@Leo - Sorry I wasn't too clear there; I was actually agreeing with you that micro-generation doesn't seem to stack up from a government-support/environmental point-of-view, but was just disagreeing with the statement that nobody ever looks into whether it does or doesn't. The likes of that paper makes it pretty clear that the best application of government money (from both a cost and environment POV) would be in supporting large centralised renewable energy projects. My understanding from other papers is that this is also the best approach for the grid.
Where are the calculations supporting EV subsidies
EV subsidies are quite different to micro-generation subsidies. There is no equivalent to pooling all the micro-generation grants and buying a giant solar farm here, unless you consider electrified public transport to be that analogy, but I think that's a bigger issue. So while a carbon tax to push up the cost of ICE cars would probably be better, subsidies are the next best bet to increase the adoption of EVs which are better for the environment and for which there is a fairly large body of evidence.
those that apply to air-to-air heat pumps
I think these are different as well. The government can encourage people to replace their gas/oil boiler with a more efficient one and save a small bit of carbon output, but if they can encourage people onto electric heating of some sort (like heat pumps) they can then centrally drive down the carbon output of electricity generation. This one I haven't thought much about, but I think it's probably self-evident comparing a gas boiler to a heatpump running on electricity that is getting cleaner and cleaner?
how do we have quite a range of such grants available?
In summary I think the EV subsidies and those that encourage people to reduce energy consumption are fairly well backed and should exist. As for why we have grants for home solar, honestly I think that is down to pressure from interest groups. You see complete uproar in the US as feed-in-tariffs are rolled-back and even here if you're in the right forums you'll see constant complaining about the lack of FITs etc., which the government have tried to avoid bringing in. Now of course if the argument is that it is more efficient for the government to do it centrally, they damn well need to actually be doing it, which they most certainly are not in Ireland and are only paying lip-service to in the US. But I think most people pushing for home solar grants and FITs would be understanding if the government came out and said "Listen we're not going to bring in a home solar grant this year, but we are going to take those X million €, add a lot more, and start building huge numbers of solar farms, wind farms and grid-scale battery storage and WILL be at 50% renewable by 2025, 75% by 2030". But if they don't do either, I can understand why some people (me included) would like to just stick up some solar panels and at least feel we are doing our bit.
Those are my 2 cents anyway