One of the challenges with the smaller electrical systems found on most boats is managing the power load. Whenever electrical loads are running on a boat, they are delivered by some power source. It might be the generator, it could be shore power, or it could be the inverter. But, whatever the power source, it has some limit on maximum output.
Different power sources have different bounds, but they all have limits. Our generator can reliably and continuously produce 42A. A common shore power configuration in North America is 50A at 240V. In Europe, 32A and 16A at 240V is common and we’ve seen as low as 6A on some shore power pedestals. Our 240V inverter can produce 25 amps and our 120V inverter can produce 33A (details on our power configuration are at A More Flexible Power System for Dirona).
Some of these bounds are quite low, and all are lower than you would expect to find in most apartments, so the electrical loads have to be managed. It’s difficult to stay within those limits, especially when there are multiple people on the boat and some might not be used to managing electrical loads. That’s why boaters frequently are resetting the breakers on shore power pedestals
|Our 240V inverter can produce 25 amps and our generator can reliably and continuously produce 42A|
Overloading the power source can range from a minor hassle to a major inconvenience. In the case of the generator, you might stall it, which results in a complete power failure until it’s restarted. Putting excess loads on a generator isn’t good for it, and stalling a generator exposes the all the electrical devices on the boat to rapidly decreasing voltage levels and the increasing current draw that results. Stalling a generator is not good for anything, but it generally can be quickly restarted. In the case of an inverter, it might lockout for a brief period after a load-driven thermal overload.
Most boaters have experience with overloading the shore power source and popping a breaker. Resolving this can be as simple as walking outside to the shore power pedestal and resetting the breaker. But in some marinas the breaker cabinets are locked, and help from the marina staff is required to reset. If this happens in the middle of the night, the shore power might be off until morning when the marina staff becomes available. Generally avoiding power source over-draw is better for everyone on the boat, and all the powered equipment on board.
|Plugged into twin 6-amp, 240V connections along Sweden’s Gota Canal|
To prevent overloading the power source, rules must be put in place. For example, nobody can run a hairdryer when the kettle is on, the microwave can’t be used at the same time as the kettle and the dryer and HVAC or oven can’t be used together. The loads need to be managed to below that of the power source capacity, which means that those on the boat have to know both the power source maximum output and everyone using power on the boat has to communicate. It’s often the case that when a hair dryer is going to be turned on, the user will yell their intent so others on the boat don’t use high draw appliances.
We don’t like that model, and want Dirona to run as much like an apartment as possible, so we implemented load-shedding. This is an approach to automated load management that applies to the generator, inverter, shore power or any other power source. Load-shedding is an automated system that knows the power source limits and prioritizes all the electrical loads ensuring the most important get the available power. This avoids over-drawing the power source, and removes the burden of load management from those on the boat.
When our 240V system is overloaded, be it the generator or the inverter, our load-shedding system first temporarily disconnects the water heater, then the second charger, then the first charger, then the HVAC system. When the 120V system is overloaded, we shed the microwave and the diesel furnace, and finally the engine room fans (with limits on how long they can be off to avoid engine room temperature problems).
|The white ‘W/H’ and ‘Chg#2’ boxes at bottom center indicate that our control system has shed the water heater and charger 2 with the generator delivering its maximum of 42 amps (upper right).|
Load-shedding gives the feel of an unbounded power source and takes away the hassle of load management. The system just turns loads off and on to keep the draw within the capabilities of the power source. Because the HVAC system always cycles on and off and the shedding through load peaks are usually short term events (often only seconds to tens of seconds long), people on the boat often don’t notice when the system is at work preventing breaker overloads. It makes the boat feel like it has no electrical bound just as most houses and apartments feel. They all have bounds but, in the cases of houses and apartments in many parts of the world, the limit is so high that it’s rarely if ever encountered. Shedding gives the same feel when power sources are more constrained.
Generally what is required to implement a load-shedding system is to have a sensor to measure how much power is being drawn, a controller to switch loads off and on, and then a set of contractors, relays, or remotely controlled breakers that can be used to control the loads. Ours is a custom design, but many commercial systems are available.
Maretron N2kView, for example, supports shedding of up to ten different loads across four power sources. The Maretron approach is quite nice and, had this been available when we implemented our system, we probably would have used it instead.
Load shedding also is showing up in heavy volume in the RV market. Usually these systems are referred to as Energy Management Systems (EMS) and you can find a lot of alternatives searching for “EMS and load shedding”. That query, however, will find some very large commercial systems used for managing multi-megawatt loads on oil rigs. The search for “RV EMS and Load shedding” narrows things down to systems that work well at smaller scale.
To learn more about load-shedding, the article Energy Management in the RV Electrical Tutorial is a good place to start. As an example, they describe an Intellitec system which is fairly broadly used in the RV world.
|Maretron N2kView load-shedding configuration page|