I've always heard 40%, though I can't back that up. I will say though that you need some humidity to reduce static electricity build up.
EDIT:
Ah, I found my documentation, good old Sun Microsystems Part No. 805-5863-13, "Sun Microsystems Data Center Site Planning Guide: Data Centers’ Best Practices"
Temperature and relative humidity
conditions should be maintained at
levels that allow for the greatest
operational buffer in case of
environmental support equipment
down-time. The goal levels for the
computer room should be determined in
a manner that will achieve the
greatest operational buffer and the
least possibility of negative
influence. The specific hardware
design, room configuration,
environmental support equipment design
and other influencing factors should
be taken into consideration when
determining the specific relative
humidity control appropriate for a
particular room. Psychrometrics can
affect hardware through thermal
influences, Electrostatic Discharge
(ESD), and increases in environmental
corrosivity.
And:
Under most circumstances, air
conditioners should be set at 72º F
(22º C) with a sensitivity range of
+/- 2º F (+/-1º C). Humidifiers, in most cases, should be set at 48% RH
with a sensitivity range of +/- 3% RH.
The set-points of the air conditioners
should always be chosen in an effort
to maintain the optimal recommended
temperature and relative humidity
levels for the room environment. These
set points should maintain appropriate
conditions, while allowing wide enough
sensitivity ranges to help avoid
frequent cycling of the units. While
these tight ranges would be difficult
to maintain in a loosely controlled
office environment, they should be
easily attained in a controlled data
center.
Numerous factors, such as heat-load
and vapor barrier integrity, will
influence the actual set-points. If
the room lacks adequate vapor barrier
protection, for instance, it may be
necessary to adjust humidifier set
points to accommodate seasonal
influences. Ideally, all inappropriate
influences on the data center
environment will be eliminated, but in
the event that they are not, minor
adjustments, made by trained
personnel, can help alleviate their
effects on the environment.
And on Electrostatic Discharge:
The maintenance of appropriate relative humidity levels is probably the most
universal and easiest means of addressing ESD concerns. Appropriate moisture
levels will help ease the dissipation of charges, lessening the likelihood of
catastrophic failures. The following chart illustrates the effect moisture levels can
have on electrostatic charge generation.
Note – Source Simco, A Basic Guide to an ESD Control Program for Electronics
Manufacturers
TABLE 6-3 Electrostatic Voltage At Workstations
Static Voltage
Means Of Static Generation Relative Humidity 10-20% Relative Humidity 65-90%
Walking Across Carpet 35,000 1,500
Walking over vinyl floor 12,000 250
Worker at bench 6,000 100
Vinyl envelopes for work instructions
7,000 600
Common polly bag picked up from bench
20,000 1,200
Work chair padded with urethane foam
18,000 1,500
Best Answer
Just convert the estimated draw from Watts to BTUs: To calculate the BTUs/hour, multiply the watts by 3.413. 1 ton of cooling is equivalent to 12,000 BTU/hour.
Most of the energy drawn will be converted to waste heat.
From what I am reading, a pro would tell you to have someone come in and do the full calculation. That would take into effect any odd heat sources (solar for example) and your humidity concern.
In practice - I have just oversized mine by 25% or so in the past - to allow for new equipment. (In a small equipment room!)