Understanding Inverters Pt 2 – Buying Guide

So you have decided that you need to run some mains powered equipment when away from home and your caravan or boat is not plugged into an EHU or Shore Power.

Before we look into the options for inverters and what to look out for, we have to ask a few questions first. What are we wanting to power? If it’s that you need to charge your mobile phone or laptop, then maybe an inverter is not the best way of doing it. There are a number of manufacturers offering a wide range of products for charging your mobile devices. An older style phone may need a dedicated 12 volt charger, but modern smart phones can usually be charged via a USB port and there are a number of products that offer USB charging from 12 volt ‘cigarette lighter’ type plugs. For laptops, it’s a bit more difficult, but there are options. Companies such as Lenovo, Kensington and Targus offer such devices. I have not put any links in as manufacturers often update their sites but you can search for them as “Universal Auto/Air Laptop Charger”

So, you still need an inverter?

Well, there are three main categories of inverter – Compact or Lightweight, Heavy Duty and Inverter/Charger. The compact or lightweight inverter is one that will usually plug into your 12 volt accessory socket in the car, caravan or boat. Compact or lightweight are usually rated up to about 200 to 300 watts and because of the cost are usually of the modified sine wave type – read through Understanding Inverters to get to know the different types of inverter and how they work. Remember, only inverters with a rating of 180 watts should be plugged into your accessory socket or you risk exceeding the fused current rating.

I’ll skip the second type for the moment and go on to the Inverter/Charger type. These are probably more commonly known as a UPS. No, it’s not one of the brown short wearing parcel delivery guy’s…. UPS is Uninterruptible Power Supply. Small UPS’s are more commonly found sitting under desks in offices and larger units in network and server cabinets. These are bits of equipment that plug into the wall socket and a computer plugs into them. They usually have a set of batteries internally and are designed to keep a  computer running for a short while in the event of a power cut. Some have the option of adding additional external batteries. UPS’s are great at running computers for a short period of time as the demand on them is fairly constant, they also have additional technology that will keep the output voltage stable and clear of transient spikes even if the supply voltage varies. To make them more efficient, usually they have 24 volt or 48 volt internal batteries, this allows the use of physically slightly smaller batteries for any given run time. The internal circuitry also monitors the batteries condition and will charge/discharge them in a maintenance cycle.

The device we are really interested in is the Heavy Duty inverter. These are available from 750 watts right up to 5000 watts – but at this wattage, only expect your 110Ah leisure battery to last a few minutes!

We decided that it wasn’t a mobile phone or laptop that we needed to charge, so what is it you want to run? Well, it could be anything… and the most common thing that people want to power is………… a hairdryer, closely followed by hair straighteners.

Well, we need to find out what size inverter we need to run our hair care devices. If we look at a hairdryer, it will have the wattage for the appliance written on it, lets say 1200 Watts for a professional hairdryer. When it’s connected to the mains supply, that 1200 watt’s is about 5.2 Amps ( P /V = I or 1200 / 230 = 5.2 ) which isn’t too bad. So how much will that be out of our 12 volt leisure battery? Well, 1200 watts is 1200 watts, no matter what the voltage, so we can do exactly the same calculation as before: 1200 Watts / 12 Volts = 100 Amps.

Wow 100 Amps! That’s about what my car takes when starting!

Yep, we are talking big bits of wire here. Now the calculation above would be great if some of the laws of physics didn’t exist and inverters were 100% efficient. Unfortunately they are not, far from it. It’s safe to assume that as an average they are round about 80% efficient, so we need to take that into account. It is always better having a 2000 watt inverter running a 1200 watt load rather than a 1200 watt inverter running flat-out. So, looking at our 1200 watt Hairdryer, we need to think of the 20% loss using the inverter, so we add another 240 watts and we have a load of 1440 watts. We need to be looking at that 2000 watt inverter really. If you are going to make the investment installing an inverter, it’s better to make sure you get one that will be comfortable doing the job.

Inverters of this size need installing permanently. The maximum current a 2000 watt inverter can draw is 166 amps… in fact it will be a bit more as the inverter itself needs power, usually to run a couple of cooling fans and its own internal circuits. The 12 volt cables that connect it to the battery are going to be as thick as your index finger, so some careful planning on its location is required.

So we have decided on the size of inverter, now what else?

Pure Sine Wave -v- Modified Square Wave

For me, there is no contest, always go for the Pure Sine Wave. It will be more expensive but you can be sure it will power absolutely everything you plug into it. Modified square wave always have a nasty habit of powering some things, but not others and it can be really annoying. In addition, some kit can be damaged if you try running of a modified square wave supply. If you are not sure what the difference is… it’s all explained in Understanding Inverters.

Inverter Efficiency

Sometimes bigger is not better. What we really need to know is what percentage of the power that goes into the inverter from the battery comes out the other side as AC power. To make this calculation even more difficult, the efficiency of an inverter changes with the output load placed on it. if a small load is put on an inverter it may be only around 50% efficient. However increase the load to near the inverters stated maximum continuous load and the efficiency will rise to around 90%.

A 3000 Watt inverter may draw around 20 Watts of power from the battery when it is connected and turned on without anything plugged into the output. This power is what the inverter needs to run itself… battery monitoring circuits, alarm circuits and maybe a small fan. Now if we plug-in a small AC load.. say 20 Watts…. the total load on the battery is now 40 Watts (20 for the inverter +20 for the load) so a bit of maths – 40 watts in – 20 Watts out shows an efficiency of 50%.

Now if we look at this again with a small 150 Watt inverter – connected to the battery with no load it takes about 5 Watts to run the inverter. Now if we plug in the same 20 Watt load we have 5 Watts for the inverter plus 20 watts for the load giving a total load on the battery of 25 Watts. So 25 Watts in and 20 Watts out gives us an efficiency of around 80%.

Practically this means that you coud run your 20 Watt appliance for much longer on the same battery by choosing to use the 150 Watt inverter rather than the 3000 inverter.

So size does matter…. but bigger is not always better. Matching your inverter size to your load size to maximize your run time of a battery is important.

What else?

Well, look for some of the following features:

  • Low voltage alarm
  • Battery auto cut out
  • Over temp alarm/cut out
  • Remote control
  • Power save sleep mode

The low voltage alarm is a must have really. It will alert you when the voltage on your battery has dropped to a level that could start to permanently damage your battery by allowing it to discharge too much. On some models, this voltage can be set differently for the type of battery you are using.  The Battery auto cut out will shut down the inverter when this critical voltage has been reached. As an inverter of this size is handling a big current, some of the internal components will get very hot, It is therefore essential when installing it to pay particular attention to the manufacturers installation instructions to make sure its located correctly. Nearly all good inverters will have an over temperature alarm and cut out that will stop your inverter being damaged if it does get too warm.

The next two are “nice to have’s”… when your inverter is switched on, even if there is nothing plugged into it, it will consume precious battery power. It can be as much as 2 or 3 amps. Some of the more sophisticated units have a sleep mode, which means you can leave it turned on and it will only consume a few milliamps…. until a load is plugged in. Some units don;t have this, instead, they have a remote control that allows you to turn on the inverter without having to physically press a button on the unit. This allows some flexibility in the location when installing it.

If you are installing an inverter in your caravan, motor home, boat or RV then make sure to follow the manufacturers instructions. Try to install the unit as close as possible to the battery to keep the big cables as short as possible and make sure you install an isolator switch and fuse between the inverter and battery. Modern leisure batteries store an unbelievable amount of energy that if discharged quickly can have a catastrophic effect and can kill. As with anything electrical… if you don’t know – ASK and stay safe.

Inverter Current and Power Table

One of the most often asked questions I get is “what current will be drawn from my battery for a XXX watt device plugged in to my inverter?”

To make it simple, I have assumed two things:- One, the rating the manufacturer gives is the inverter INPUT rating so a 300 Watt inverter is actually a 300 Watt input, not output. The second is that the efficiency is 80%.

So if you have a 300 Watt appliance you want to run, look down the right hand column (Output Wattage) for 300 Watts…. as there isn’t one for 300 Watts go to the next line – 320 Watts. Now look across to the left hand column (Inverter Rated Wattage) – 400 Watts. This is a good starting point for the size of inverter you would require.

Click on table to view full size

Click on table to view full size

As we have seen earlier, some inverters are more efficient, some less dependant on the load, so the table doesn’t account for that. Also, some equipment has a higher starting current at switch on than when running, again the table can’t work that out for you, but it does give you a good starting point. If you think you know which inverter you want, check out the manufacturers specifications on-line. Most publish full details, including temporary peak loads – which is useful to know if you anticipate using something that has a higher starting current.

Top Tip

If you want to use an inverter using two batteries to provide extended run time, read my  “How To: Connect Two Batteries in Parallel”  as there is a right way and a wrong way.

I hope you found the guide useful, keep looking out for some of my other guides here on Caravan Chronicles

Copyright © 2011 – 2015 Simon P Barlow – All rights reserved

21 thoughts on “Understanding Inverters Pt 2 – Buying Guide”

  1. Hello Simon.

    What very helpful information, thank you.
    I’m new to this and I now realise i’ve had our twin batteries and 2kw inverter in our MotorHome connected incorrectly.
    Could you please help in wiring up 7 identical batteries. Is this the same as 2 batteries ie connecting down the line and taking power feed from bat 1 and neg from bat 7.
    I’m a retired marine engineer/fabricator/inventor (old school) and keen on playing with powering my home/workshop with Gods sunshine plus alternative energy supply.
    Kind regards

    • Hi Graham

      Funnily enough it was a retired marine engineer (former submarine electrical engineer from WWII) who worked for Oldhams Batteries before retiring that told me about battery banks back in the early 70’s.

      You are correct, +ve from battery 1 and -ve from battery 7 (or the last battery) will always balance out volt drop across a battery bank.

      Apparently this also allowed them to use battery link straps of different cross sections saving copper, each link getting thicker as it approached the main feed (or return).


  2. Pls I would like to know if I can use a transformer from a UPS rated 400watt to build a 400watt inverter thanks I must say your site has really been of help

  3. say I had a 12V 125A leisure battery and wanted to run a fridge which uses 99kWh per year (based on standard test results for 24 hours`). How long would the fridge run for using a 1500kv inverter before the battery was flat?

    • There are a lot of variables… but to make any meaningful calculation you need to know the wattage of the fridge (volts and amps) when it is running.

    • I would use the following math:
      Fridge consumption = 99kWh / 365 days = 0.271kWh = 271Wh/day
      Inverter standby consumption (@1amp) = 12V x 1A x 24h = 288Wh/day
      Total consumption = 288Wh (inverter) + 271Wh (fridge) = 559Wh/day
      Battery energy = 12V x 125Ah = 1500Wh
      Battery run time = 1500Wh / 559Wh/day = 2.68 days

      Inverter standby usage will vary, so insert your figure instead of 1A.
      Furthermore as Simon pointed out there are many further factors that might affect runtime in a negative way, such as high amp draw from the battery. Fridge compressor runs only for a short time but it may draw quite a lot of power. The higher the amp draw the less energy you will get out of fully charged battery (effectively less Ah). Another factor is how deep you want to discharge your battery. Fully draining battery will reduce the longevity.

      • Hi Andrius S
        I’d also cone up with about 2.5 days, but the volume of the fridge and how much was in it, plus the ambient air temp and how many times the door is opened and for how long will all have an effect on the run time. I think that based on a 50% full fridge capacity and not trying to freeze anything and only discharging the battery to 50%, the calculated run time will be about 24 to 30 hours.

  4. I hope this does not sound too stupid, but I need help. I want something lightweight that I can charge at home, carry to a dog show and be able to plug a hair dryer or a straightening tongs into it, but I don’t know what this device would be called. Can you help me please and advise me where to get one? I am in Ireland. Thank you.

    • Hi Maureen
      To power a 1500 Watt hair dryer for 15 to 20 minutes you would need something like a car battery and 2000 Watt inverter. I haven’t seen anything like this available commercially in one box. It would be heavy and ‘transportable’ rather than ‘portable’.

      Nearly all the portable mains power packs are limited to just 200 or 300 watts and designed to run phone chargers or laptops. I don’t know of anything that would be big enough to run a hairdryer.

      As an alternative, there are hair straighteners that work on gas… they are usually refillable from the cylinders used to refill gas lighters. For the hair dryer, there are also 12 volt versions available that could be used with the emergency car jump start power packs that have an accessory socket. I’m not sure how good a 12 volt hair dryer would be compared to a mains powered hairdryer though.

      Sorry I can’t be more helpful.


      • Thanks a million Simon. I don’t find the gas straighteners very good, but have ordered a re-chargeable one. It is the hair dry that I could do with the most and bought one a couple of years ago that also runs on gas, but it is useless. Oh well, I will keep on the look-out for something better. I appreciate your response. These dogs will leave me in the poor house lol.

        Best regards,

  5. Dear Simon, I want to run a 240 volt pulling 4.7 amps to start and runs on 4.0. I wanted to use a 12 volt. DC to 240 volt ac using it with my car idling and getting the extra push from the alternator, which at idle is 55 amps. I calculated the inverter at around 1600 watt with a surge to 3500 watt. I figure I would need 0 size wire no longer than 3 feet. The odd part is the run to the pump is probably 70′. Did I do that right. We thought that we could run the pump connected to do all the water things necessary and shut it down for the day ( when the power goes out) Did I calculate the power right? Do you see any issues that I need to take care of? Thanks all of your info was very helpful.

    • Hi Patrick
      You need to check the figures for the pump first. The current draw you gave might be a “no load” rating. You need to check what the start up current is with all the pipework connected and primed, and check the current when running with all the pipework full. Fluids are heavy and have a mass when trying to get them moving with a pump.
      Assuming the figures given are correct, 240 x 4.7 amps = 1128 watts startup
      240 x 4.0 amps = 960 watts continuous
      I’d look for a 2000 watt inverter.
      The startup current drawn from the battery based on 1128 watts will be 94 amps and (960 / 12) 80 amps continuous when running.
      I would be cautious about running an alternator at the same time as trying to run an inverter. The normal installation and set up in vehicles is based on the alternator only having to supply the vehicle electrics and charge a starter battery. If you suddenly put an additional load of 90 amps on top of what the vehicle system is designed for could potentially cause issues with the alternator electronics. Plus you will always be discharging the starter battery when the inverter is running.
      It is more usual to install a second (or dual) battery to run inverters and have a split charge unit to allow the alternator to charge the inverter battery and this would protect the starter battery from being discharged so that you will always be able to start the engine. Some split chargers have the facility to switch a link to connect the two (starter and inverter) batteries together for emergency use.

  6. Hi,I have a 1unit – 200 watt solar panel, 1 unit – 100ah battery, 40ah controller and 4000watts inverter. What do I need to add for me to run a 140 refrigerator.

    • Hi

      You need to work out the total consumption of the fridge over a 24 hour period. Iv’e used 200 watts for the fridge to keep the maths simple.

      If the fridge is rated at 200 watts = 200/12 volts = 16 amps. So we can say the fridge needs 16 AmpHours and a total of 324 Ah per day.

      Just running of battery you would need at least a 324 Ah battery (and that would be discharging it to zero) As you are only supposed to discharge a battery to 50% you would therefore need at least a 650Ah capacity.

      The solar panel produces 200 watts = 200/12 = 16Amps or 16 Ah. However as the panel is only effective for a max of 12 hours per day, it will only produce 12 x 16 = 192 Ah in total, so you can see that you would need at least 350 watts of solar panel (350/12 = 29Ah x 12 hours =350 Ah) to supply the same energy required by the fridge in a day returning the batteries to fully charged.

      I haven’t included any losses…. inverter efficiency loss, battery and charger loss etc.

      Realistically to run a 200 what load for 24 hours, given only 8 hours of sunlight per day you are looking at a solar panel setup of around 800 watts and a battery bank capacity of 600 Ah.

      The other things to factor in are charging times of the batteries… can you actually charge the batteries back up to full in the given time.

      Here is a great explanation of it, although it is for a domestic fridge installed in an RV… (watch the video) http://www.gonewiththewynns.com/rv-residential-refrigerator-power

  7. Hi Simon,
    your site is very educative. I wish I had seen it before buying my inverter. The problem I’m having now is I use my inverter to power my computer. whenever there’s power failure the computer restarts due to inverter delay to switch to battery backup. Can I use a UPS combined with the inverter to compliment for this so the computer won’t go off/reboot?

    • Hi
      Personally I’d look at a UPS (Uninterruptible Power Supply) from APC (American Power Conversion ) for your computer. Plug your computer into a UPS and plug that into your inverter. If there is a power outage, the UPS will power your computer until the inverter kicks in. You can get inverters that will run a computer and screen for 20 to 30 minutes. UPS systems work slightly differently than basic inverters in two much as the output is already being run by the battery which is being maintained by the main electrical supply. Added advantage is they also smooth out any voltage fluctuations and suppress any voltage spikes.

  8. Hi Simon. Excellent article but I think I’m falling between the cracks! I have a coffee machine on my van which is gas powered for the boiler and has a 350w electrical rating (the pump and dosing). It is used to make approx 450 cups of coffee a week. I have a 220a leisure battery through a 1000w inverter which has powered it all nicely for about 5 years (the battery last around 2 years). Lately the machine is cutting out with no audible alarm from the inverter – the battery never gets to full discharge anyway. At the weekend I powered the machine from a 1000w pure sine wave generator and it never cut out once – am I right in assuming that I need to replace the inverter, or should I be looking elsewhere? Out of interest the machine will cut out sometimes just being in standby

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