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 Consumer||Amps||Hr/day||Amp-Hr/day|
|Refrigerator||30 amps||2 hours/day||60 amp-hours/day|
|Water Maker||6 amps||4 hours/day||24 amp-hours|
|Autopilot||6/0 amps under way/at anchor||?||24/0 amp-hours (est) under way/at anchor|
|Instruments & GPS||2 amps||24 hours/day||48 amp-hours|
|Interior Lighting||5 amps||4 hours/day||20 amp-hours|
|Anchor Light||2 amps||0/8 hours/day under way/at anchor||0/16 amp-hours under way/at anchor|
|Navigation Lighting||4 amps||8/0 hours/day under way/at anchor||32/0 amp-hours under way/at anchor|
|VHF||0.5 amps (RX)||24 hours/day||12 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.
- 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:
- 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.
- 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.
- If instead you find a rating in amps, multiply it by 10 and use that as the 12V amperage required.
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.