There are instances of where sprinklers have been of value in protecting houses in wildfires. As a defensive measure, you might want to consider adding sprinklers in strategically vulnerable spots. This home survived the 2003 San Diego fires by using relatively inexpensive series of sprinklers along the ridge line (an adjacent home was destroyed):
They were turned on when the homeowner left. Of course, this doesn't work well if the water pressure drops too much or electricity is lost. To sprinkler-protect a house, first look at everything in the "home zone" to see what could be protected. Very high on that list has to be a deck, porch, patio, or landing. These features need sprinklers that can adequately "mist" all of the flat area and any places that butts against vertical surfaces, such as siding or steps.
You might sprinkle some vulnerable glazing as well, such as large first-story windows or doors having a lot of glass. You shouldn't go too far with this in terms of water volume; you don't want to create water damage. On the other hand, the exterior envelope does get exposed to rain, so it should be able to tolerate this.
If you have a shake roof, definitely look at sprinkling it. A number of homeowners have saved houses by hosing down the shakes (but then you have to be there!). This also has the advantage of getting water into gutters that still might have debris, especially the upper level gutters that are seldom cleaned. If you go with sprinklers, be sure they are designed to fully drain so that no freezing occurs. Any outbuildings are good candidates
However, there are some major problems or considerations:
1. Without a large cistern and a rechargeable battery-operated pump, you are at risk from the loss of water pressure or electrical power, and could impact water pressure that is needed for firefighters.
2. Placement and types of sprinklers is critical.
3. You may need a remote or temperature-sensitive means of activating the sprinklers if you are not at the house.
4. Inadvertent or extended activation could cause serious water damage.
There are some jurisdictions that are requiring internal sprinklers to extinguish a fire in the attic or garage.
You might want to consider one (or more) of the following sprinkler systems:
1. Ad hoc. A simple, quick system using hoses and area sprinklers. These can be installed at the last minute, but you are limited by the number and length of hoses.
2. Installed manual system. Lines installed using irrigation system components, with sprinklers that are matched to the desired spray pattern, using line water pressure. The sprinkler heads could be mounted on roof ridges, deck corners, near windows, etc. These could be activated using a manual valve, or using a spare line on your sprinkler control unit.
3. Automatic activation. This can be done rather easily using a thermostatic control unit in control line to the pressure header. It is important to set the thermostat well above ambient temperature and in the most appropriate (shady) location.