Inverters And How To Size

So you have decided that you need to run some electronics when away from home and your caravan/motorhome or boat is not plugged into mains, what do you do? Fit an inverter! But what size inverter?

Before you look into inverters you may need to ask yourself a few questions first. What are you wanting to power or charge? If it’s something simple like a mobile phone or laptop, then an inverter is an expensive way to do it. Lets face it, a modern day mobile phone uses USB to charge, that’s 5v maximum so no point in switching even the smallest inverter on for, much easier to run a cigarette USB type device. Laptops are slightly more difficult because most run at 19v so if your on a 12v system your a little short, however there are plenty of ‘buck inverters’ around that are dirt cheap and again plug into a cigarette socket, these work by upping the voltage usually adjustable between 12v and 22v, if your on a 24v or higher system then you don’t need anything!

Still hankering for an inverter?

We can group inverters into three main categories – Small, Large and Inverter/Charger. The small type inverter is one that will usually plug into your 12 volt accessory/cigarette socket and not draw a lot of amperage, the positive and negative wires are usually quite thin. Small inverters range from 100 to 600 watts and usually of the modified sine wave type, although nowadays you can easily get pure sine wave as well, these would be the preferred type and we won’t go into the differences here, just take my word for it, pure sine wave will not restrict you on what you can run. Remember, inverters with a rating of 180 watts or less are the only power to be plugged into your cigarette socket as the fuse current rating is usually very small.

So large type inverters, these usually range from around 800 watts right upto 5000 watts i believe is the biggest i have seen (Victron 12v) however! Your 120ah AGM leisure battery will last seconds if you were to run something needing 5000 watts through it! So going back to the beginning now you can see it would be foolish to switch a 5000 watt inverter on to charge your laptop, the inverter idling with nothing plugged in would use more power than it takes to charge the laptop!

Ok so you still need 240v and if your household is like mine, err indoors will be moaning about not being able to dry her hair or straighten it (what did we do before?) after having her expensive shower!

So your average hair dryer is around 1200 watts, at home that’s about 5ish amps (W/V=I (1200w / 240v = 5a)) however, we aren’t at home! Now we are running off batteries and in this instance (because it’s most common) 12 volt batteries, so same theory (W/V=I) (1200w / 12v = 100a) that’s you buggered for a night in front of the TV!

Not so bad you say……well hold on! Inverters are no-where near 100% efficient in fact your probably looking at around 80% efficient at best, so that means on top of your 1200 watts while the Love of your life is sat in front of the mirror you have to add another 240 watts to the power that she is using, so rather than a 1200w inverter now realistically you need nearer 2000w, this has now turned from an easy portable plug in job to a properly fitted inverter with cable size to suite and in a nicely ventilated, flame proof area! (inverters can get very hot remember) Not only that but generally an inverter of this size will pull around 160 amp minimum so that’s the battery storage gone (cheaper to send her to the hairdresser’s)

So is bigger always better? Well no not really…..going back to efficiency let’s look at a 3000 watt inverter, these can take 20 watts at idle (nothing plugged in) my Victron does! So let’s say we plug something small in that takes 20 watts, that’s 40 watts disappearing straight away, that’s assuming you switch your inverter off when not using it, that’s only 50% efficient, meaning 40 watts coming out of the battery pack and only powering a 20 watt item on AC! Crikey you might gasp, but generally it goes off load as well, so the nearer you get to using an inverters peak wattage the more efficient it gets but you get the gist that if your running something small then a smaller inverter will still power it but use less from the batteries or run it for longer! A note here as well is, make sure your inverter is what it claims! Plenty of inverters say 1500w on them but when you read the instructions that’s a surge power for so long, not the long term running! Our Epever inverters are rated 1500w (with a peak of 3000w) but only run 1500w for 6 min’s then return to a nominal running of 1200w, many of the manufacturers try to confuse you like this! My own Victron 3000kva Mk3 you would assume runs at 3000w, no it doesn’t it’s actually 2400w

Below is a rough idea of what amperage and inverter will consume for a given wattage, obviously if your 24 volt this can basically be halved on amperage draw, quartered on 48 volt etc, it’s only a rough guide!

On another note, sometimes it is also cheaper and easier buying wise with electrical goods, joking aside i bought a hair dryer and straighteners that work perfectly well but use a lot less wattage than regular ones, it’s a matter of reading the label before buying! Russell Hobs do a 1000w 240v kettle for example and no it doesn’t take all day to boil, holds 4 mugs full but consumes a lot less power than a normal 3000w kettle!

Something to think about!

Table of 12 volt inverter efficiency and output power

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