Inverters can be very useful, allowing you to run 230v AC appliances from your 12v DC house battery. So you can run a toaster, hair dryer, printer, or even a washing machine, just as long as you have an inverter with sufficient capacity and house batteries that are able to supply the amp-hours that will be required.
This is pretty much a motorhome lifestyle decision. You don't need an inverter, but it can make possible some very nice to haves if that's what you want.
FITTING AN INVERTER
This should really be done by an auto electrician who will ensure that the appropriate heavy duty cables are used for connecting to the house battery, that the inverter is located appropriately and fitted with both a fuse and an accessible on-off switch.
Connecting the 230v AC output into your existing 230v circuitry is a job for a qualified professional, as you must comply to the appropriate regulations. In our case, we have simply made use of an extension cord for plugging appliances into the inverter's 230v output - which is completely separate form the existing 230v circuitry.
It's important to have your inverter installed in location that is not likely to attract spiders. We had our first inverted installed next to the battery compartment, which made connecting to the battery very convenient. You can imagine our dismay on turning it back on, some time later, when there was a loud bang and a cloud of acrid smoke. On opening it up, we found the remains of a spider, who got fried, along with our expensive inverter.
So, we had the replacement inverter installed inside the lounge seat storage, which was immediately above the house battery. That has worked reliably ever since.
Note, that it is a very good idea to have an easily accessed on-off switch, as the inverter will draw current from your batteries even when it's not being used.
RUNNING COMPUTERS AND RECHARGING DEVICES
The only 12v DC outlet in our motorhome was part of the Fiat engine console, which was only available while the engine was running. So, when we installed our inverter, I plugged the 230v adaptor for my laptop computer into the 230v inverter output, which I then used for charging my computer and mobile phone (using the USB port).
It was later pointed out to me that this was a grossly inefficient way to do both tasks. Depending on the size of the inverter, it might only be 50 or 60% efficient when providing 230v AC, and then there is loss again when my laptop adapter converts the 230v AC to 19v DC. The same applies for using the USB port to charge my phone.
The solution was very simple - to get an 12v DC cigarette lighter output installed and to make use of a 12v DC to 19v DC adaptor, Choosing the right one also provided a USB socket for charging our phones as well. These are very much more efficient, and don't draw power from the house battery while they are plugged in but not being used.
You can get an auto electrician to install the 12v DC socket, and you can buy a suitable adaptor at either a Jaycar Electronics store, PB Tech, JB HiFi, or online at www.elive.co.nz.