1. Your electric bill

Start here, not with the roof. Solar pays you back by cancelling out electricity you’d otherwise buy. If you don’t buy much, there’s less to cancel.

Pull out a recent bill and find two numbers: how many kilowatt-hours (kWh) you used, and what you paid per kWh. The average US home uses about 900 kWh a month.

• High bill, high rate: the strongest case for solar. Every panel displaces expensive power.
• Low bill, cheap power: the hardest case. If you pay $40 a month, a $25,000 system has very little to save you.

This single factor outweighs almost everything else on this page. Two identical houses in different utility territories can have wildly different answers.

2. Shade — the real dealbreaker

Shade is the one factor that can genuinely rule out a roof. And it’s not proportional: shading part of a panel does more damage than you’d expect, because a shaded cell restricts the whole panel’s output the way a kink restricts a hose.

Go outside and look at your roof three times in one day — mid-morning, midday, and mid-afternoon. What matters is the middle of the day, roughly 9am to 3pm, when most production happens.

• Clear at midday: good, even if there’s shade early and late.
• A tree you could trim or remove: a cost, not a dealbreaker. Get a quote for the tree work and add it to the project.
• Shade from a neighbor’s tall trees or an adjacent building: the hardest case, because you can’t fix it.
• Chimneys, dormers and vents: minor. The design just works around them.

3. Which way your roof faces

In the US, south-facing is best. But the penalty for other directions is much smaller than most people assume, and this is where a lot of homeowners rule themselves out unnecessarily.

An east or west roof at 80% of ideal doesn’t mean solar fails. It means you need a slightly larger system for the same output. If you have the roof space, the difference is modest.

West-facing has a hidden advantage worth asking about: it produces later in the afternoon, which lines up with peak demand. If your utility charges more during late afternoon hours, west-facing panels can be worth more per kilowatt-hour than the raw production figure suggests.

4. How old your roof is

This is the factor most likely to change your timing, and the one homeowners most often skip.

Panels last 25 to 30 years. If your roof has fewer than about 10 years left, you will eventually pay to have the array removed and reinstalled so the roof can be replaced underneath it — commonly a few thousand dollars, for nothing.

• Roof under 10 years old: proceed.
• Roof 10–15 years old: get a roofer’s honest opinion on remaining life before committing.
• Roof over 15 years, or visibly worn: strongly consider replacing it first. Many people do both projects together, and some installers coordinate it.

5. Roof space and material

Each panel takes roughly 18 to 22 square feet. A typical 8 kW system of around 19 panels needs somewhere near 350–400 square feet of usable space — and “usable” excludes areas too close to edges, vents, or anything the fire code requires be kept clear.

Material matters less than people expect:

• Asphalt shingle: the easiest and cheapest. Most US homes.
• Metal standing seam: excellent — panels clamp on without drilling holes.
• Tile, clay or slate: workable but more labor, and tiles break. Expect to pay more.
• Wood shake: the most difficult, and some installers decline it.
• Flat roof: fine. Panels go on tilted or ballasted frames.

6. Your electrical panel

Your home’s breaker panel has to be able to accept the solar connection. Older homes — especially those with 100-amp service — sometimes need an upgrade, typically $2,000 to $4,000.

You can check quickly: open your breaker panel door and look for the main breaker number, usually 100, 150 or 200. Two hundred amps is generally comfortable. One hundred may or may not be, depending on system size and what else you run.

This is a cost, not a barrier — but it’s a cost that should appear in a quote up front rather than surfacing after you’ve signed.

What if the answer is no?

A roof that doesn’t suit panels doesn’t end the conversation:

• Ground mount — if you have the land. Costs 10–20% more, but you can aim it perfectly, so it often out-produces a compromised roof.
• Replace the roof first — and revisit in a year.
• Community solar — you subscribe to a share of a large off-site array and get credits on your bill. No equipment, no roof needed. Common for renters, condos and shaded homes, though availability depends on your state.
• Reduce first — if your bill is modest, insulation, air sealing and appliance upgrades may return more per dollar than panels would.