Inverters

Inverters are one of the most useful bits of power electronics around, but they are also one of the biggest consumers of 12Volt power, so we need to know what we’re doing when we invest in one of these beasts. In short the inverter’s job is to take the 12Volts DC we have in our battery, and convert it to a 240 Volt AC supply like we have at home.

This means we can power all the must-have items we so love and adore, even when the only power source we have is a 12Volt battery. Well this is true in theory, and as always we’ll explore this idea against the background of some practical realities and a bit of science too.

Sizes:

Inverters come in all shapes and sizes – from small 150 Watt ones to those that will deliver 2000 Watts, and beyond. And then there are those that only claim they can deliver thousands of Watts. Yes, I’m afraid this is one area where we need to tread really carefully to make sure that we actually get what we think we’re getting. So a little history might be useful to us here.

The Truth and nothing but…

Anyone remember the days of Peak Music Power Output (PMPO)? Until then we just measured music output in Watts RMS (root-mean-square) and everyone was on a level playing field. Then came these “adventurous” makers of portable stereos claiming thousands of Watts PMPO. Despite PMPO being unfounded marketing hype[1], it took some years for the facts to seep through. Let’s throw some science at it and see how it comes up.

Let’s say I have a portable radio that claims 1000 Watts PMPO and takes 4 D-cell batteries. Now a 1.5V D-cell battery can deliver about 15 Amp-hours, so four of these will give us around 90 Watt-hours to play with (4 x 1.5V x 15Ah =90Wh). So if we ignore inefficiencies for a moment, at 1000 Watts we would deplete the batteries in 9/100 of an hour (90Wh/1000W) or about 5 minutes – sounds about right doesn’t it? Not! In fact the batteries last for weeks, even months – 5 minutes versus weeks & months?!? – something’s wrong, very wrong – I smell b…

So a quick check-in with the science is a pretty good bullshit filter, so let’s try that on inverters. Here we are dealing with the same sort of marketing hype, and there are a number of things to watch for, so in fact we’ll need a few bullshit filters – but we’ll take it slowly – stay with me.

Sinewaves, Modified ones, and Squares:

Sine Wave

First we’ll get the sinewave versus modified-sinewave issue out of the way. Unless you have very simple loads like heating or incandescent lighting (the old glow-worm bulbs) then it’s sinewave all the way, and true-sine and pure-sine are the same thing as sine-wave. Modified sinewave is actually misleading too – the  waveform starts out as a square wave – then they try to modify it to look something like a sinewave – so it’s actually a modified square-wave. Anyway, the output is nothing like a sinewave and can damage electrical and electronic equipment.

Square Wave

Think of the waveforms as a road, and imagine driving your car over each of these road surfaces – the nice smooth sinewave surface, or the kerb-like square-wave. This is what it’s like for the 240 Volt equipment that you connect to the inverter.

Oh, and the big attraction is that modified square-wave inverters are cheap, because the electronics inside is really simple – in fact I’ve even made a few myself. But unfortunately they also fit with the old saying: cheap-and-nasty. So if you value your equipment at all, don’t risk it – the inverter will survive, no problem, it’s your 240V equipment that won’t.

Toroids, Ferrites and THD:

Then it’s about how we produce that sinewave – basically there are two ways – either using a toroidal transformer, or a high-frequency ferrite transformer. The toroidal transformer is wound like a doughnut and is very efficient – a great start – but it’s made of iron so it’s heavy, and quite bulky. The ferrite transformer is basically a ceramic core and therefore light and also small, but it only comes alive at high frequencies, so some electronic trickery is needed to make it work.

Overload and Surge ratings:

Now as far as overload is concerned, the toroidal iron-core transformer is big and it takes a long time to heat up, so its overload (surge) rating tends to be around 3 times its continuous rating. This is really useful if I have a motor running off the inverter, as motors draw about 5 times their normal current on startup. In contrast the ferrite-cored transformer is smaller and heats up quite quickly, so its overload ratings are more modest, usually about 2 times their continuous rating. These overload ratings are typically for a short time too, from 1 to 5 seconds, and must be long enough to get our motor through its initial startup phase.

So, what does all this stuff about toroids and ferrites mean in practical terms – well we’ve touched on most of it already, so let’s summarise it in a table. We’ll look at a 1000 Watt model in each topology so that we’re comparing apples with apples.

Sometimes it’s not easy to tell what internals you’re getting and you may have to go scratching in the specs to get that info. Usually the weight and size will be a pretty good guide too. I’ve often heard the toroidal topology described as “built like a brick shithouse” and I reckon that about sums it up. The ferrite-based inverters are less robust, but will still do the job very nicely in many cases, and are usually cheaper too, which can be persuasive at times.

Oh, and in terms of measuring the quality or purity of the sinewave output, the term Total Harmonic Distortion (THD) is a widely-used standard, and anything under 5% is good.

Overload claims (aka Surge or Intermittent):

This is another area where marketing spin can creep in so let’s have a quick look at that. We touched briefly on continuous versus overload ratings, and both are measured in Watts. It’s what these mean to the average camper that we are interested in.

The most important is the continuous rating, and this is simply the power (Watts) that it can deliver for a long time without getting upset. This a key figure, and is always lower than the overload or intermittent figure, so it’s sometimes hidden or in brackets in the advertising brochure somewhere – in fact if it is hidden, keep sniffing around, you may find a rat.

The intermittent (surge) power is just that – the inverter can only supply that for a very short burst, so this will only be of interest to those of us that have motor-driven appliances that need to work off our inverter. Otherwise the only figure that really matters is the continuous output power.

Even when you have that continuous figure, I would still suggest some caution. Unfortunately there are manufacturers, particularly those brands only available online, that choose to label their products “optimistically”. A quick search for the brand on the ‘net will typically reveal a few cases where a buyer is airing a grievance after buying a huge inverter that can’t deliver the stated output. A recent case I viewed was by an electronics technician who measured his 1500 Watt inverter’s output to be a good few hundred Watts less than stated on the label. Not nice.

So how do we protect ourselves and make sure we’re getting what we’re paying for? Well one sure-fire way is to buy from a place where you can “see the whites of their eyes” – over the counter. The other is to look for a well-known brand, and as far as inverters go there are some very good Australian brands out there. Myself, I’m sick-and-tired of paying for something, and when I find it’s junk I have to rip everything out, buy a second time and re-install, so I just look for Australian brands where I can. It’s seldom the cheapest, but in the long run it saves me on time and sanity. Others of us may have more patience than I do – in that case, well done, use it to good effect!

What 240V items do I want to run?

Matching an inverter to our loads is also part of the mix, and now that we know a little about these inverter-beasties themselves we can get onto that bit.

Well this is usually a case of: you can’t always get what you want, but if you try some time, you just might find, you get what you need.[3] So it becomes a 3-way balance between what we want, what we actually need, and what we can get from our 12Volt battery.

When we invert 12Volts to 240Volts, that’s a 20-fold increase in voltage at the output, and to get that right we have to obey the formula of Watts = Volts x Amps. So this means on the 12Volt input side our current shoots up, reaching 100 Amps and more for big inverters. So this is where it comes down to what we can get from our 12Volt batteries.

So what do we want to run from this inverter of ours? Let me start with the two most mentioned items – a kettle and a microwave. Both of these have huge draw power draws – in fact the kettle is out of reach even for a 2000 Watt 12Volt inverter as it draws no less than 2400 Watts.

The microwave is not much better – a small 850 Watt microwave draws 1100 Watts or more from the 240V supply – the 850W is what it delivers into the cooking space. So this means an inverter with a continuous rating of 1500 Watts. Also, while running the microwave you’re looking at over 100 Amps coming from the 12 Volt battery, so it needs to be a really big bank for the inverter to work properly, and without damaging the batteries.

Maybe just leave the microwave and other high-power stuff to when 240 Volts is available at a caravan park or from a gennie? Anyway, something to think about, and this is where opinion and personal choice come in – so that’s over to you now.

What size Inverter?:

What helps a lot with sizing and inverters is that they are measured in Watts, so all we need to do is look at the wattage of the 240V things we want to run, and size the inverter accordingly.

Things like camera and phone chargers are typically less than 50 Watts, and most laptops are under 100 Watts. So if we can restrict ourselves to those few things, a small efficient 150 Watt inverter will do just fine, and the current draw on the 12Volt side will be under 15 Amps – all quite manageable. And most CPAP machines are around 50 Watts too, although the recent models have 12Volt adaptors available – a much more efficient option than an inverter. (see also How Much Power does 12Volt Stuff use?)

Standby Mode and No-Load current:

The other thing that affects our inverter decision is the no-load current and the standby current. These are both currents that are being drawn while the inverter is idle and not running any loads.

The standby mode is a low-current mode that some inverters have – some do this automatically, and others do it via a remote switch. Standby mode minimises the 12Volt current when the inverter is not needed, by suspending the main electronics in the inverter. The inverter can quickly “wake” from this mode when called on, either manually or automatically.

The no-load current is drawn when the inverter is “awake” but has no 240V load connected. This is higher than the standby current but the 240V is instantly available if required. Have a look at the table earlier on to get an idea of typical values for these two currents.

Why do we care about these currents anyway? Well the standby and no-load currents both increase with the size of the inverter. So if we have a 1000 Watt inverter but it is only running a laptop, this is a very inefficient way of doing things.

Generally, all we need is the smallest inverter that will comfortably run all our 240V stuff. And perhaps some of the “wants” will win out too – who knows?

Anyway, now you know how to choose, and what the implications of those various choices are.

Battery for the inverter:

In the article about deep-cycle batteries we saw that most manufacturers recommend a maximum current draw of 10-15% of the battery’s capacity. So if we have a 100 Ah deep-cycle battery then to maximise its life expectancy we would keep the charge and discharge currents to around 10 to 15 Amps.

A rough guesstimate of the 12Volt current drawn by an inverter is to take the wattage and divide by 10 – this is an easy one, just knock off a zero – so a 150 Watt inverter will draw up to 15 Amps[4]. The far end of the scale is a 2000 Watt inverter, which is up there at 200 Amps!

While we’re talking about inverters it’s useful to note that battery manufacturers also specify deep-cycle discharge times down to 5 hours as well. So discharging our typical 100 Ah battery in 5 hours takes us up to 20 Amps, or 20% of its capacity (100Ah/5hrs =20A). This is beyond our usual 10-15% figure for deep-cycle batteries, but if it’s just for a short while, the effects might not be too severe.

So let’s look at the 1100 Watts again that we need for that microwave we mentioned before. Using our divide-by-10 rule that gives us 110 Amps at 12 Volts. If we stretch a friendship with our batteries to 20% then we need a capacity in the order of 550 Amp-hours – and that is just huge. On the other hand, if we have a weekender or a beach shack where we don’t have to lug the batteries around, then that size battery bank is quite do-able.

However for those of us who are mobile, the choice would appear to be: have a microwave but shorten our battery life, or do without and save the batteries. Life’s all about choices isn’t it?

Inverter close to Battery

Finally, given the massive currents on the 12Volt side, we need to make sure the old Volt-Drop doesn’t catch us out. Australian Standards say we should keep our volt-drop under 5% or 0.6 Volts on a 12Volt system, but with high-power inverters it’s best to keep this around 0.2 Volts so we don’t waste power in the cables.

The volt-drop calculator is useful here, and allows us to choose a cable that will maximise the power into the inverter. Keeping the 12Volt cables short is essential in this case, so if distance is a problem, rather lengthen the cables on the 240 Volt side of things[5].

So, inverters hey? Useful beasties when we’re far from a 240V supply, but pretty hungry little things too. Anyway, knowing a bit about them certainly helps us to make sensible choices – different choices depending on our circumstances and preferences, but also savvy choices because now we know what we’re up to, and the benefits and drawbacks of each choice.

[1] Wikipedia has a pretty good go at PMPO and its factless origins too. Fortunately PMPO is used very little any more.

[2] At 50 Hz there are 50 cycles each second, so 30 cycles is 30/50 = 0.6 sec – that’s not long…

[3] With thanks to Mick Jagger for these wise words

[4] It is actually the Watts divided by 12Volts but dividing by 10 allows a bit for inverter inefficiency – and makes it an easy calc!

[5] On the 240V side, dropping a few volts is not a problem – the current is 20x less than on the 12Volt side.