How much of your Pepco bill can solar actually offset? It depends on your roof

Solar companies love to say "eliminate your electricity bill." The truth is more nuanced. Some DC homes can offset 150% of their electricity use. Others can only manage 50–60%. The difference comes down to three things: your roof size, your roof's sun exposure, and how much electricity you actually use.

Why offset percentage varies so much in DC

Washington DC has some of the most varied housing stock of any American city. A Capitol Hill rowhouse with a narrow south-facing roof is a completely different solar proposition than a detached colonial in Petworth with a large unshaded roof plane. And a family of five running central AC, an EV charger, and electric heat is using three times as much electricity as a single person in the same house.

Solar installers size systems based on the available roof space — not based on what you use. This means two neighbors with identical Pepco bills can end up with very different offset percentages depending on what their roofs can physically hold.

Real DC home scenarios

What determines how many panels fit on your roof

Each standard solar panel is roughly 22 square feet (about 3.5 ft x 6.5 ft). But usable roof space is always less than total roof area because installers need to avoid edges, ridges, valleys, vents, chimneys, and skylights — typically leaving a 3-foot clearance around all obstructions.

In DC, roof orientation matters enormously. A south-facing roof at a 30-degree pitch is ideal — it captures the most sun throughout the day and year. East or west-facing roofs produce about 15–20% less than south-facing. North-facing roofs are generally not worth installing on at all.

The electricity usage side of the equation

Offset percentage is a ratio — it's not just about how much you produce, it's about how much you use. Two homes with identical 10kW solar systems will have very different offset percentages depending on their consumption.

In DC, electricity usage varies enormously. A couple in a well-insulated rowhouse with gas heating might use 6,000–7,000 kWh per year. A family of five in an all-electric home with central AC, an EV charger, and electric water heating might use 18,000–22,000 kWh. The same 10kW system offsets 150% for the first household and only 50% for the second.

What happens when you produce more than you use

In DC, net metering allows homeowners to send excess solar production back to the grid and receive a credit on their Pepco bill. So a system producing 150% of your usage doesn't mean you waste 50% — it means you bank credits during sunny months (spring and summer) that you draw down in cloudier months (winter).

For free PPA customers, the solar company owns the system and earns the SREC income from excess production. You benefit from the Pepco savings on the electricity you do use — but any excess generation credit goes toward keeping your bill low year-round, not to you directly. This is still a good deal — you're saving $0 on a system you don't own — but it's worth understanding.

What about partial shading from trees or neighboring buildings?

Shading is the single biggest variable after roof size. A tree that shades even one panel for part of the day can reduce a whole string of panels' output by 20–30% without microinverters or power optimizers. In DC's dense neighborhoods — particularly in areas like Columbia Heights, Brookland, or Takoma — mature street trees and neighboring rowhouses can dramatically cut solar production.

Modern panel systems with microinverters (where each panel operates independently) handle shading much better than older string inverter systems. If your roof has any shading, ask specifically whether the quoted system uses microinverters or string inverters — it makes a significant difference in real-world production.

Is a partial offset still worth it with a free PPA?

Yes — for most DC homeowners, even a 50–60% offset with a free PPA is a strong deal. Here's why: you're paying $0 for the system. Any electricity your panels produce is electricity you're not buying from Pepco at 23.9¢/kWh. If your roof only fits a 6kW system that offsets 60% of your usage, that's still $100–$150/month in savings with zero cost or risk to you.

The calculation changes if you're considering a solar loan. With a loan, you're paying $150–$200/month regardless of production. If your system only offsets 55% of your usage, your loan payment may exceed your electricity savings — leaving you worse off. This is why the loan vs free PPA math matters so much, and why you should always run the real numbers for your specific roof before deciding.

Frequently asked questions

How do I know how many panels will fit on my roof?

A solar installer will do a site assessment using satellite imagery and sometimes an in-person visit. They measure usable roof space, identify obstructions, assess shading, and check roof age and condition. You can get a rough estimate yourself using Google's Project Sunroof tool — search your address and it will estimate your solar potential based on roof size and local sun data.

Can I add more panels later if I want more offset?

Sometimes, but not always. With a free PPA, the solar company owns and sizes the system — you generally can't add panels on your own. With a purchased or loaned system, you can expand later if you have remaining roof space and your inverter can handle additional capacity. Plan for your future electricity needs upfront if possible.

What if my roof isn't big enough for a worthwhile system?

If your roof can only fit 3–4 panels, a solar system may not make financial sense. Some DC homeowners in this situation look at community solar subscriptions instead — you subscribe to a share of a larger solar farm elsewhere in DC and receive a credit on your Pepco bill without needing any panels on your own roof. The savings are smaller (typically 10–15%) but there's no roof requirement.

Does roof age matter for solar installation?

Yes. Most solar installers won't install on a roof with less than 10 years of remaining life. If your roof needs replacement in the next 5–7 years, it's worth replacing it before going solar — removing and reinstalling panels for a roof replacement costs $2,000–$5,000 extra. With a free PPA, this cost falls on the solar company, but they may decline to install on an aging roof.