What happens to the solar power my home doesn't use?

It flows back to the grid. If your plug-in solar system generates more than your home is consuming at that moment, the surplus leaves your property. In most cases that energy is used by nearby homes and displaces generation from another source, so it is not wasted in a physical sense. Financially, it usually is. Compensation for exported electricity requires a net metering or interconnection agreement with your utility, and most plug-in solar owners do not have one. The practical conclusion is to size your system to what you actually use continuously during daylight hours.

Does my meter handle this correctly?

Some meters are configured to register exported electricity as consumption. This is not very common, but if you are unsure, check with your utility before adding a second panel.

Do I need to split kits across both legs of my panel?

Generally not. Most meters net consumption and export across both legs (phases) of your service, so a surplus on one side offsets draw on the other.

Should I fit an export limiter?

Only alongside a battery. Without storage, an export limiter just throttles production, so you generate less rather than saving more. With a battery, it lets you hold the surplus for later instead of giving it away.

How do I know how much I'm exporting?

The inverter app shows live and historical production, but it doesn't know how much went back to the grid. Some utilities allow you to download consumption data in hourly or 15-minute intervals using the so-called Green Button in your online account. Alternatively, you can use a whole-home energy monitor, e.g. from Emporia, if you want to see exactly where your energy goes.

This topic produced a spirited thread in the Bright Saver Community, the kind of back-and-forth you only get from people who have wired the thing themselves.