On the high end, a phone charger consumed 252% of the nominal power, and on the low end, the stand mixer consumed a maximum of 48% of the nominal power. No large appliance drew more than 130% of the nominal power continuously. Therefore, it’s safe to use the rated, or nominal power as an indication of the typical current draw of a product. If this is less than the rating of your inverter, so long as it can handle the peak power, it should be able to handle the typical power.

We hope this article helped you navigate the confusing topic of inverter peak power, and you now know the typical current surges of some common products. This information should help you make an informed decision when buying an inverter. Let us know in the comments if you’ve bought a product that uses way more current than advertised!

If you’re still not sure which components to buy for your electrical system, let Nohma do the hard work for you. We employ scientists and engineers, who, alongside our clever algorithm, have designed thousands of bespoke campervan electrical systems. Let us design and ship yours, for no extra cost over the parts themselves.

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