I have been living off grid since may 2013 and I figured it informative to let others know what to look for in specifying an inverter prior to purchase. Based on my personal experience I will relate to you what are some good qualities of an off grid, or stand-alone, inverter.
Step 1: The Waveform.
I highly recommend using only a pure sine wave inverter. All homes have motor based appliances and these need clean sinusoidal power to operate properly and ensure long life. Do not be tempted into buying those cheaper modified square wave inverter (wrongly advertised as modified sine wave!).
The best quality pure sine wave inverters produce a clean waveform and are actually safe for medical grade equipment.
Step 2: The Power Rating.
The bigger the better. Why? For motor loads like fridges, there is a high starting current (4 to 8 times the running current) and you want your inverter to safely handle such loads. For example if your fridge pulls 300watts, buy as a minimum a 600watt inverter. In fact a 1000watt is even better but it depends on your budget.
An oversized unit will have a bigger idle current but it will last longer due to less thermal stress.
Step 3: High Frequency or Low Frequency?
Basically the high frequency design is lower weight and more efficient than the low frequency type (this uses a bulky transformer). If the high frequency pure sine wave inverter has no transformer then it's dc supply cannot be grounded.
The low frequency type will offer a neutral output for the ac voltage and this can be tied to the utility neutral and also local ground. The same cannot be said for the high frequency type and as a result the switching of a load between the inverter and utility requires a 2 (or 3 pole for 240/120v output) transfer switch.
Simply put, make sure you read the documentation (prior to purchase) that comes from the manufacturer and if there is none then you may consider buying another well documented brand.
Step 4: The Internal Cooling.
I learnt this first hand that the cooling fans and or integral control circuit can fail fast and the rest of the inverter work until it overheats.
Honestly you can't tell if the cooling system will even last long. I had to remove the failed fans and externally mount new ones for easier future replacement. I also installed new programmable digital temperature controllers to manage the fans.
In the end you need to be observant with your inverters (and other off grid equipment) to know if the cooling is working.
Step 5: The DC Supply Voltage.
It is far better for voltage regulation and to reduce wiring costs, to specify a high dc voltage input inverter. A 48vdc input unit will use smaller supply conductors than an identically power rated unit at 12vdc.
Just be very careful around battery voltage above 30vdc. Always best to wear electrically insulated gloves around all live conductors.