Sizing an Inverter To A Service Panel

Let's say that your home's existing electrical system contains a main service panel that has a busbar rating of 200 amps.  The busbar rating is listed on the door of the panel.  Most houses' service panels are either 100 or 200 amps, unless you have something old and funky, or massively huge.

Every house electrical system has a main breaker.  That breaker has a rating, whether it's 200 amps or 250 amps.  That is the point at which the load from your house will shut off the power for safety.

To determine the amount of backfed solar power your house can withstand, there's a really simple calculation you can perform, and it's extremely simple.

Take the rating of the busbar.  Multiply it by 120%.  Subtract the main breaker.

Busbar = 200 (x120% = 240) - Main Breaker (200) = total of 40 amps to backfeed onto.

let's say it's a 225 amp panel, and a 200 amp main breaker.
225 (x120% = 270) - Main Breaker (200) = total of 70 amps to backfeed onto.  

What does that mean?  
If you can backfeed 40 amps, then multiply the 40 amps by the house power coming in.  That means you can have 7,680 watts backfed into the house, because 40 amps at 240v AC is 9600w.  But then you must divide by 125% for overcurrent protection, leaving you with the 7680 watts, or at most a 7000-watt inverter that can tie into the existing home electrical system.

Can you lower the rating of your main breaker? 
Yeah, technically you can but it's not recommended.  It's better to upgrade your service panel.

What's a Line-Side Tap? 
Please don't ask that question.