Grid-tied vs off-grid: two different sizing problems
Updated July 2026
The same house needs roughly twice the array off-grid that it needs grid-tied, and the reason is a calendar, not a catalog. Grid-tied systems size to the annual average because the grid absorbs every mismatch. Off-grid systems size to the darkest week of December because nothing absorbs anything.
Grid-tied: the average is the whole story
With net metering, the grid is a perfect, free, infinitely large battery (billing-wise): July surplus becomes credit, December runs on credit, and the design target is simply annual production ≈ annual usage. That is what the calculator and every table on this site compute, using annual-average sun figures. The fine print worth reading is the export rate. Utilities that credit exports below retail shift the optimum toward smaller systems plus load-shifting, and a few tariff designs make the last 20% of offset nearly worthless. Size to the tariff, not to pride.
Off-grid: December is the whole story
Off-grid, the question flips from "how much over a year" to "enough on the worst week." Two substitutions in the math: use your location's December peak sun hours (often 40 to 60% of the annual figure; PVWatts lists monthly values), and add battery losses, about 10 to 20% round trip depending on chemistry. A cabin drawing 6 kWh a day in a place with 2.0 December sun hours needs 6 ÷ (2.0 × 0.77 × 0.85 battery factor) ≈ 4.6 kW of array, where the annual-average math would have said 2. That gap is why annual averages sell off-grid kits and December returns them.
Battery sizing, the short version
Days of autonomy times daily kWh, divided by usable depth of discharge. Two cloudy days' autonomy on the 6 kWh cabin with lithium (90% usable): 6 × 2 ÷ 0.9 ≈ 13 kWh of nameplate battery. Lead-acid at 50% usable doubles that. Autonomy is a judgment call (generators exist, and a backup generator is almost always cheaper than the third day of battery), but sizing with no autonomy at all just builds a system that browns out at the first overcast Tuesday.
The hybrid middle
Grid-tied with battery backup sizes like grid-tied (annual math) with a battery scoped to outage priorities rather than daily cycling: fridge, well pump surge, comms, some lights, which is a 5 to 15 kWh battery for most homes, not a December-proof bank. Our single-load pages (the fridge and well pump in particular) carry the surge details that decide whether a given battery inverter can actually start what it must start.
Questions people ask
Can I just unplug from the grid with a normal solar system?
No. Standard grid-tied inverters shut down during outages by design (anti-islanding protection, a legal requirement), and a grid-tied array has no storage. Going off-grid is a different system: bigger array, battery bank, off-grid or hybrid inverter, and usually a generator for the dark weeks.
How many panels do I need to go off-grid?
Take your daily kWh, divide by your December peak sun hours times 0.77 times a ~0.85 battery round-trip factor, and divide by panel kW. For a modest 15 kWh/day home at 2.5 December sun hours, that is about 9.2 kW, or 23 400W panels, roughly double what the annual-average math suggests for the same usage.