Hi, I had to dig out my 1900 Flight Safety manual to try and come up with the answer to this one.
The outflow valves are pneumatically operated by vacuum pressure from the pressurization controller, so an electrical failure shouldn't cause them to open. If the pressurization switch is placed to "DUMP", the outflow valve is opened via a dump solenoid. However, this probably wouldn't work during a complete electrical failure.
"If a complete electrical failure should occur, both environmental regulator/shutoff valves would fail to the closed position. No more bleed air would enter the pressure vessel and cabin pressure would leak down." (Flight Safety Manual, p.8-4)
However, if you still had battery power, you could still open the left bleed valve, since it has that white ring around it.