Aotearoa / New Zealand

The issue with it is that the grid has to consume the energy in the same instant that it is generated.

Picking some easy numbers to make the example clearer; say my store uses 24 units of electricity per day - 1 unit per hour - and say there are 12 hours of useful sunlight per day. I can install enough solar panels to generate 2 units per hour of sunlight, generate 24 units in a day, and be "100% renewable". Except in reality it's hard to actually be using 100% renewables, because for the 12 hours per day when my panels aren't doing anything I still need power, and that power has to come from somewhere.

Electrons are fungible - power generated by solar is just as usable as power generated by burning gas - but generation capacity isnt. A kWh of stable renewable capacity that can reduce the demand on a non-renewable generator is much more "green" than a kWh from a renewable source that can only generate when capacity is already high - you'd need to keep the non-renewable generator around to pick up the slack when the capacity drops.

The situation in NZ isn't as bad as it could be because we get a lot of our energy from hydro, but there is still the problem that solar generation isn't constant, and the times of the day where it peaks don't line up with the times of the day where demand is highest. This is going to get worse over the next ~20 years as more people switch to EVs that they'll want to charge at home overnight. Systems like grid-scale batteries and other storage technologies might mitigate this, but they are a lot less cost effective than solar panels kWh for kWh, so projects like this tend not to include them