That 20mi/day value they claim on the site is more of an optimistic "up to" value than a realistic one, especially depending on weather and actual sunlight hours each season.
Most newer EVs are coming out with ranges > 250mi, so that 20mi/day is below the 10% reduction for 80kpm. 80kpm (49.7mph) is below the typical highway speed in the US, so unless you're only driving in cities or generally go slow, it will likely hurt more than it helps in daily use.
That said, it would be great if you frequently leave the vehicle sitting outside for an extended period of multiple days, or potentially as a generator for off-grid use (e.g. camping). Still not convinced it is worth the cost for that, though.