Tuesday, May 11, 2010

Watt's Your Energy Budget?

Now that we have common ground on the meaning of energy, we can discuss an energy budget. In this discussion we will use the "bastard unit" of amp-hours as a stand-in for energy.

Why not use the real energy unit: watts? Because it is easy to say something like: "My refrigerator draws 30 amps, and I need to run it for about 2 hours/day. Thus it uses 60 amp-hours per day." In the real energy units of watts, the refrigerator uses 30 amps x 12 volts x 2 hours = 720 watt-hours. Tho this is correct, it is one step removed from those things that are easily measured, and differs from the amp-hour value only by a constant: the system voltage.So we take a step back from correctness in favor of transparency.

So let's run thru an analysis. On our hypothetical boat, Hypothesis, we find these things which use electrical energy:

12 Volt ConsumerAmpsHr/dayAmp-Hr/day
Refrigerator30 amps2 hours/day60 amp-hours/day
Water Maker6 amps4 hours/day24 amp-hours
Autopilot6/0 amps under way/at anchor?24/0 amp-hours (est) under way/at anchor
Instruments & GPS2 amps24 hours/day48 amp-hours
Interior Lighting5 amps4 hours/day20 amp-hours
Anchor Light2 amps0/8 hours/day under way/at anchor0/16 amp-hours under way/at anchor
Navigation Lighting4 amps8/0 hours/day under way/at anchor32/0 amp-hours under way/at anchor
VHF0.5 amps (RX)24 hours/day12 amp-hours

Wow. Hypothesis shows a total daily consumption of energy of about 220 amp-hr while under way, and about 180 amp-hr while at anchor. These are substantial numbers. If Hypothesis carries (let's say) 600 amp-hours of battery capacity, her owner might still be feeling pretty smug. But most experts seem to agree that regularly drawing down your batteries more than 50% will severely shorten their lives. Uh oh... now we are looking at a daily energy consumption which runs 60% to 73% of the available storage capacity.

So unless we replenish energy, we'll run out sometime in the second day. Not looking good for that long cruise at all. Here are some ways to replenish that 180-220 amp-hours every day:
Run the engine.
If you have a 180 amp alternator, you'd think that an hour would do it. But the alternator won't be delivering energy at it's rated output for that full hour - it will fall off as the batteries charge up. Figure two hours at a minimum

Solar Panels
Assuming that Hypothesis owner bought 4 large panels that are rated at 15 amps, all she'd need is 3.6 hours of sunshine per day. But that rated output is at "full sun", which you can read as "noon in the tropics". If Hypothesis were in the tropics, those 4 panels would probably take care of her pretty well, given that she will have a lot more than 4 hours of fairly direct sun and even more hours at a less than optimal angle. The crew's job would be to keep adjusting those panels so that they are perpendicular to the incoming sunlight thru the day, and to make sure that nothing shadows a panel. Cloudy days present a problem, of course

Wind Generator
Most of these are rated at 300-500 watts - let's convert to our units: dividing by system voltage, 25-42 amps. Now that rating is probably at 25 kt, which we hope Hypothesis is not spending days on end in. So let's cut that in half for a more realistic estimate of power output - say 10-20 amps. The beauty of the wind generator is that it continues day and night, as long as there is wind. The wind generator could easily handle the load.
But the first thing that should be done aboard Hypothesis is to try to reduce the energy consumption.
  • Replacing all the lights with LED bulbs will make a very significant dent in the budget.
  • Drink warm beer and turn off the refrigerator. The refrigerator is the largest single energy consumer on this boat.

Many cruisers seem to use a strategy employing all of the above: They reduce consumption as far as possible. They run the engine occasionally. They have solar panels, and they have a wind generator. Hypothesis should be able to make her cruise after all.

Note that in all of this, I have purposely left out mention of an inverter. Inverters are wonderful inventions, allowing use of 120V appliances with a 12V energy source. But here is where our "bastard unit" of amp-hours can get us in trouble. Remember I said that we could just ignore the system voltage because it was constant? Well with the inverter in the equation, that is no longer true. Here's how to figure your 120V energy usage:
  1. Look for the tag on the 120V appliance where it's rate of energy consumption is specified. It will most likely be in watts (or kilowatts: 1000 watts). Sometimes it is stamped or molded into the bottom or back of the appliance.
  2. If the tag denotes watts, just divide the wattage by 12, and you will be pretty close to the 12V amperage required to feed this 120V appliance thru the inverter.
  3. If instead you find a rating in amps, multiply it by 10 and use that as the 12V amperage required.
(Here, near the end where most people will have given up reading this, I shamefacedly admit that Eolian's energy budget includes 16 amp-hours/day for the making of lattes in the morning, using the espresso machine thru the inverter.)

Also note that an inverter draws power all the time it is on, even if there is no 120V load. But most likely there will be a few "wall bricks" plugged in somewhere, increasing the load. Turn off the inverter when it is not in use.

If you are doing an energy budget (or cruising!), there is one instrument you will want to install: an amp-hour meter. These directly read out the amp-hours consumed from (or fed into) your batteries. Having one on a cruise is having a "fuel" gauge for your electricity storage. And having one before the cruise allows you to get a firm handle on your consumption. I highly recommend it.

Those 600 amp-hour worth of batteries on Hypothesis could store a total of 600 x 12 = 7200 watt-hours, or 7.2 kilowatt-hours. You could buy this energy from your local power company for about $1. Instead, managing it yourself onboard is a *BIG* deal. Life is sure easier ashore.

But life offshore is well worth it.


Unknown said...

Do you leave your VHF on all night?

Robert Salnick said...

Estrellita: Oops - no I do not while at anchor. But I typically do leave the instruments on, so that if I get up in the night to check water depth or wind speed I do not have to fumble around as much.

But thanks for bringing this up, as it shows the kind of thinking that we all need to be doing when budgeting energy: do I really need to run that VHF all nite? Couldn't I turn it off? Should I turn it off? What are the risks of having it off, balanced against the energy it consumes?


(BTW, 0.5 amp is probably way high for squelched RX, or perhaps Hypothesis has a tube-based radio...)

Mike said...

As mentioned on my blog, we have the same inverter as you but we have the 1000 series remote that does not have the digital display that you showed in your photo. I have toyed with the idea of installing this to give me a better idea of our consumption:



Drew Frye said...

Nicely done.

Next to our electrical panel I have 2 small charts:

* Amp-hours remaining at a given voltage. Yes, it is only approximate for a number of reasons (temperature and draw) but it is a good reminder of what is left in the tank. A line is drawn across where I consider our reserve to start. Another line indicates where the engines may fail to start.

* Amp-hours for various systems. A reminder to me and my crew about what we have to run with how much battery.

We find these 2 charts very useful in terms of everyone understanding the situation, rather than just being told to "cut back." In a way, it actually lets us be more free with power, since we can see the daily budget.

Robert Salnick said...

Mike: Holy Cow! That Link 2000 sure has gotten expensive! That is way more than I paid, like 12 years ago. It sure is a big deal to manage that $1.00 worth of electricity we keep on board.

Drew: Thanks! I haven't yet found that line you mentioned: the one that says the engine may not start. I should tho. But we have a backup: we have a genset on board, with a separate battery bank to start it. So if I run down both the house banks, I still have a separate bank to start the generator (presuming I have remembered to throw the "connect all the banks" switch to off). Still, I should know where that line is, just in case the genset doesn't start.

Unknown said...

@Mike - We have the two battery bank Link remote monitor and find it extremely useful. We check in with it all of the time to see where we might be drawing energy that we've forgotten about or to remind ourselves how much something draws (I like the idea of a chart written down). With that being said, the PO paid for it.

Drew Frye said...

Actually, the outboards on the PDQ can be hand started, but you have a wrap a line around the flywheel. Though I've never had to do it under fire, I have tested the practice in fine weather.

I found the "line" on our delivery trip; the batteries were on their last leg and it was quite cold. It is actually a good story about having a plan "b".
The boat wasn't prepared, but we were.

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