Solar Panel System Sizing Guide

A solar system loses some of its theoretical maximum output at several real-world points: inverter conversion losses (DC to AC), wiring resistance losses, panel soiling (dust, dirt, occasional debris), a small amount of panel-to-panel mismatch, and gradual panel degradation over the system's service life — combined, these commonly reduce actual delivered energy to somewhere around three-quarters of the theoretical maximum a would-be perfectly efficient system suggests. The array is sized larger than the theoretical minimum specifically to compensate for this expected combined loss, so the SYSTEM's actual delivered output (not the panels' theoretical rated output) meets the target usage.

A grid-tied system connects directly to the utility grid with no battery storage — excess production during the day can be exported to the grid (often for credit, depending on local policy), and the home draws from the grid normally at night or when production is insufficient; it's the lowest-cost configuration but provides no backup power during a grid outage, since it's required to shut down when the grid goes down for safety reasons. An off-grid system has no utility connection at all, relies entirely on battery storage sized for the location's worst realistic weather stretch, and requires more conservative, larger sizing since there's no grid to fall back on. A hybrid system combines grid connection with battery storage, providing both the ability to export/import from the grid AND backup power during an outage — generally the highest-cost configuration but the most flexible.

Solar offset is the target percentage of your electricity usage the array is sized to produce — 100% offset targets producing roughly as much energy annually as you consume (though not necessarily matching consumption hour-by-hour without storage), while a lower offset percentage (say 50-70%) sizes a smaller, less expensive array that meaningfully reduces but doesn't eliminate grid electricity purchases. Choosing offset percentage is a genuine cost-vs-benefit decision, not just a technical calculation — a smaller offset array costs less upfront and still delivers real savings, while 100%+ offset (sometimes chosen to also account for future usage growth, like adding an EV) costs more but targets closer to eliminating the electricity bill entirely.

Panel efficiency (the percentage of sunlight energy a panel converts to electricity) primarily determines the physical AREA needed for a given total wattage, not the wattage itself — a higher-efficiency panel produces the same rated wattage in a smaller physical footprint than a lower-efficiency panel of the same wattage rating. Efficiency matters most when available roof or ground space is limited relative to the required total array wattage; if space isn't a binding constraint, total wattage (and its resulting energy production) is the figure that actually determines whether the array meets the target usage offset, not the efficiency percentage on its own.