Construction on National Grid's new, five-acre solar farm on Hilldale Ave. is nearly complete. The farm, which broke ground last August, will produce enough electricity to power 175 to 200 households every year. The farm is National Grid's biggest solar farm in Massachusetts this year. Its other three farms are located in Whitinsville, which opened last summer, and in Everett and Revere, both scheduled for completion by the end of this month. National Grid also plans to build a fifth solar farm in Dorchester next year. The company estimates the combined cost for the five farms at approximately $31 million. National Grid says that by the end of the year, the farm will be producing 3.5 megawatts of electricity, making the company the largest owner of solar generation in the state in 2010. The farm will have 3,670, five by three foot solar panels. The site was site previously owned by a 19th century-era coal-to-gas manufacturing plant to store gas, but has been empty since the early 1970s. Once complete, National Grid plans to plant bare areas around the farm with shrubs and grasses to make it more aesthetically pleasing and to better blend it in to the surrounding residential area, which is located just outside Lafayette Sq. Customers will need to pay approximately 6 cents per month over the life of the Haverhill farm in order to fully pay for it.