A household in Canada would typically need more panels than one in Mexico. Canadian homes tend to consume more electricity and the Canadian rooftop will get les

A household in Canada would typically need more panels than one in Mexico. Canadian homes tend to consume more electricity and the Canadian rooftop will get less sunlight. If you are space-constrained, like on a rooftop, 10% more power from the same area of panels could be significant. It's a step function. It goes from "not enough room for desired wattage" to "enough room for desired wattage" very abruptly. Once you've reached "enough room for desired wattage," further efficiency improvements matter much less. You don't care a lot whether you have 25% or 40% leftover roof space after installing the panels. Paying for further efficiency improvement is worthwhile only if it offsets enough costs elsewhere to lower systemic costs (from using e.g. less mounting hardware, less connecting wire, less installation labor.) The higher the industry-average efficiency, the less of a constrained-space advantage there is for expensive, higher-efficiency modules. That is, SunPower had a more compelling advantage when they were selling expensive 19% panels and cheap Chinese panels were at 12%. But now that SunPower offers expensive 22% panels and cheap Chinese panels are at 15%, more households can now reach their desired wattage-per-roof targets without paying for premium-efficiency. If you just want to zero out your household's annualized average electricity consumption, because e.g. you have a net-metered grid connection, adding batteries to a PV system will increase the expected financial time-to-payback. If you are off-grid you need batteries to use solar power after dark but should make every effort to cut your non-daylight power needs before sizing the battery system. Batteries are currently a lot more expensive per watt-hour delivered than the instantaneous output of solar panels. In an intermediate case, where you don't get full net metering but are still grid connected, there are a few but not many cases where household batteries yield positive financial ROI at current prices. You could make the numbers work for a modest fraction of Hawaiian households and smaller fractions of households in Germany, California, and Australia.