Solar Panel Calculator

Because a grid-tied array is an energy offset, not a power supply. The grid still carries every instantaneous peak — the dryer, the AC compressor, the EV charger — and your system simply pushes kilowatt-hours back over the year to cancel out what you pulled. Sizing to peak demand would mean building an array three or four times larger than you can ever use. Off-grid and battery systems are the exception: there you do size to peak load and to the worst production month, usually December, not to an annual average.

Usually yes. Most net metering tariffs credit exported power at retail rate up to your own annual consumption and then pay wholesale, or nothing at all, for anything beyond it. Building past 100% therefore adds cost with a much longer payback. Many utilities also cap system size administratively at 100 to 120% of historical usage. The main reason to go above 100% is a known future load — an EV, a heat pump, or a pool — and even then, check whether your utility will approve the larger interconnection first.

More than panel brand, by a wide margin. A true-south array at a tilt near your latitude is the benchmark. East or west facing typically gives up 15 to 20% of annual production, and north facing in the US is rarely worth installing. Shade is worse still: with string inverters, one shaded module can drag down its entire string, which is why microinverters or DC optimizers are standard on complicated roofs. An installer will run a shade analysis on site before quoting real numbers.

Budget about 21 square feet per module including racking gaps, so a 19-panel array wants roughly 400 square feet of clear, unobstructed plane. That is not the same as your roof area. Fire codes in most jurisdictions require setbacks at ridges and along at least one side, and vents, chimneys, skylights, and dormers all carve into usable space. A practical rule is that you can use 60 to 75% of a given roof plane. Complex hip roofs land at the low end.

It should land in the right neighborhood, usually within 10 to 15% of a proper design, but treat it strictly as a planning number. A real proposal uses hourly weather data for your specific address, measured shading, actual roof planes and tilts, inverter clipping behavior, and your utility rate structure. Before anything is ordered you also need a structural review of the roof framing for the added dead and wind load, an electrical review of the panel and busbar rating under NEC 705.12, and permits with utility interconnection approval. The locally adopted code and a licensed electrician or solar contractor govern the final design — nothing here substitutes for that.

Yes, and it is predictable. Mainstream modules carry a production warranty of roughly 0.4 to 0.55% loss per year, so most are guaranteed at 85 to 92% of nameplate output at year 25. Real-world median degradation measured across large fleets is closer to 0.5% a year. That is already partly absorbed in the 20% loss figure this calculator defaults to. If you want to be sure of covering your usage in year 25 rather than year 1, size 5 to 10% above your target offset.