Photoluminescent signs absorb and store energy from ambient light (requires 54 LUX or 5-ft candles of fluorescent, metal halide or mercury vapor light to charge material for 60 minutes). These signs require NO AC power (no wiring!), no maintenance, and are easy to install. (A light source is required for "charging").
NYC-EXIT.COM TIP: Consider using power free signs in any location where an egress path needs to be marked but there is no easy access to wiring.