What actually determines production
A panel converts sunlight to electricity, so everything about output comes down to how much sunlight reaches it and at what angle, across a year.
| Factor | What it does | What to check |
|---|---|---|
| Orientation (azimuth) | In the northern hemisphere, a south-facing surface receives the most sun over a year. East and west facing arrays produce less annually, though a west-facing array shifts production later in the day, which matters where afternoon power is worth more. | Which way the usable roof planes face, in compass degrees, not "roughly south" |
| Tilt | The angle from horizontal. There is an optimum for a given latitude and season, and a flat or very steep roof departs from it. The penalty is usually gentler than people fear. | Roof pitch. The roof pitch calculator converts a rise-and-run measurement into degrees. |
| Shading | The one that punishes disproportionately. Partial shade on one part of an array can drag down output well beyond the shaded fraction, depending on how the modules are wired and whether power optimizers or microinverters are used. | Where the shadows fall in December, not in July. A neighbor's tree that clears the roof in summer can cover it at a low winter sun angle. |
| Local sun resource | How much solar energy arrives per square meter per day at your location, which varies substantially across the country and by season | This is a site-specific figure. Get it from a location-based tool, not from a national average. |
| Usable roof area | Plumbing vents, chimneys, dormers, setback requirements for firefighter access, and obstructions all subtract from the plane you thought you had | A layout drawing, not a square footage |
Because all six vary by site, nobody can state an output figure for your roof without looking at it, and any number you are given before a site assessment is an assumption dressed as a result. The array sizing calculator will show what array size a given daily kWh load implies, which is a useful sanity check against a proposal, but it is working from inputs you supply and cannot see your roof either.
Start with the roof, not the panels
An array is expected to sit on the roof for decades. If the roof under it has ten years left, the panels come off and go back on partway through, at a cost nobody put in the original arithmetic. Reroofing first, while the roof is bare, is straightforward. Reroofing under an existing array is not.
So the first question in any solar conversation is the age of the covering: inspect the roof, reroof if it is near the end of its life, then install. Confirm too that the structure carries the added load and that the workmanship warranty on penetrations is written down. Penetrations are where solar installations leak, and they leak years later — finding a water leak is a page you would rather not need afterward.
The decision that dominates the economics
People spend weeks comparing module brands and minutes on the ownership structure, and the ownership structure is worth far more money. There are three broad forms and they are genuinely different products.
| Structure | What you own | What determines whether it is good |
|---|---|---|
| Cash purchase | The system. You own the hardware, the production, and any incentives that flow to the system owner. | The largest up-front cost and the simplest arrangement. Whether it works out depends on installed cost, production, and what your utility does with power you export. |
| Solar loan | The system, with debt against it | The interest rate and any dealer fee built into the price. A low advertised rate is sometimes paid for by a higher system price — compare the cash price and the financed price side by side, and ask directly what the dealer fee is. Also check whether the loan payment assumes you will apply an incentive as a lump sum, and what the payment becomes if you do not. |
| Lease or power purchase agreement | Nothing. A third party owns the system on your roof; you pay for the equipment or for the power it produces. | Contract term, any annual escalator in the rate you pay, who is responsible for maintenance and roof work, what happens if you sell the house, and what the buyout terms are. An escalator that rises faster than utility rates do turns a good early year into a bad later one. |
Two details deserve a pen regardless of structure. Transferability: if you sell, a lease or PPA has to be assumed by the buyer or bought out, and that is a real negotiation on a real closing date. And what happens if the installer closes — who honors the workmanship warranty, and is the manufacturer warranty separately claimable.
Incentives, quoted nowhere on this page on purpose
Federal, state, local and utility incentives for residential solar exist, they stack, they have eligibility conditions, and they change on schedules that have nothing to do with each other. Any specific figure written into an article ages badly and can be wrong in a way that costs somebody real money.
So this page names none. Look them up for your own address at the time you are deciding, from primary sources: your utility, your state energy office, and the federal program documentation itself. Where a salesperson quotes an incentive, ask which program, ask the eligibility conditions, and verify it independently before it enters your arithmetic. If a proposal only works because of an incentive, that is worth knowing explicitly rather than discovering at tax time. Where the benefit depends on your tax situation, that is a question for a tax professional who knows your return, not for an installer and not for this page.
What happens to the power you do not use
An array produces most at midday, which for many households is not when they use most. The value of the surplus depends entirely on your utility's rules for exported power, and those rules are neither uniform across the country nor stable — several states have changed them substantially in recent years, generally in ways that reduce what exported power is worth and increase the value of using power at the moment it is produced.
Because that is both local and moving, find out what your specific utility currently does with exported energy, and what it would do for a system installed now, before you model anything. Ask whether the arrangement is locked for a period once you interconnect, since a grandfathering provision can be a significant part of the value, and ask what interconnection costs and how long it takes — a system installed but not permitted to operate produces nothing.
If exports are worth little, that changes what a good system looks like: a smaller array matched to daytime use, or storage.
Batteries, and what they are for
Storage is often sold as a way to improve the economics, and sometimes it is, but its clearest justification is keeping the house running when the grid is down. A grid-tied array without storage or specific backup capability shuts down during an outage, by design, so that it does not energize lines utility crews are working on. Panels on the roof do not mean lights during a blackout unless the system was built for it.
If backup is the goal, the questions are which circuits are backed up rather than the whole house, how long the bank carries them, and how the array recharges it. Battery bank sizing and inverter and charge controller sizing cover the arithmetic; the appliance power calculator helps identify what actually needs to stay on. A generator is a different tradeoff — cheaper capacity, fuel dependency, noise — and generator sizing covers that side.
Reading a quote
Ask for these in writing, from every bidder, and compare like with like.
| Ask | Why |
|---|---|
| System size in kW DC and in kW AC, and the module and inverter model numbers | Otherwise you are comparing prices for unspecified objects. DC and AC ratings differ, and quoting one against the other hides a difference in size. |
| Total installed price, and the price per watt derived from it | Price per watt is the only way to compare two proposals of different sizes. Compute it yourself from the total rather than accepting a stated figure. |
| The production estimate and the assumptions behind it — shading analysis, orientation, tilt, degradation rate, and the sun resource data used | An estimate without assumptions is a number someone chose. Two proposals with different assumptions are not comparable. |
| Whether a shading study was done on site, and with what | Shading is the factor most often glossed over, and the one most likely to make a real installation underperform its proposal |
| Warranties, separately: module, inverter, and installer workmanship including roof penetrations | Three different documents from up to three different parties with different terms and lifespans |
| Who pulls the permit, who handles interconnection, and what happens to the schedule if either is delayed | These are the usual source of a long gap between installation and operation |
| The full contract, including any escalator, buyout schedule and transfer terms if it is a lease or PPA | The financial structure, not the hardware, is where the outcome is decided |
Get more than one bid, and treat pressure to sign at the visit as information about the company rather than the offer. Any guaranteed-savings claim deserves a written definition of what is guaranteed, against what baseline, and who pays if it is missed. The payback calculator will run break-even on numbers you supply — the point being that they must be your numbers, from your quotes and your bill, since the result is only as good as the inputs.
What is worth doing before any of this: reduce the load. Every kilowatt-hour the house does not need is a kilowatt-hour of array you do not have to buy, and sealing, insulation and water heating measures cost a fraction of what generating capacity costs. The energy audit is the cheaper half of the same project, and it should come first.
Questions people ask
How many panels will my house need?
Nobody can answer that without seeing the roof and your consumption, and any figure offered before a site assessment is an assumption presented as a result. What determines it is your annual kilowatt-hour use, the usable roof area after obstructions and setbacks, the orientation and tilt of the planes available, the shading across the whole year, and the sun resource at your location. Start by pulling twelve months of electricity bills and adding up the kWh, which is the one input you can produce yourself. An array sizing calculator will turn a daily kWh figure into an array size, which is a useful check against a proposal but is not a design.
Is it better to buy solar outright or lease it?
That choice moves more money than any hardware decision, and it depends on things specific to you. A cash purchase costs the most up front and means you own the system, the production and any incentive that flows to the owner. A loan buys the same thing with debt, and the details that matter are the interest rate and any dealer fee folded into the price — compare the cash price with the financed price side by side and ask what the fee is. A lease or PPA means a third party owns the system on your roof; there the terms that decide the outcome are the annual escalator, the length, maintenance responsibility, roof work, transferability on sale, and the buyout schedule. Read all of it before comparing panel brands.
Does shade from one tree really matter?
More than its area suggests. Depending on how the modules are wired, shade on part of an array can reduce output well beyond the shaded fraction, because the affected string is limited by its weakest module. Power optimizers and microinverters mitigate this, which is precisely why they exist and why a shaded site should be asked about them specifically. The check that gets skipped is seasonal: the sun sits much lower in winter, so a tree that clears the roof in July can shade it for hours in December. Ask whether an on-site shading study was done across the year, and with what.
Will solar keep my lights on during a power outage?
Not by itself. A standard grid-tied system disconnects when the grid goes down, deliberately, so that it cannot energize lines that utility crews may be working on. Panels on the roof and sun in the sky make no difference to that. Backup requires equipment built for it — storage with an appropriate inverter, or in some systems a limited daytime-only backup outlet. If riding out outages is the actual goal, say so at the start, because it changes the design and the price, and ask specifically which circuits would be backed up and for how long rather than accepting a general answer.
What should I ask before signing a solar contract?
Ask for system size in both kW DC and kW AC with module and inverter model numbers, the total installed price so you can compute price per watt yourself, and the production estimate together with every assumption behind it, including the shading analysis and the sun resource data used. Ask for the three warranties separately — module, inverter, and installer workmanship covering roof penetrations — and who honors each if the installer closes. Ask who pulls the permit and handles interconnection. If it is a lease or PPA, read the escalator, the buyout schedule and the transfer terms. Verify any incentive claim independently with the program itself, and get more than one bid.