Why a full year of data changes the calculation
Solar production swings hard with the seasons, and so does home electricity use in most climates. A single winter or summer bill catches your usage at one extreme; a full year averages across both, along with the shoulder months in between. That's the real advantage of sizing from an annual figure rather than one month.
How this tool reads your annual number
Enter your total kWh for the past 12 months and this tool converts it straight to a system size, no seasonal guesswork required on your part. For 10,800 kWh a year, close to the national average the EIA's residential energy consumption survey reports, with full net metering, it recommends a 9.6 kW system, a 10% buffer above the 21 panels your raw usage needs, since banked summer credit at retail rate has real value. Without net metering or on time-of-use billing, the recommendation tightens to 8.4 kW, since surplus production sold back cheaply or wasted outright isn't worth paying for.
When a year itself was unusual
A renovation, a new household member, or months of the house sitting vacant can all throw off a 12-month total the same way one hot month can throw off a single bill. This tool asks directly whether your year was typical, then trims an atypically high total by about 15% or bumps an atypically low one up by the same amount, so one unusual year doesn't lock in an oversized or undersized system. Prefer working from a single representative month instead? The monthly usage tool handles that version of the same question.
Building in tomorrow's load today
An annual total captures your past year, not what's coming. Adding 3,600 kWh a year for a planned EV on top of an already-high year raises the estimate from 8.4 kW to 11.2 kW, enough of a jump that it's worth accounting for now rather than discovering the gap after the panels are already up. A second installation later means a second round of permitting and interconnection paperwork on top of the equipment cost. The Department of Energy's interconnection roadmap describes the steps and timelines that second round involves, which is why sizing for known future load now is usually cheaper.
The gap between this estimate and a site-specific quote
Even with a real annual figure behind it, this tool still leans on typical panel wattage and national average sun hours rather than your roof's actual orientation or your preferred equipment. The full solar panel calculator swaps those assumptions for your own numbers if you want a tighter estimate.
Turning this number into next steps
Bring your estimated system size to the payback period calculator to model what it saves over time, and check what incentive programs apply to a system of this size before you get quotes.