How To Calculate Amp Usage Aboard a Boat

Almost every cruising boat, including us, seems to underestimate the amount of electricity we use per day when planning alternative energy – for us solar panels and wind generator.  Very frustrating when your goal is to live by the sun and wind and not have to run the diesel or generator daily!

DISCLAIMER:   I am NOT an electronics guru – I don’t really understand much of this stuff other than basic principles.  What I DO know is that our alternative energy has NEVER kept up with our electrical consumption and this analysis shows why!    🙂     Keep in mind the power usage figures I’m using is what we’ve determined by using our Link 10 and turning things on & off to isolate individual items – not necessarily what’s quoted by the manufacturer. Hopefully, unlike us, you’ll figure out your alternative energy needs BEFORE you spend six years cruising frustrated every morning by the morning ritual of “how can we POSSIBLY be down that many amps!!!”   🙂

Before you can start to plan for a cruising vessel’s alternative energy plan, you need to know how many amp hours of electricity you use each day.  Do you need to replace 100 amp hours … or 150 …. or 200 ???   Unfortunately there’s no shortcut easy way to calculate this information — I can’t tell you because it’s vastly different for every cruising boat.  Even individual systems, such as radar, can consume vastly different amounts of juice based on how old they are (ours are ancient) and a variety of other factors, so there’s no one size fits all equation.

When we started outfitting Winterlude, we had a 55 amp alternator with our 30 hp diesel engine and that was it.  No alternative energy at all.  We didn’t pay as much attention as we should have to our alternative energy plan, and as a result, on an average we only replace about half of our daily usage via alternative energy.    Entirely preventable if you plan correctly.

STEP 1:

The first step is to calculate how many amps you use daily.  To do this, you must go through meticulously and calculate amp usage by hours used by anything aboard.  If you don’t know how to calculate amp usage for each individual thing that uses electricity, you’re going to have to be a bit of a sleuth – you need to find the watts and voltage for each light bulb, each appliance and so on.  Most packaging will contain the information you need, often you can look it up online.  As an example, look on the bottom of an appliance – here’s our coffeepot.   You can see that it uses 1425 watts and it’s 120 volts.  WATTS divided by VOLTAGE = AMPS is the formula.  So 1425 divided by 120 = approximately 12 amp hours PER HOUR.

THE INVERTER FACTOR

HOWEVER,  it’s not this simple — of course not!   There’s nothing worse than running the noisy Honda 2000 generator before I even get my first cup of coffee!  So, we use the inverter.  Ours is modified sine wave and we’re lucky we’ve never had anything electronic break – a better choice for us would be pure sine wave, but it still works, so se la vie.  Problem is, all inverters are different but ALL inverters waste power – it gets really really complicated to calculate how much wasted power there is in converting DC to AC so we use a “wasted” ratio of 2 to 1 — i.e. it takes twice as many DC amps to power that AC through the inverter.  So for items powered via the inverter, I double the amps usage.  I know this is not scientific, but it seems to come pretty close for our alternative energy calculations – if you’re more technical than I, feel free to jump in and let us know a better way to do this by leaving a comment below!

Here’s our at anchor list as a starting point, note that you need to know amps per hour AND an estimate of how many hours you use the item to calculate daily amp usage.

LED Anchor Light .25/hour X 10 hours = 2.5 amps/night

Adler Barber Cold Plate – Refrigerator & Freezer    6 amps/hour X 24 hours = 144 amps/day (theoretically it cycles on & off, which it does, but it still seems to take between 120 – 150 amps a day)

Bilge Pump:  3 amps/hour X 5 minutes + .25/day

Sensibulb LED Interior Lights:  avg 3 lights on for 3 hours each = .20 amps/hour X 9 = 2 amps/day

VHF Radio:  6 amps transmit/ .5 amps receive – 12 hours = 6 amps + 10 minutes talk time = 1 amp = 7 amps total

SSB Radio:  1 hour receive @ 3 amps/10 minutes talk time @ 30 amps = 8 amps total

(NOTE:  Both radios are different based on whether we’re using low power or high power, these are just guesstimate averages)

Coffee Pot:  6 cups of coffee – 3 amps each (including the inverter) = 18 amps

Cabin Fans:  2 amps/hour X 2 hours each or 4 hours = 8 amps (obviously if it’s hot and we run the pullman berth fan all night we use 20 amps, we probably need to replace these fans with more energy efficient fans, but they move SO much air!!)

Laptop Computer:  2 hours/day 5 amps/hour = 10 amps

Printer:  10 minutes/day at .35/hour

Charging Stuff (from phones to drills to handheld VHF’s etc):  5 amps/day average

So you can see with just the basic daily “stuff” we run, we average over 200 amps a day.  Unfortunately, we only have 4 55 watt solar panels and a KISS Wind Generator.  Figuring 14.5 amp hours from our solar (my simplified rule of thumb is 1 amp per every 15 watts) – and there are maybe 4 total hours when we generate max amps – so 14 amps X 4 hours = 56 amps, plus less  for the remaining 5 hours or another 25 amps – total solar on a good day is 81 amps.  Significantly short of our 200 amps/day usage.  YIKES!  Add wind — if it’s blowing 12 knots, we get maybe 3-4 amps/hour for a total of 72 amps a day – and rarely do we ever see this many amps in a day from the wind generator. Under perfect conditions, with our alternative energy choices, we would add 153 amps — and we’re using over 200.   Bottom line is we can go about 3 days without having to use either our Honda 2000 portable generator or the diesel to recharge our 6 Trojan T105 wet cell batteries.

We didn’t do our homework well enough when equipping the boat for alternative energy!  If we did it over again, we’d have at least 450 watts of solar – or 30 amps/hour from 450 watts or 120 amps from peak hours and another 55 from off peak hours = 174 amps on a good day for solar, plus the 72 from the wind generator for a total of 246 amp hours a day.  Unfortunately perfect days are rare, but with 450 watts of solar and the wind generator, we should be able to keep up with our 200 amp daily usage.

Other stuff:

Spectra Catalina 300 Watermaker:  15 amps/hour, 13 gallons/hour takes 3-4 hours to fill one tank or 52+ amps – we usually run the Honda 2000 or make water underway.  Technically we CAN make water using our alternative energy and sometimes do if the wind is blowing 15-18 and we’re generating significant wind amps as well as solar.

Don’t forget to calculate an under sail version of this same analysis – underway we run our navigation lights (since we’ve switched to LED, these are not a big draw, but before we switched they were HUGE – don’t leave them out of your analysis), our radar close to shipping lanes, our GPS – actually 2, a Garmin 541 at the helm and our laptop consistently for the Nobeltec electronic charts plus our Tridata Depth, Wind & Speed instruments. Update:  if we were to do the electronics over again, we’d include AIS, so that’s another energy draw underway … who knows, we may add one soon anyway! 🙂

If we’re sailing these add up – especially the laptop with it’s 5 amps an hour draw = 120 per 24 hour period, plus another 7 per hour below = 288 amp hours in 24 hours NOT including the daily stuff such as the refrigerator…. YIKES!  We limit the amount of time we need the laptop and radar actually running since we have a Garmin at the help and the radar only needs to be on if we’re dodging either ships or thunderstorms.

One last point …. we don’t include our electric windlass, electric autopilot or other items that are only used when the diesel is running because the alternator is charging and we’re not draining our batteries.    If you use an electric autopilot underway when SAILING, not under power, then you’ll need to calculate the usage because they can be huge amp hogs & you don’t want to run your batteries down while enjoying your sail!

Please leave a comment and chime in to the discussion!  THANKS!  Jan