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  #1  
Old 04-10-2010
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How much is a watt-hour per day worth?

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).

Regards,
Brad
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Last edited by Bene505; 04-10-2010 at 12:44 AM.
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Old 04-10-2010
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The guesses are probably a bit high... a Kyocera 135 watt panel is only about $365, and mounting it isn't going to cost $635... especially on a boat the size of yours, where the bimini or dodger could easily support a panel.

Add two panels, an SB2000E Blue Sky MPPT controller and mounting, and you're about $1500 or so, not counting shipping...

While I can justify refrigeration, and have a small Engel refrigerator that runs completely off of the solar panels on my boat, which is on a mooring—I don't think I could ever justify having a power hog like a microwave oven aboard. If you consider a small one is 900 Watts, and with an inverter losing 15% or so... you're looking at 1100 watts or so.

A typical halogen bulb is 10 watts. A typical LED bulb is .2 Watts or so.Replace all your bulbs and you might have a 200 watt savings....but not all will be on at the same time, so it really is a lesser savings.

Quote:
Originally Posted by Bene505 View Post
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).

Regards,
Brad
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Old 04-10-2010
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Quote:
Originally Posted by sailingdog View Post
While I can justify refrigeration, and have a small Engel refrigerator that runs completely off of the solar panels on my boat, which is on a mooring—I don't think I could ever justify having a power hog like a microwave oven aboard. If you consider a small one is 900 Watts, and with an inverter losing 15% or so... you're looking at 1100 watts or so.
I'd go just the opposite on that, a small microwave is only used for minutes and doesn't use any power when it isn't cooking, I have had one hooked to an inverter before and it worked great, and was very inexpensive. Contrast that with a refrigerator which never stops needing power. I don't want a microwave on the boat, but I'd have a microwave long before I'd have refrigeration.
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Old 06-01-2010
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Been thinking about this a lately. My take on it is that purely comparing dollars-per-watt-hour-per-day gained isn't sufficient. The problem is that it leads to only one of two possible conclusions: more DPWHPDs for switching to LEDs (conclusion in your case) or more DPWHPDs for installing panels.

Maybe the solution that maximizes DPWHPDs is a mixed solution, e.g., less wattage of panels, and convert only the lights of highest daily cost to LED. Maybe the light in the head is only used for a couple of minutes per day, for example, whereas the anchor light may be used for eight hours. If you buy a few LEDs where it counts, then a smaller wattage of panels may be sufficient to cover the remaining incandescents and other consumers. In the end the savings might be greater than either fully converting to LEDs, or installing panels to fully cover your budget.

Of course it's not really necessary to fully cover your energy budget with off-grid solutions, unless you plan on being off-grid indefinitely. What I did was to set a safety margin that represented the maximum discharge I ever want to get to on the battery. I picked 30% (i.e. draw down to 70% of full charge). Obviously 50% is the absolute minimum, but I wanted extra room on top of that as a safety margin. Then I looked at what my daily surplus energy from off-grid sources was. In other words, daily generation from panels - daily consumption, in Ah/day. For me it was -6.67 Ah/day, meaning a deficit. If I'm only comfortable using 30% of my total battery capacity, that's 45 Ah, which would be consumed in six days by my system.

In other words, the setup I've got right now gives me six reasonably sunny days off the grid. If somebody were to suggest a different setup to me, I would have to see how many days it gives me off-grid in order to determine whether it's worth it to switch.
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Old 06-07-2010
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I am off grid 100% ie on a mooring. I have no solar or wind. I would like to get solar but $1000 would be 1/4 the cost of my boat and more than I can afford. To us having lights and a radio this year is considered a luxury. All last summer we used 2 wind up flaslights. Does your 6 days figure mean you are cruising or your boat is just sitting there? If cruising wouldn't you have to count motoring time as charging time?
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Old 06-07-2010
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Really doing a financial analysis means accounting for capital investment, maintenance, operating costs, and equipment life. It's a pain. *grin*

Sunk costs don't count - you generally can't "unspend" money already gone.

So for me, running the batteries on a charge cycle between 80 and 50% and accounting for maintenance but not ultimate replacement, my 6kW diesel generator costs about 2/10 cent / Whr. Pretty hard to justify spending more money on renewables. The equation might be very different starting from scratch.

Interesting to think about.
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Old 06-08-2010
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ekenna: in picking my numbers, I assumed that I'm cruising; mostly not underway at night; running an autopilot, depthsounder, and VHF most of the time while underway; and have an oil lantern and battery-powered anchor light. Mostly these are guesses because mostly I have not done that kind of multi-day cruising yet. In principle I could run the outboard to charge but I'm not counting that, as I'm keen to minimize use of the motor (mostly for fun).

SVA: not at all surprised to hear that a fuel-burning generator is cheaper than renewables. However, there are other factors to consider, such as whether you've got an inboard (I don't) or space for a generator and its fuel (I don't). The panel may have cost more ($3/W) per W-hr/day, but it's out of the way, silent, and I don't have to do anything at all to turn it on so it keeps doing its silent, out-of-the-way thing while I'm away.

Also, getting back to why I like the days-off-grid method. I can afford to take six days off from work. I can probably afford twelve, if less frequently. But until my life changes, more days off-grid are just not useful to me.
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Old 06-08-2010
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If you keep your boat on the hook, you'll never get a really good full charge on a generator. The acceptance rate goes down to where you batteries may only be accepting a few percet of what the generator is putting out.

Dog or others may have more to say on this. IMHO, you typically won't be running your generator for long enough get up to a full charge, which needs occasionally for the health of the battery. Dockside when plugged into shore power you get this. On the hook you need a renewable energy source to get you there if you aren't on the boat using power. (A generator can certainly help you get close though, and that's useful.)
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Bene, true but SVA did specify cycling between 80% and 50%, so maybe he doesn't care about topping off his bank. But like you said, undercharging can cause other problems. Is 80% enough or do you really need to get it to the high 90%s?
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Old 06-08-2010
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I think it's all related to where you drop your hook.

If I'm off some uninhabited island and the nearest diesel is 80 nm to windward, having solar or wind power generation seems cheap.

If I've got a small diesel tank, then I can't run the engine/genset as long, so maybe having those big solar panels and wind generator will compensate for the load on the watermaker.

If it's a 2 minute run in the tender to the fuel dock to get another 5 gallons of diesel then it's going to be hard to break that habit. However, fuel prices are going up, maintenance costs are going up, and the noise and air pollution a genset or long running diesel may not make many new friends.

Going green isn't cheap but it seems quieter, cheaper over the long run, and causes less enviromental damage.

A 270W (2x 135W) solar array with regulator is about $1000 with basic mount. Using your 5 hours a day, I get 1350W/day for the solar panels, and assuming a 25% recharge penalty, that's 1000+ watts of charge to the batteries or about 15 Amps for 5 hours @ 13.2VDC. So as long as you keep the total boat load below that you're ahead of the game.

The initial cost is $3.70/w for the entire system, but the panels are warrantied for 25 years. Figuring $1000 for the complete system tells me that the true cost is $40/year. If we assume 5 hr/day x 270 days/year looks like 273.3KW/year after that generous 25% recharge penalty. Not bad for your $40 assuming you can control your power demands.

Chances are you'll replace the batteries several times over that 25 years, not including other gear that fails or you replace.

I'm of the belief that it's a balancing act. By reducing the load the vessel requires, the cheaper you can "repower". Having multiple repower sources can help the overall repower time, prevent single point failures, and allow you to stay in that sweet location longer.

You need to look at the loads on the system, then find ways to reduce them. Some you can't, some you won't, but some are a "we should do that".

Only then can you do a cost benefit and decide, based on your vessel, your lifestyle, and your requirements, where the best compromise is for you.

Edited: June 9 - changed 15AH for 5 hours
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Last edited by oceanscapt; 06-09-2010 at 01:08 PM.
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