Instead, a simple approach is to download the daily power consumption for a year and size the battery for about your 80th-90th percentile consumption. You tend to find the sizing is not that sensitive to whether you go for 80th or 90th percentile, and in any case the batteries come in standard sizes.
If you've sized your battery system economically, it should be empty a good proportion of the time, but that just doesn't "feel right" to consumers.
* Yes I mean frequency not probability but I didn't want to cause confusion with electrical frequency
Although I suppose it is cheaper to oversize solar and not the battery, but that's usually already maxed out and limited by roof space. Maybe a small wind turbine to compensate for stormy days...
I think they're talking about the situation where you're still connected to the grid, in which case you don't need to handle the backup capacity.
