The cost part is easy. Your electric bill is based on wattage. Add up the wattage of all the bulbs used in each arrangement, and compare the numbers you arrive at. Ignore voltage.

Hmmmm, so your telling me that voltage has nothing to do with it?
Low voltage is being drawn from a transformer, in this case 160 watts (8 lights x 20 watts). Isnt this wattage being drawn from the actual transformer and not from the acutaly 120v source for the transformer?

Thats where get confused.....

"Isnt this wattage being drawn from the actual transformer and not from the acutaly 120v source for the transformer?"

No, the transformer is not an energy source, it just steps the voltage down from the 120V line voltage to the appropriate voltage for the landscape light system - maybe 12V, maybe 24v. It doesn't matter, in terms of power consumption, that the transformer stepped the power down to 12 volts because, as the voltage goes down, the amperage must go up to achieve the same wattage. It's still the wattage of the bulb that determines how much power is being consumed.

It might help to know that Watts=Volts*Amps. 160W of 12V bulbs will be drawing 13.3 amps from the transformer. Notice that 12V X 13.3 Amps =160 Watts. In order to supply 13.3 amps at 12 volts, the transformer draws 1.33 Amps at 120 Volts; the line-voltage side of the transformer is working at ten times the voltage and one-tenth the amperage of the low-voltage side, but the wattage, 120V * 1.33 Amps, still works out to 160W. The particular voltage at which the load is applied really doesn't matter.

Take a look at your electric bill. The units used to measure power consumption are Kilowatt-Hours, abbreviated kWh. This is the amount of power consumed by, for example, a 1000-watt light bulb if left on for one hour.

Imagine that you left your 160 watts worth of low-voltage lights on all night - we'll call it 8 hours. 160 watts/1000 is 0.16 kilowatts. 0.16 kW X 8 hours = 1.28 kWh, which is what the power company bills you for. It might cost you fourteen cents for that night.

If you'd had, instead, two 23w CFLs, you'd have used .046 kW X 8 hours = 0.368 kWh, for which you might pay about four cents.

Wow, awesome breakdown. I never looked at it that way with low voltage.
I knew how to determine the line voltage but always thought the lv was much different because of the transformer in the way.
I keep my lights on for about 6 hours per night. Sounds cheap doing it day by day, but I'm trying to save on electrically as here in Long Island its real expensive.

Thank so much for the help! Broke it down very nicely and clearly!

-Chip

I'm glad. I wasn't sure how much sense I was making.

I'm in Ohio, and my electricity is running around 11 cents/kWh. I don't have a good sense of how much electric rates vary around the country.

Jon: In Southern Calif. my electric rate ran exactly 12 cents/kWh last month.

(That rate is comprised of 6 separate lines of Delivery Related Charges, 8 separate lines of Generation Related Charges, and 8 other Component lines for fees, taxes, other charges, etc. GEEZO!!!!! All this makes my billing statement come to two full pages of calculations. Unreal!!

Wow, teedup1. My city utility bill is a postcard, with a tear-off stub I enclose with my check. On the card is one line for water, one for sewer, one for trash pickup and one for electricity. If I want more detailed info I drop into the city office which is about 5 blocks from my house.