Definition: On-Grid Systems are solar PV systems that only generate power when the utility power grid is available. They must connect to the grid to function. They can send excess power generated back to the grid when you are overproducing so you credit it for later use.
Benefits: These are are the simplest systems and the most cost-effective to install. These systems will pay for themselves by offsetting utility bills in 3-8 yrs.
Downside: These do not provide power during a grid outage.
Conclusion: They are simple enough that you can install it yourself in a weekend with a friend. They are cost-effective enough to pay for themselves and put money in your pocket long term. If you want to reduce your electric bill and your carbon footprint this is a good choice.
Definition: These systems allow you to store your solar power in batteries for use when the power grid goes down or if you are not on the grid. Hybrid systems provide power to offset the grid power whenever the sun is shining and will even send excess power to the grid for credit for later use.
Benefits: Provides power for your critical loads when the power grid is down.
Downside: Cannot be expected to provide power for all your loads since the cost and volume of batteries would be prohibitive. Off-Grid systems require a lot more specialized equipment to function that is more costly and more complex to install. Specifically, they require a central/string inverter, a charge controller as well as batteries.
Sizing the solar array and the batteries required is complex. Detailed analysis of your requirements will be needed to provide for your minimal critical needs. You’ll also need to rewire your main electrical panel to isolate the “critical loads” so that only they are provided power in an outage. This means that your good pump, refrigerator, and a few lights are provided power while your air conditioners and TV and other non-essential loads are not.
This is definitely more complex to install as well. There are dangerous components, mostly dealing with high amp batteries so caution needs to be exercised. Also, batteries are expensive, require ongoing maintenance and periodic replacement.
Given the additional specialized equipment required and the fact that it requires come complex installation, expect an off-grid system to cost four(4) times as much to install per watt and to require ongoing maintenance outlays.