2 possibilities – they have a 100% offset system. In this case, they overgenerate electricity during the day, and feed that power into the grid, and at night they buy power from the grid like a normal energy customer. If everyone did their math right during design, the total amount of power they generated will match the total amount of power they used in a year. Of course this requires a connection to the power grid at all times.
Alternatively, they could have a battery bank that gets charged up all day, and drains out overnight. Tesla Power Walls (Li-ion battery bank) are the most likely candidate for smaller operations, but there are other battery systems out there that you can buy. These setups can be totally self sufficient, if there’s enough power generation and storage in the system.
The simple answer is… batteries. They can be one of many kinds, but tend to be large banks of lithium ion or lead acid.
I have a little vacation cottage and it’s powered by solar, a small wind turbine and tiny water wheel. Plus, it’s also on the grid so I can sell extra juice back to the electric utility company.
If they’re 100% solar, then they tend to have a battery array to collect excess power into he day for use at night. Otherwise they’re mostly solar, using main grid power at night and solar in the day, but in that case they often still make more power than they need in the day so that power is flowing into the grid in the daytime since they’re connected to it so they break even, and if not break-even come pretty close.
Edit: I can’t spell godamnit.
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