Main Inver ter Types
OFF-GRID
Typically ranging in size from 500 watts to 5,000 watts, off-grid inverters serve remote cabins, homes, barns and other facilities with no access to the public utility grid. Electricity from solar modules or wind turbines is stored in large battery banks from which the inverter draws power when needed.
Off-grid inverters come in two primary varieties:
1. Square wave or modified sine wave.
2. Sine wave (sometimes described as "Pure Sine wave")
Square Wave or Modified Sine wave
The square wave form will be as shown below. The modified sine wave form will make some attempt to round the corners off though the waveform will still have some sharp corners or spikes.
Sine Wave
Compare this to the Sine wave form to the right, which replicates the waveform provided by the utility grid. Most appliances are
designed to work with this pure sine wave rather than a modified or square form. Many AC appliances will work perfectly well with a modified sine 
wave form wave. Some appliances such as computers, televisions, radios or music centers have built-in power supplies that reduce the voltage, rectify it to produce a DC current, and smooth it to give a steady DC voltage. This process will often smooth out any noise that was in the original AC supply. However there is the possibility, without a sine wave inverter, that a television picture may not be as clear as it should be and anything
with a timer (e.g., a bread maker) may not run at the correct speed. There may also be a noise problem. Any equipment that may give a quiet hum when connected to the utility grid, is likely to give a more annoying buzz when using a modified sine wave inverter or square wave inverter. This effect is most prominent in home appliances such as ceiling fans, particularly when running at lower speeds. These potential problems will need to be balanced against the price difference (modified sine wave inverters will be significantly cheaper than pure sine wave) taking into account the appliances you expect to be using.
GRID-TIED
A grid-tied inverter or utility intertie inverter converts DC current from solar modules into AC current and feeds the user's electricity needs directly. To the extent that the solar modules produce more than the user requires, excess power is pushed by the inverter in the public utility grid where it is purchased by the utility company according to local rules and regulations. Grid-tied systems use the utility grid, effectively as the storage bank, drawing from the grid when necessary and pushing into the grid when there is excess power. These inverters are designed to typically operate between 200 and 600 volts DC, requiring that solar modules be grouped into strings that combine to reach this voltage window. As a
result, grid-tied inverters are also called string inverters. At these voltages, electricity is very dangerous and should be managed in a careful manner. Because grid-tied inverters cannot handle storage systems such as batteries, grid-tied systems are subject to the
weaknesses of the local grid. When the grid goes down the grid-tied solar energy system will also go down.
DUAL FUNCTION
Dual function inverters work both as an off-grid and grid-tied inverter in one box, allowing battery storage for use when the grid goes down, and also pushing power to the grid when the batteries are full and the solar panels are producing excess electricity. These inverters are typically used where there is an unreliable grid that goes down for extended periods of time or in situations where even a short grid blackout is intolerable. These systems are called Grid-Tied Battery Backup systems and are the most expensive choice for customers wishing to go solar. They
necessarily require the expense of both the off-grid and grid-tied systems.
Sizing Inverters
Off-grid inverters will necessarily be sized according to the maximum requirements of the home or business of which they serve. The inverter must be able to supply enough power at any given instant to supply the expected combined anticipated draw even if that draw will only occur occasionally. If appliances in the home simultaneously combine to require 4000 watts the inverter must be sized at or above 4000 watts. Grid-tied inverters on the other hand, are not required to match the simultaneous maximum draw wattage like off-grid inverters must. Instead the system draws power from the grid when the requirements of the home or business exceed the capability of the inverter. As a result, grid-tied inverters are sized according to the maximum power output of the attached solar modules, which may or may not be near the maximum requirements of the home.