There are two factors that I think you're leaving out (at least).
- Peak power consumption occurs during system boot, when all your drives are spinning up. During spin-up, a drive may require as much as 2-3x the power it does on average (the numbers on the drive are usually average power consumption for a particular usage profile). Power supply calculators almost always give what's called a "steady state" or "continuously available" rating, not a peak rating. So while you may only require <250 watts during normal operation per the calculator (but see below), that could be more like 300+ watts during system boot.
- Capacitor aging. Power supplies grow weaker as they age; they are less able to supply their full rated power than when they were new. If you take aging into account (figure you're going to keep the WHS PC substantially as built for a year or more of 24/7 operation), you need to allow for at least 30% greater initial power.
Those two factors, taken together, suggest that a 450w power supply is probably somewhat marginal for your use. I would probably choose a power supply of 500+w.
