How installers decide system size
Most installers start with your annual electricity use (kWh) and then layer on site factors. Key inputs are your recent utility bills, roof area and orientation, shading, local sun hours, and the specific panels and inverter they plan to use. Some companies prioritize higher production per roof area by using high-efficiency panels; others design around cost or aesthetics. That’s why two well-known brands like SunPower and Tesla Energy Solar can produce different-sized proposals for the same home — they may choose different panel models, panel layouts, or production targets. Remember: the quoted size will reflect choices about how much of your load you want covered, how much roof space is available, and local rules about interconnection and export.
Common sizing approaches and tradeoffs
Installers typically follow one of a few approaches: match current annual use, cover only daytime consumption, or aim for a target percentage of your load. Each choice has tradeoffs. Designing to match 100% of current use may mean a larger system and higher upfront cost but reduces grid purchases over time; smaller systems cost less now but leave more grid dependence. Adding batteries changes sizing and placement. Oversizing panels relative to the inverter can increase midday output but may lower efficiency at times. Also consider future changes — an EV or new heat pump raises demand. Local net-metering rules, export limits, and permitting can also push installers to recommend different sizes. All of these factors depend on your location, roof, monthly bills, financing path, and installer quality.
What to verify on the quote before you sign
Treat the quote as a set of assumptions you should confirm. Key items to check: system size in kilowatts (kW); estimated annual production in kWh and the assumptions behind that estimate (tilt, orientation, shading); exact panel make and model and the number of panels; inverter model and configuration; assumed performance degradation rate and any production guarantees; site work (roof repairs, flashing, penetrations); interconnection and permit fees; timeline for installation and who handles inspections; and what monitoring or maintenance is included. Also check the installer’s license, insurance, references, and how they handle service after install. If a quote references specific brands like SunPower or Tesla Energy Solar, make sure the paperwork names the exact equipment to be installed.
How to compare multiple quotes
Compare like with like. Match system kW, expected annual kWh, panel wattage and model, inverter type, and any assumed production losses from shading or orientation. Ask how each installer measured your roof and which bills they used to size the system. Compare line-item costs and financing offers, and note who will handle permits and utility paperwork. Look at installer reputation, response times for service, and whether monitoring is included. Getting at least three site-verified quotes helps reveal differences in assumptions and gives you leverage to ask specific questions.