Posted by Peter Kennedy on 4/10/2019 to Techniques and How to...
ABYC Standard E11 proposes a way to do DC load calculations for boats. The results of the basic load calculation can be used to determine the amp rating for switch panels. The data collected can then be used to calculate daily power consumption and ultimately the results can be used as a way of calculating battery size.
Load calculation for panelboards
The first part of the calculation divides up the loads into essential and non-essential loads. How you make the division is up to you, for some people having cold beer might be essential, others they might go with the more conventional definition of essential items like emergency radios and bilge pumps. Here is Table 2 from ABYC E11
The total load used for calculating breaker panels is the sum of the essential loads plus a portion of the non-essential loads as defined above. Click here to see an actual completed example.
Daily power consumption
The ABYC doesnt cover the next part of the load calculation but it involves using some of the same numbers you used above to come up with a daily power consumption figure. This might be done for a couple of different scenarios, for example a boat underway would have a much different load profile that a boat at anchor. At anchor the electronics may be off but the lights will be on much longer, the stereo may be cranked up, the engine will usually be off, and this is typically the most energy intensive time. You can do your calculation in Amps and end up with Amp Hours as a total or you can do your calculation in Watts and end up with Watt Hours as a total, it really doesnt matter. For the purposes of this calculation we are using Amps and Amp Hours and we are using the nominal voltage of 12 volts even though the actual voltage may vary a bit.
In the simplified example above I have only filled in a few of the values just to give you an idea of how it works. The example boat uses more power for nav lights underway but overall consumption is greater at anchor because they have the cabin lights on longer and the stereo cranked up.
Daily power generation
Now we know how much power we use every day we have to figure out where it is coming from. Again there might be differing scenarios, maybe the boat has a lot of solar but we also have to figure for cloudy days.
If we take our examples above together we see a boat that consumes on average 70 Amp Hours per day and can produce more than that so the totals look good, we just have to figure out a battery size. I normally size the battery based on one full days consumption. We wont have any solar charging at night and we may not be popular running the engine at night either. At this stage you have to realize that these calculations are all a bit vague. We don't really know how long the stereo will be on, or how long the autopilot is running but we are just doing our best shot at it and when it comes to sizing the battery and other important decisions a bit of wiggle room would be good and a lot of wiggle room would be better. What happens if you have two days without sun? What happens if the engine breaks down?
The last part of this blog post is what inspired me to write it in the first place. It is a detailed calculation done for a large catamaran that was proposing to install a lithium battery system. The boat has a 24 volt DC system but also runs quite a bit of 12 volt DC equipment as well as having an inverter to produce 120 volts. Because of the variety of voltages it made a lot more sense to do the calculation in Watts and Watt hours and again they are using the nominal voltages of the 12 and 24 volt systems even though the actual voltage may be a bit different. The formula to relate amps to watts is shown below:
Watts= Volts x Amps
On this example boat a lot of solar is available, the battery proposed is a high performance Lithium Ion battery, and there are two alternators and a hydro generator available. The loads are quite extensive too and include watermakes and some limited running of the Air Conditioning system from the battery using an inverter. The image below is a snapshot, click on the image to enlarge it to full size.