Trying to find a mobo and CPU combo that idles < 20w and has enough power for expansion over the next 5 years is proving difficult.
Really? What does idle at >20W DC these days?
Why don't you simply get a regular server board with a more powerful CPU you could even upgrade down the road without changing the board if need be? Intel-branded boards tend to be efficient. Have you looked at their low-end server boards?
Modern Intel CPUs consume very little when they idle. It's the board that consumes most of the power. So you shouldn't be afraid of using a mainstream CPU (and conversely you shouldn't automatically trust boards bundled with low-power CPUs to be very efficient because the various features vendors stick on the boards such as many NICs, remote management and so forth can end up burning up quite a bit of power). The advantages of using a low-power CPU is that they're easy to cool and they consume little *under load*. Very low *idle* power consumption, you find by looking at feature-poor boards, boards using mobile parts, DC-powered boards and so forth... in other words consumer or industrial (appliance) boards rather than server, typically.
I was thinking of the Pico because I can't see it using more than 100w
The thing is, if you add many drives you'll have to spin them up and that takes some juice (unless the board has staggered spin up... does it?). And if you don't add drives, what's the point of spending so much? If it's only for development, you can get cheap performance in a less reliable and efficient computer you won't have to keep on 24/7.
Most of the other PSUs look like they lose efficiency with lower loads.
Picos do it too. But you're right: most PSUs have terrible efficiency at low loads.
You only need to use one of the best and the loss will be small enough. The problem is obviously the price. But since you're spending so much on a server board, I figure you might as well also spend on a quality PSU since the PSU affects reliability as well.