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What happens in the middle of the day when nobody is at home and the sun is at its brightest? Your solar system generates a ton of electricity, but it’s not getting used. Where does it go, and how does this affect your finances? And what happens at night when there is no sun, but you want to watch TV?

It all depends on your utility and your solar system.

If you’re living off-grid and have an expensive and complex battery backup system, the extra you produce during the day gets stored to use at night. But for everyone else, your solar system is inextricably tied to the grid, and electrons pass back and forth between your roof and the rest of the world according to where they’re generated and where they’re being used. If you’re making energy and not using it, it will flow outward to get used up by your neighbors. If you’re using energy and not making enough, you’ll be supplied by the grid, just like everyone else. The exchange is automatic; you don’t need to worry about it once it’s set up.

If you feed the grid during the day and draw equally from the grid at night, you’ll wind up costing your utility exactly $0 in energy used. So it would stand to reason that your bill would say the same thing, but such isn’t always the case. In fact, many energy providers have a rate at which they sell to you, and another rate at which they’ll buy back, and it probably won’t surprise you to know that they’re the ones making money on the transaction.

Net-zero metering

But some utilities, particularly the ones with an eco-centric mission, will credit your account at the same rate as they charge it. This is called “net-zero metering”, because your bill is for your net usage, or the amount of grid energy you consume minus the solar energy you feed back. If you use 1,000 kWh in a month, and produce 1,000 kWh in a month, you won’t be charged anything. As a result, most folks who get a solar system wind up changing their electricity provider immediately; otherwise the savings you plan to get from your investment are constantly gobbled up by your utility.

If, however, you only use 900 kWh in a month, and produce 1,000, they aren’t going to write you a check. They’ll credit your account, and if you need extra later on, they’ll apply the excess and you may wind up breaking even in the long run. This is why it’s important to get the right size system for your needs–otherwise you’re just giving a bunch of money to the utility.