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.
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.
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.
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.
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.
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