OwlWhat Boston Solar publishes about its guarantee
Boston Solar describes the guarantee in a blog post with a published date of July 15, 2024, and repeats it in its residential solar FAQ. The key sentences, quoted from its site:
“Our Production Guarantee offers assured energy production of at least 90% over the first ten years that your system runs — or we’ll pay the difference.”
“When we install your system, we’ll generate a ‘Guaranteed Cumulative Production’ chart, which outlines a guaranteed amount of energy your system will produce over a specified time period.”
“If for some reason, your system produces less than that amount, we will pay you the difference using the guaranteed power production price for that time period.”
“Our Production Guarantee covers energy production for the first ten years of your system’s lifespan and is assessed every two years.”
The site does not say when the guarantee took effect or whether its terms have changed over time. A web page is a summary: the agreement you signed, and the chart attached to it, are the documents that control.
What each term means
| Published term | What it means in practice |
|---|---|
| “at least 90%” | A floor stated as a percentage. In Boston Solar's own example, the kWh figure on the chart is the number actual production is compared against, and no further percentage is applied to it. The post does not show how the chart figures relate to the original production estimate in a proposal. Your agreement answers that. |
| “Guaranteed Cumulative Production” chart | A schedule of running totals, not a single annual number. Each checkpoint is the total kWh expected since the system started, so a strong year can offset a weak one inside the same window. |
| “assessed every two years” | The comparison happens at two-year checkpoints. The unit of measurement is the two-year (or cumulative) total, not a single month or season. |
| “first ten years” | Ten years assessed every two years works out to five checkpoints, if your agreement follows the published description. Production after year ten is outside the published guarantee. |
| “guaranteed power production price for that time period” | The payment is shortfall kWh multiplied by a price per kWh listed for that period, rather than your utility's current rate. |
Boston Solar's own worked example
The post includes a sample chart. These are Boston Solar's illustrative figures, not figures from any real system:
| Contract year | Guaranteed cumulative kWh / guaranteed price per kWh |
|---|---|
| 2 | 17,713 kWh / $0.22 |
| 4 | 35,249 kWh / $0.24 |
| 6 | 52,610 kWh / $0.25 |
| 8 | 69,798 kWh / $0.27 |
| 10 | 86,814 kWh / $0.28 |
The post then works the first checkpoint: if the system produced 17,000 kWh over the first two years, the gap is 713 kWh, and Boston Solar writes the payment as “$0.22/kWh x 713 kWh = $156.86”. The post works only that first checkpoint, so how a payment made at year two is counted against the later cumulative totals is a question your agreement answers. It also does not describe how weather, outages, or time the system was switched off are treated; that is in the agreement.
How to check your own system against it
- Find your chart. Locate the Guaranteed Cumulative Production chart and the guarantee section of your agreement. Note your system's start date, each two-year checkpoint, the kWh at each checkpoint, and the price for each period.
- Get your actual production for the same dates. If your system uses Enphase microinverters, the Enphase Enlighten app records production over any date range you choose. Use the same start date as the chart, not the calendar year.
- Compare totals, not months. Because the chart is cumulative and assessed every two years, a low winter says little on its own. Compare the running total at each checkpoint.
- Read the fine print on adjustments. Check whether your agreement adjusts for weather, shading added after installation, grid outages, or time the system was switched off. The published post does not mention adjustments, so the agreement is the only source for them.
- Keep a dated record. Write down the figures you compared and where each came from. If you have a question about a checkpoint, contact Boston Solar through the channel your agreement names and bring those figures.
For the general version of this process, including weather adjustment and degradation, see how to tell whether a performance guarantee is being met and how production guarantees work.
Where OwlWatt fits
Enphase Enlighten records what your system produced. The guaranteed figures live in your agreement, so putting the two side by side is a separate step. OwlWatt does that step for Enphase systems: you enter your guarantee terms, OwlWatt reads your production from Enphase, compares it contract year by contract year with your guarantee terms and with a weather-adjusted expected baseline for your location, and updates as new production data arrives. You can then hold the running total up against your chart's two-year checkpoint yourself, before the checkpoint date rather than after it.
OwlWatt works with Enphase systems today. SolarEdge support is on the roadmap and not yet available. If your Boston Solar system uses different equipment, the manual steps above still apply.
Common questions
Does the published guarantee apply to my system?
Boston Solar's post and FAQ describe the guarantee for residential PV systems it installs, and do not say when it took effect. Whether your system has it, and on what exact terms, is set by your agreement. If you can't find a Guaranteed Cumulative Production chart in your paperwork, ask Boston Solar for a copy.
My production looks low this season. Does that mean the guarantee is missed?
Not on its own. The published guarantee is measured on cumulative totals at two-year checkpoints, so one season is a partial reading. Compare the running total to the chart at the checkpoint dates, and read how your agreement treats weather.
Is this the same as my panel or inverter warranty?
No. Equipment warranties cover defective hardware and come from the manufacturer. A production guarantee is a promise about kWh output from the company that sold the system. See what the Enphase warranty covers.
Is OwlWatt affiliated with Boston Solar?
No. OwlWatt has no relationship with Boston Solar and takes no referral or partnership revenue from any installer. We quote Boston Solar's public site so you can read the terms in one place.
See the running total before the checkpoint
Connect your Enphase system, enter your guarantee terms, and OwlWatt keeps a weather-adjusted record of your production against them, so the two-year checkpoint is not the first time you look.
OwlWatt provides information and documentation, not legal advice. For advice on your specific situation, consult a licensed attorney.