Average energy use in the Netherlands is 3340 kW-hr per household per year, which amounts to about 9.15kW-hr per day (and seeing that these are averages, I gath

Average energy use in the Netherlands is 3340 kW-hr per household per year, which amounts to about 9.15kW-hr per day (and seeing that these are averages, I gather that the type of household to pioneer this sort of tech will be sufficiently energy-conscious to use quite a bit less than that). Either way, I think you'd have to round your estimate to order-of-magnitude anyway, because of the rather big assumption "that your solar power system can supply that over a 12 hour period we need to store half of it, 72MJ, to keep us going after dark", because getting max solar power for half the day and no solar power for the rest is not exactly how it works, nor does one use the same amount of energy during day and night (which is why I can opt in to an energy plan that gives you discount on electricity at night, because there's a surplus then).