You’ll still be a customer of National Grid’s. And your delivery, transmissions, etc charges will still be from national grid. It’s simply the supply rate portion of your billl that will state the third party supplier name and their price. They coordinate with national grid (or any utility) directly.

Read your contract carefully before signing. Third party electricity suppliers are great and offered in MA because we have a deregulated energy market.

You should see if your town or city (like Boston is, most of the cape is) is a part of a Community Choice Aggregation (CCA). If your town is then they have an agreement in place with a third party electric supplier already and you can opt into it. Your local town’s department of utilities should have more information.

Energy prices are extremely high right now and the rates are in effect for 6 months. Nation grid until may. And who knows what the summer rate will be which will be in effect for may-Nov. If you can lock in a lower rate with a third party, you should. Just, again, read the contract carefully and look out for escalators which are atypical of CCAs since the town/city does so much vetting of the offer first for their residents

(I work in energy, close to this sector)

The other poster’s comment was on point, but to answer your question, you fill out a form online and they notify national grid for you. All you need is your national grid account number to make the change. Here’s the link for the municipal aggregation program, https://www.mass.gov/info-details/municipal-aggregation - select your town and then enroll in the program.