A question that we are often asked is, “Which Inverter should I get?

There are, however, a few questions to ask first.

1. What do you want to be able to use while out and about on your cruise?

2. What type of Inverter do you need? Pure sinewave? Modified?

3. Inverter or Inverter charger?

4. Do you have enough space to fit your chosen inverter?

5. Do you have a sufficient battery bank to supply the inverter correctly?

6. Is your engine charging system good enough to recharge your battery bank ?

Meanings -

W = Watts - Value of Power required or being used

A = Amp - Value of current being used

V = Voltage - Mains power 230V Battery most commonly 12V

Ah = Amp hour - Capacity of a battery how many Amps it can give for 1 hour. 

Most batteries, however, only have half available as things don’t work as the voltage drops. So a 100Ah battery has 50 Ah available.

W Watts divided by the V supply voltage equals the A current used. So a 12W bulb powered from a 12V battery would use 1A.

Question 1. What do you want to be able to use?

Look at the equipment that you want to be able to use, all equipment has a label telling you how much power it uses.

The best example of checking the label is a Microwave. Most will have 700W or 800W on the front so people think that this is the power that it uses, but if you look at the label on the back, it will say 1200W or 1250W possibly more, this is the power that the microwave uses to cook at 700 or 800W. Washing machines and Dryers are probably the highest power consuming item - washing machines 2500W, Dryers 3000W.

So let’s start with a microwave and 1250W. We need to choose an inverter that can produce enough power for it to work, so we should add an extra margin so the inverter is not working at it’s maximum, say 10 to 15%. So we should be looking for an inverter rated above 1375W / 1438W. The most common would be 1500 or 1600 units. If we are thinking of the washing machine, then a 3000 unit. Dryers are very difficult because of the amount of power and how long they want it. A washing machine only needs the high power while it heats the water, and as we seem to wash cooler and cooler this is not so much as it used to be. Dryers, however, use heat the whole time and so use a lot more energy. A Dryer using 3000W on for 1 hour would use 250Ah, and this would flatten 5 standard 100Ah batteries. So a dryer really needs to be only used when you are on the Land line or using a generator.

Question 2. What type?

There are two main types of inverter - ones that will produce a pure sinewave, and others that produce a modified or quasi sinewave. Pure sinewave are better, and so more expensive, but generally they do not have any problems in making things work. Modified sinewave inverters sometimes have problems , especially powering processor controlled items, and have been known to give ghosting lines across TV screens, although this is much, much rarer than it used to be.

Question 3. Inverter or Inverter Charger?  

An Inverter just produces 230V mains power from your batteries. An Inverter Charger will also charge your batteries when connected to shore power or a generator, but this is all one unit. It also has an inbuilt transfer switch to select the power automatically. Because of this, the wiring is much simpler and the charging capability is much higher than most separate battery chargers. A Victron 12/1600/70 Multiplus has the ability of charging the domestic batteries at 70Amp and a trickle charge output for the start battery. Combi or Multi units are available from many manufacturers - Victron, Mastervolt, Sterling, Vetus, and Trace to mention a few.

Question 4. Space

Space is always a concern in a boat. The higher capacity the unit, the larger it will be and where can you mount it. It needs to be close to the batteries to keep the battery cables short, but not directly above them. When a battery is working, it will give off gases and if working hard the inverter can draw then in with the cooling fans, so you should always make sure there is good ventilation to allow any gasses to escape. So making sure you have a large enough safe place to mount the unit is very important.

Question 5. Large enough battery bank?

A 1000W unit would need 200Ah, so 2 batteries. 1600W would need 300Ah, so 3 batteries, and a 3000w unit would need 400Ah, so 4 batteries. These are quite approximate sizes, and the manual for any unit will give the minimum requirement. If too small a bank is used on a large inverter, it may very well show a fault like overload, this is because the unit cannot draw the amount of power that it requires.

Question 6. Charging the batteries

We have to be careful not to increase the battery bank size without thinking about how the batteries are charged from the engine. We need to assess the size of the alternator, so if you have an old Lister SR or ST 2 or 3, for example, these were often fitted with a quite small 35A alternator which would probably just manage to look after 2 batteries. If you have, say, a much newer Beta, you may very well have a 150A or even higher alternator which is fine for the larger 4 or 5 battery bank. We have to make sure that the batteries are charged properly.  

So, there are many things to consider when deciding on the best inverter for you, I hope this guide helps to point you in the right direction, to ask the questions to find the best solution for your requirements.