How much is a watt-hour per day worth? About $1.93. Here's the way I figure it...
I was thinking about whether it's worth replacing my bulbs with LEDs, or better to beef-up my system with wind and/or solar. I'm looking to add refrigeration and/or an inverter for my microwave. That means reducing power usage or adding passive charging (solar or wind).
For solar power, it costs about $1000 to buy and mount a 135 watt panel. Add another $300 for a voltage controller and wiring, and you get $1300. The panel will get the equivalent of 5 solid hours from it, which means 5 x 135 = 675 watt-hours per day. $1200/(675 watt hours/day) = $1.93 per watt-hour per day.
By the way, that leaves out the storage costs, but I have plenty of storage right now, so you may figure a higher cost. I'm also ignoring the fact that the 135 watt panel would go where my current 40 watt panel is. For wind (instead of solar) I get a different number, but the above number is good enough for estimating.
I have 8 lights in my salon. (See sistership here.)
The lights burn 5 watts each = 40 watts. We leave them on for about 4 hours a night when on the hook. That's means 160 watt-hours for those lights, versus maybe 10 watt-hours a day for 8 LED lights (pure guess), for a difference of 160-10=150 watt hours.
From the above, 150 watt-hours x $1.93 per watt-hour = $290 for solar panels to support that.
If I buy LEDs for $35 each, the cost for 8 LEDs = $280. In other words, it's cheaper to replace my halogen bulbs with LEDs, than it is to add solar panels to support the halogen. Also, not adding a bigger solar panel certainly makes for less clutter.
This also means that optimally, any liveaboard boat that is kept on the hook ought to have LED interior lighting, since it costs more to have the solar to support non-LED lights.
By the way, my cost numbers are guesses above. Wanted to post this to show what I'm thinking, in terms of freeing up amp-hours for a fridge and/or inverter for the microwave. If anyone has done a similar analysis, please post it (or a link to it).