Calculator
Run your own numbers. Then check our math.
Every input and formula behind this tool is documented on this page, below the calculator itself. If a competing estimate disagrees with ours, you should be able to find exactly which assumption caused the gap. That is the point.
Slider bounds come from our own PVWatts v8 runs across five Massachusetts cities. Pittsfield sits at the bottom, Hyannis at the top.
LBNL pegged the 2024 national cash median at $3.50 a watt. New England bids tend to land above that line, so plug in your real number.
Assumptions you can change
Type in whatever rate appears on the contract in front of you. We refuse to hardcode a statewide figure nobody audits.
Cash purchase
$22,800 after state credit
- Year 1 benefit
- $242/mo
- Payback
- 8 years
- 20 year net position
- $46,297
Upfront price reflects the LBNL cash median. Post-2025 purchases earn zero federal credit, so payback depends on your utility rate holding or climbing.
Solar loan
$278/mo 15 yr payment
- Year 1 net
- -$36/mo
- Upfront
- $0
- 20 year net position
- $19,007
Financed systems carry roughly $1.20 more per watt than cash deals once dealer fees are rolled in. A low advertised APR usually means a higher system price.
Lease / PPA
$0 down you buy the power
- Year 1 net
- $37/mo
- Upfront
- $0
- 20 year net position
- $13,238
You never own the hardware. The provider claims the 48E credit and production incentives, then sells you the output at a contracted rate with an annual escalator.
This is a model, not a quote. Real bids move with roof complexity, shading, panel brand, and how hungry the installer is that month. Solar Learning Lab installs systems and routes work to vetted partner companies; our disclosures page spells out how that works.
Data behind this tool: retail rates from EIA Electric Power Monthly (5.6.A, May 2026), consumption baselines from EIA annual Table 5.A, production from our NREL PVWatts v8 runs (August 2026), pricing medians from the LBNL Distributed Solar 2025 update, incentive terms from Mass.gov and IRS guidance.
The formulas, in plain English
System size starts from your bill. We divide your annual spend by your state's residential rate to get usage in kWh, then divide that by local production per installed kilowatt. In Massachusetts we use PVWatts v8 figures we generated ourselves, which range from about 1,250 kWh per kW in the Berkshires to 1,400 on the Cape. Most calculators grab one national constant. That single shortcut can swing a quote by 12 percent.
From there, year one savings equal the electricity you no longer buy, plus performance payments where a state offers them, minus any loan payment. We degrade panel output half a percent per year, which is the industry standard assumption for modern modules. The 20 year net position sums all of it against what you paid.
What this model deliberately ignores
Four things, and each one matters. First, utility rate inflation. We hold today's rate flat for 20 years, which is conservative; New England rates have not gone sideways in decades. Second, panel brand and inverter choices, which shift price per watt. Third, roof condition. If your roof has a decade left, budget the tear-off now, not after panels are on it. Fourth, batteries. Storage changes the math enough that it needs its own tool, and bundling it here would blur both answers.
Where estimates usually go wrong
The mistake we see most in the field is trusting a savings figure without asking which incentives it assumes. Plenty of tools still bake in the dead 30 percent federal credit, which inflates purchase savings by thousands. The second mistake is comparing a lease quote against a cash quote as if they were the same product. One is an asset on your house. The other is a power contract with an escalator clause. Read both before signing either.