I am installing outlets in my patio room. What is the standard height recommended above the ground for these to be installed?
The direct answer to your question is "There is no standard". Outlets can be installed at whatever height is comfortable for the intended use. The only NEC requirement is that you must be within 6 feet of an outlet along any wall (12' between outlets, A unbroken wall space of 2 feet or more including space measured around a corner requires an outlet. ref. NEC 210.52.A.2. A common height for a wall outlet is between 12 to 16 inches to the bottom of the device. In rare instances, outlets are still installed in baseboards and in the floor with proper box/covers.
In the UK, Part P of the Building Regulations 2010 states that for new builds (emphasis mine):
Approved Document M recommends that in new dwellings only, switches and socket-outlets for lighting and other equipment should be between 450mm and 1200mm from finished floor level. Approved Document M does not recommend a height for new consumer units. However, one way of complying with Part M in new dwellings is to mount consumer units so that the switches are between 1350mm and 1450mm above floor level. At this height, the consumer unit is out of reach of young children yet accessible to other people when standing or sitting.
In practice, in areas where there is limited risk, it tends to be the case that unless otherwise specified, a home builder will install light switches at 1.2m above finished floor level, and sockets are at 450mm (approx 18inches) above finished floor level. Isolating switches such as those for a extractor fan for a shower are exempt from those heights (and instead would be installed close to the ceiling level).
When I had my new room built onto my house, the electrician used a standard claw hammer for the height. The electrician stated that this is a general rule of thumb that many electricians use. In fact, a simple google search revealed that someone at the DoItYourself.com forums, recommended a standard 16 oz hammer standing on its end as a reference height. The eHow.com site recommends using an "electrician's hammer" as the point of reference. A forum post a the DIYChatroom recommends also to use a 16 oz hammer that gets you about 18 inches off the floor.
So, I would use the hammer as a guide.
While I agree with @matthew because it answers your question of, “What is the standard heigh...?”, and I like matching the height of the existing outlets.
However, more and more of the homes we are doing we install outlets 18” AFF to match ADA requirements (not because we have to, but because the clients are older and acknowledge their “reach range” is decreasing).
We are also installing elevators (or shafts for future elevators) in multi story houses too. As the population ages, we’ll all be thinking about designing “things” and “uses” for this aging group.
There is no code on the height of electrical outlets. You have a standard height of 16" to the top of the outlet box, and a standard height of 48" to the top of the switch box. Then, there are custom heights in which different company use accordingly to the home owner or builder. Spec homes generally get standard height, and custom homes usually get standard height as well, unless other wise specified by the blue print, home owner or builder.
A hammer length or the front part of a spade is good rough approximation. The exact answer according to the code of practice SANS 10142 states that it must be 500 mm above floor level.