Key takeaway
In Massachusetts, each kW of solar makes about 1,100 to 1,200 kWh a year. To size your system, add up 12 months of kWh from your electric bills, divide by about 1,150, then divide by the panel size. A home using 9,600 kWh a year needs about an 8.3 kW system, or roughly 21 panels.
Who this guide is for: Massachusetts homeowners who want to know how big their solar system should be, or want to check the size on a quote.
How many solar panels you need comes down to one thing: how much electricity your home uses. Not your square footage, not your neighbor's system, and not a round number on a sales sheet.
Here's the simple math, with Massachusetts numbers, so you can figure out your own system size in about five minutes.
At a glance
| MA production | About 1,100 to 1,200 kWh per kW of solar per year |
| Typical panel | About 400 watts (0.4 kW) |
| Formula | Yearly kWh ÷ 1,150 = system kW, then ÷ 0.4 = panels |
| Example | 9,600 kWh a year ≈ 8.3 kW ≈ 21 panels |
| Good offset | About 90% to 110% of your yearly usage |
How do I calculate how many solar panels I need?
- Find your yearly usage. Add up the kWh from your last 12 Eversource or National Grid bills. Most bills show a usage history chart, and your online account usually lists all 12 months.
- Divide by about 1,150. In Massachusetts, each kW of solar makes roughly 1,100 to 1,200 kWh a year. Dividing by 1,150 gives you the system size in kW.
- Divide by the panel size. Most home panels today are around 400 watts, or 0.4 kW. Divide your system size by 0.4 to get the number of panels.
Example: a home using 800 kWh a month uses about 9,600 kWh a year. 9,600 ÷ 1,150 = about 8.3 kW. 8.3 ÷ 0.4 = about 21 panels.
How many solar panels do I need by monthly usage?
| Monthly usage | Yearly usage | System size | Panels (400 W) |
|---|---|---|---|
| 500 kWh | 6,000 kWh | 5.2 kW | 13 |
| 750 kWh | 9,000 kWh | 7.8 kW | 20 |
| 1,000 kWh | 12,000 kWh | 10.4 kW | 26 |
| 1,250 kWh | 15,000 kWh | 13.0 kW | 33 |
Based on about 1,150 kWh per kW per year in Massachusetts. Your number depends on your roof direction, pitch and shade.
Why square footage is the wrong place to start
Lots of guides list panel counts by home size. In Massachusetts, that can be way off.
Two 2,000 square foot homes can use very different amounts of electricity. A home heated with oil or gas might use 7,000 kWh a year. The same size home with heat pumps and an EV could use twice that. That's why your bills are the only reliable starting point.
What else affects how many panels you need?
- Roof direction: south-facing roofs make the most power. East and west roofs work well too, but may need a few more panels to make the same amount.
- Shade: Massachusetts has a lot of mature oaks and maples. Heavy shade lowers production, which can mean more panels or a smaller offset.
- Roof space: each panel takes up roughly 20 square feet, and you need room around vents, chimneys and skylights.
- Panel size: higher-wattage panels mean fewer panels for the same system size.
Your roof matters for more than space, too. Asphalt shingles and standing seam metal are the best roofs for solar panels in Massachusetts.
Want to know if your quote is the right size?
Send it to Mark. He'll check the system size against your actual usage and tell you if it fits.
Mark, review my quoteShould I plan for a heat pump or EV?
If you're planning to add a heat pump or an electric car in the next few years, say so up front. Either one can add thousands of kWh a year.
It's usually easier to size the system for it now than to add panels later. Just make sure the quote shows the extra usage it's planning for, so the size makes sense on paper.
What about winter?
Your panels will make a lot more in June than in January. That's expected. Under net metering, extra power you make in the long summer months earns credits on your bill that help cover the shorter winter months.
That's why solar here is sized around a full year, not one month, and it's how net metering works in Massachusetts.
How to check the system size on your quote
- Find the system size in kW and the yearly production estimate in kWh.
- Compare the production to your yearly usage. About 90% to 110% is a good fit.
- Divide the production by the system size in watts. In Massachusetts, the answer is usually around 1.1 to 1.2.
If the system covers far less than you use, you'll still buy a lot of power from the utility. If it's much bigger than you need, ask why. The production estimate on your quote is the next thing to check, and it's a big part of whether your solar quote is a good deal.
Got a quote and not sure the size is right?
Send it to Mark. He'll run the numbers against your bills and tell you honestly.
Mark, review my quoteFAQs About Solar System Size
How many solar panels do I need in Massachusetts?
Divide your yearly electricity use in kWh by about 1,150 to get the system size in kW, then divide by the panel size in kW. A home using 9,600 kWh a year needs about an 8.3 kW system, or roughly 21 panels at 400 watts each.
How many solar panels do I need for 1,000 kWh a month?
In Massachusetts, 1,000 kWh a month is about 12,000 kWh a year. That works out to roughly a 10.4 kW system, or about 26 panels at 400 watts each, depending on your roof and shade.
How many solar panels for a 2,000 square foot house?
Square footage alone won't tell you. Two 2,000 square foot homes can use very different amounts of electricity depending on how they're heated, whether they have central air and how many people live there. Start with your electric bills instead.
Should I size my system for an EV or heat pump I might add later?
If you're planning one in the next few years, it's worth telling the solar company now. A heat pump or EV can add a lot of usage, and it's usually easier to size the system for it up front.
About Mark Featherstone
Mark has been in solar for 14+ years, working at some of the biggest solar companies in the country, before co-founding Your World Solutions. His goal is simple: help homeowners get into solar deals that actually work for them, and make sure they understand every part of it before they sign.
This article is for general informational purposes only and isn't legal or tax advice. Every home, quote and contract is different, and production, savings, timelines and program rules vary and can change. Read your own contract carefully and talk with a qualified professional before making a decision.