Simple low-powered Arduino controlled on/off device circuit

I've been doing a lot of research trying to figure out the way to control a 3 amp load (at 12 volts) using an Arduino in a way that consumes the least amount of power. I was given the advice in another thread to use a MOSFET (IRL540), but I understand these consume 0.6 watts when in an "on" state. I need something more efficient.

How can I construct an Arduino-based ultra-low power timer that turns something on at 6:00 a.m. and off at 10:00 p.m. when the load is 3 amps at 12 volts?

You could use a 5v latching relay. Then the Arduino could sleep most of the time.

I’m not a MOSFET expert, but I picked up a few of these, have only played with them a little, but RDS should be < 4.2m?, so at 3A, dissipation should be < 40mW:

http://www.digikey.com/scripts/DkSearch/dksus.dll?WT.z_header=search_go&lang=en&keywords=IRLB8743PBF-ND&x=14&y=11&cur=USD

Can I power a latching relay with an alarm pulse from a DS3234 DeadOn RTC? If so, I just found the perfect Arduino on/off timer.

DS3234: https://www.sparkfun.com/products/10160

You can't beat zero power consumption in the timer!

Another thought - a low power relay that closes two contacts to trigger a Pololu power switch. Unfortunately, this consumes power when in an "on" state.

Switch: Pololu Pushbutton Power Switch SV (old version)

What power exactly are you trying to minimize, and why? I'm confused, because if we have a load that consumes 36W for 16 hours a day, I don't see how a dozen or so milliamps for a microcontroller can matter much. Is the microcontroller powered from the same source as the load, or from something different (what?)

If it were me, and I wanted to absolutely minimize power, I wouldn't use an Arduino, as the support circuitry cannot be shut down. Search this forum for discussions on that topic. I would design a custom circuit based on the ATmega328P, probably with a real time clock, and a MOSFET for a switch. The MCU could sleep 99%+ of the time, whether the load was on or off, and therefore consume a negligible amount of power.

Panasonic Electric Works make a bunch of latching relays that can be controlled directly by an Arduino (5V, <31mA).

From my reading of the DS3234 datasheet I think the INT (alarm) output is open-collector and can only handle 3mA. I can’t find any latching relays that run on 3.3V 3mA so you would need some kind of driver like a transistor.