Should I buy one solar panel or two?

Buy the second panel only if you have significant continuous daytime electrical loads. Plug-in solar reduces your bill by offsetting electricity you are using at the moment the sun is shining. Unless it includes a battery, it does not bank the surplus.

A single 180W panel produces roughly what a fridge, a router, a computer and standby loads draw during the day, so most households absorb all of it. A second panel takes you to 360W, and that extra output only saves you money if something in the house is actually consuming it: electric heating or cooling, a heat pump water heater, several large fridges or freezers, a pool pump, a spa that heats continuously, or daytime EV charging.

Without loads like those, part of your generation flows back to the grid, and in most states you are not compensated for it.

How much electricity does an average home use?

A typical American home draws around 1.2 kW continuously across a 24-hour period. However, much of this load occurs in bursts, e.g. when you turn on your drier or dishwasher. Daytime baseline draw is usually much lower than that, which is why a single panel is enough for many households.

Could exporting power to the grid actually cost me money?

It can. Some meters do not distinguish between electricity coming in and electricity going out, and will count exported generation as consumption.

Can I add a second panel later?

Yes. The panels use standard MC4 connectors and pair to the microinverter with a Y connector, so a single-panel system can be extended.

This topic comes up a lot in the Bright Saver Community, where people installing plug-in solar right now compare notes on what actually works.