Solar planning that shows its working
Size a system, estimate production, fit panels to a roof and test whether it pays back. Every assumption is visible and editable, and nothing you enter leaves your device.
Solar System Designer
Six guided steps that join every tool below into one design: your electricity use, the sun your site gets, whether it fits the roof, storage and economics. It tells you plainly whether your roof or your energy use is the thing setting the size — which is the distinction most quotes leave unsaid.
Open the designer- GoalsWhat the system is for
- UseYour kWh, a bill or appliances
- SunPeak sun hours for your site
- RoofWhether it fits
- Storage & moneyBattery, payback, finance
- DesignA report you can take to installers
Solar in thirty seconds
Pick a home like yours, or move the sliders, for a first rough answer. It runs the same sizing engine and default performance ratio as System Sizing, so the numbers carry straight over.
Pick what you have
Tell us what you're starting from and you get the two or three tools to use, in order.
Every solar tool
Thirteen calculators on one engine, so a number means the same thing everywhere.
How these tools work together
They share one calculation engine, so a number does not change meaning as you move between them. A typical path runs usage → array size → production → finance: start with what your meter says, size an array against a target offset, estimate what that array actually generates month by month, then test whether the money works. If you already know your roof dimensions, start at panel fit instead and let the roof cap the array rather than your consumption.
What these tools will not do
They produce planning estimates, not engineering. Nothing here is permit-ready or code-compliant, and none of it substitutes for a licensed installer, a structural assessment or your utility's interconnection rules. Specifically:
- No incentive is ever assumed. Tax credits, rebates and net-metering rules change constantly and vary by country, state and utility. If you want one counted, you enter it — and the tool says plainly that it cannot check whether you qualify.
- No equipment specification is built in. Module and inverter figures come from your datasheet, not from a list that may be years out of date.
- No setback or fire-access rule is assumed. Spacing defaults to zero and warns you, because those distances are set locally and getting them wrong is expensive.
- No hourly weather simulation. These are peak-sun-hour estimates. They are useful for sizing and for deciding whether a project is worth pursuing; they are not a substitute for a modelled year of weather data when you are signing a contract.
Your data stays in this browser
Every calculation runs on your own device. Your electricity use, roof dimensions, address, equipment and quotes are not uploaded, not stored on a server, and not placed in analytics. There is no account, no email gate and no installer referral. Files you open — a quote PDF, for instance — are read inside the browser and never sent anywhere.
To be precise rather than vague about the exceptions: like most sites, these pages load Google Analytics and Google AdSense. Those make requests of their own, carrying the page address, its title and the ordinary details any web request includes, plus AdSense's own ad-serving and invalid-traffic checks. None of them carry anything you type into a tool. We test that: every field on every page is filled with a marker value and every outbound request is inspected for it before each release.
Common questions
How many panels do I actually need?
It depends on four things: how much electricity you use, how much sun your site gets, how much of your use you want to cover, and how efficient the panels are. The sizing calculator does that arithmetic and shows each step, so you can see which input is driving the answer.
Why does the estimate use "peak sun hours" rather than a weather file?
Peak sun hours is a simple, checkable number you can look up for your region, and it makes the whole calculation something you can follow by hand. A full hourly simulation is more accurate but harder to audit. For deciding whether a system is roughly the right size, the simpler model is usually enough — and we show a range rather than a single confident figure.
Is a bill enough to size a system?
It will get you a rough answer, and the tool accepts it — but it marks the result low confidence. A bill contains fixed charges, taxes and sometimes demand charges that are not energy, so working backwards from the total tends to overstate consumption. If you can find kWh on the bill, or read your meter across a year, use that instead.
What if I have no bill and no meter reading?
Add up your appliances. The sizing calculator has a fourth mode where you list what you run, for how many hours and on how many days a week, and it works out the annual total. There are examples for common appliances and three starting households you can adapt.
Two things to know about it. First, a tally like this almost always comes out below the real bill, commonly by 10–30% — standby draw gets missed, hours get underestimated, and there is always something nobody thought of. Nothing is added on your behalf to compensate, because an invented uplift would be indistinguishable from a measurement in the result. Second, the breakdown is often more useful than the total: it usually turns out that two or three things account for most of the bill, and knowing which they are changes what is worth doing about them.