⚡ How Rooftop Solar Works

This page uses real data from a 12.95 kW rooftop solar system in Sidney, Maine to explain how net metering, credits, and cascading work for CMP customers.

1. What Does a Solar System Actually Do?

Solar panels convert sunlight into electricity. During daylight hours, the panels produce power that flows directly into the home. Any excess production that the home doesn't need immediately gets sent back to the grid.

Key concept: Solar panels don't store electricity — they produce it in real time. At night, or when demand exceeds production, the home draws from the grid as normal.
☀
Solar Panels
Produce power during daylight
Powers home directly →
Excess goes to grid →
🏠
Home
Uses solar first, grid when needed
← Grid supplies nights & cloudy days
⚡
CMP Grid
Always available as backup

2. What is Net Metering?

Net metering is a billing arrangement that lets solar customers send excess electricity to the grid and receive credits on their bill. In Maine, net metering uses a 1:1 ratio — every kilowatt-hour (kWh) you send to the grid earns you one kWh credit on your bill.

Maine's 1:1 Net Metering: 1 kWh exported = 1 kWh credit. Credits are applied at the full retail rate — not a wholesale buyback rate. This is one of the most favorable net metering policies in the country.

Real Example — Summer Month

Home consumed from grid 739 kWh
Solar exported to grid 1,129 kWh
Net billed kWh (739 - 1,129) -390 kWh (credit!)
Result 390 kWh banked as credit for future use

Real Example — Winter Month

Home consumed from grid 1,064 kWh
Solar exported to grid 88 kWh
Banked credits applied 88 kWh (from summer bank)
Net billed kWh 976 kWh
Result Still paying for most usage — solar doesn't eliminate winter bills

3. How Do Banked Credits Work?

When you export more than you consume in a billing period, the extra credits don't disappear — they bank for future use. This is how a solar customer can generate surplus in summer to offset their higher winter bills.

Important — Rolling 12-Month FIFO Expiration: CMP applies a rolling 12-month window to banked credits. Credits are consumed oldest-first (FIFO). Any credit that has not been used by the end of its 13th month is forfeited. There is no single annual expiration date — credits expire on a rolling basis, 12 months after they were earned. Customers who consistently over-produce should monitor their oldest credits.

4. Self-Consumed Solar — The Hidden Value

Not all solar value shows up on the bill. When a panel produces power and the home uses it directly — before it ever touches the meter — that's called self-consumption. It doesn't appear as an export credit, but it's still saving money by replacing grid power.

Total solar produced (lifetime) 33,456 kWh
Exported to grid (credits earned) 18,483 kWh
Self-consumed (used directly, never metered) 7,486 kWh
Combined value at $0.264/kWh $6,847.69
Customer talking point: "Your solar saves you money two ways — credits for what you send out, and avoided purchases for what you use directly. Both count."

5. Why Does a Solar Customer Still Have a Bill?

This is one of the most common customer questions. Even with a well-performing solar system, customers will still receive a CMP bill every month. Here's why:

🌙
Nighttime usage
Panels don't produce at night. All evening and overnight consumption comes from the grid.
❄
Winter production drop
Maine winters have shorter days and lower sun angles. A system that covers 100% of summer usage may cover only 10–20% in December and January.
💰
Fixed delivery charges
CMP's delivery charge includes a flat monthly fee (~$30) that solar does not offset. Customers are paying for the infrastructure that keeps them connected to the grid.
📊
Credits may not cover all usage
If banked credits run out before spring, the customer pays for grid consumption at full rate until production picks back up.

6. What is Cascading?

Cascading allows excess net metering credits to flow to a second account — on a different meter — after the primary account's usage is satisfied.

⚠ Cascading does NOT happen automatically. Customers must contact CMP to request cascading and designate their secondary account. Without requesting it, excess credits stay on the primary account and may expire unused — oldest credits first, after 13 months.

Real Example — This System

☀
Solar Panels
1,129 kWh produced
(August)
→
🏠
Home Account
(Priority 1)
Uses 739 kWh
390 kWh surplus →
Surplus cascades →
👵
Cascade Account
(Priority 2 — Cascade)
Credits offset
account's bill
The cascade account does not have to be at the same address. The cascade account does not need to be at the same address — it can be several towns away and still receive the overflow credits once cascading is set up. Any CMP account can be designated as a cascade recipient.

What Happens Without Cascading?

Surplus after home account satisfied 390 kWh
Without cascading Credits bank on home account, oldest expire after 13 months (FIFO)
With cascading 390 kWh reduces the cascade account's bill instead
Rep talking point: "Do you have a second meter — a barn, a garage, a family member's home? If so, ask about cascading. Any credits that your panels generate beyond what your home uses can offset another account's bill instead of sitting unused."

7. Quick Reference — Common Customer Questions

Why am I still getting a bill if I have solar?
Solar covers part of your usage — but not nighttime consumption, not fixed delivery charges, and not winter months when production is low. You'll always have some bill.
Why did my credits expire?
CMP uses a rolling 12-month FIFO model. Credits are consumed oldest-first, and any credit not used within 13 months of when it was earned is forfeited. There is no single annual reset date — expiration happens on a rolling basis. This typically affects customers whose system consistently over-produces relative to total usage across both accounts.
Why is my other account getting credits?
That's cascading — the customer requested that surplus credits flow to a second account. This is intentional and means their solar is offsetting two bills.
How do I get cascading set up?
The customer needs to contact CMP and request it. It does not happen automatically. Both accounts must be in CMP's service territory.
My solar company said I'd have no electric bill — why do I still have one?
Fixed delivery charges (connection to the grid) are not offset by solar credits. There will always be a minimum monthly delivery charge regardless of how much solar is produced.