I just power wash my 1998 S90 with clear water
every spring.  Battery included.

I dont touch my 1994 850 with a hose in the engine bay anymore, last time I
did that it cost me $60.00 to have it all dried out, and have the check
engine light turned off.  Maybe it's just specific to the 850 line, but I
was told on (my car anyway) to just use brake cleaner and spot clean.

This is what I've learned washing my V70 engine compartment:  Don't wash
if it's even a little bit warm, water will get sucked into the
distributor cap as it cools.  (You don't have a distributor cap.)  Keep
all water away from the top of the engine, as it will get under the
plastic cap (the cap is the one that says "20 Valve" on my car,
presumably "24 Valve" on yours) as this will end up in the spark plug
wells and cause misfiring.  Also, I don't blast water at anything that
has wires going to it, but I don't worry about low pressure water
washing down over them.