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Understanding Your Boat's Electrical Power Needs - Part II

August 24, 2017

Of course, evaluating your power consumption is only half the job when it comes to understanding your boat’s electrical capabilities and limitations. You have to know the supply side—how much power you can generate in your AC and DC systems—to get the full picture.
Let’s look at the DC side first, which means taking stock of your battery bank. The vast majority of marine battery banks consist of flooded-cell, AGM or Gel Cell lead-acid batteries. Each battery has slightly different charge and discharge characteristics, which must be accounted for, especially when charging. Batteries for powering DC loads (as opposed to starting batteries) should be of deep cycle batteries, optimized to survive many charge-and-discharge cycles at a deeper discharge level than a typical engine starting battery.
Deep cycle batteries carry an amp-hour rating. For example, a rating of 200 Ah means that battery could theoretically provide 200 amps for one hour or 1 amp for 200 hours before being completely discharged. As a rule of thumb, lead acid batteries should never be discharged below 50 percent, which means, in this example, the battery would provide 100 amp-hours. Deeper discharge levels shorten battery life considerably.
If you conduct an energy audit, you should have a strong idea about your consumption in amp-hours. The next step is to find out how much battery capacity—also in amp hours—you have aboard, accounting for the 50-percent discharge rule. Look at your batteries, or the manuals, and do the math.
On the M/V Slo-Poke, our example from the energy audit, the consumption is 248.6Ah per day. So, given a 50 percent discharge, that equates to a battery bank with about 500 Ah of battery capacity. This would be sufficient to run the boat for a day without additional energy to recharge the batteries using shore power, solar or wind power, running the engine (to drive the alternator) or the generator. A typical Group 31 marine deep cycle battery is rated at 105 Ah. That means that a 500-Ah battery bank aboard the boat would require five such batteries.

If you are depending strictly on a shore connection for AC power, you are largely finished with this process. However, very few boaters these days limit themselves thusly. All that onboard AC power has to come from somewhere, and the two most common options are from a generator or an inverter.
Inverters are fine for variable demands up to about 3000 watts. Generators are better suited for larger continuous loads, such as air conditioning, cooking and refrigeration. A general rule of thumb is if your daily consumption in watt-hours exceeds 2000, that’s a load worthy of a generator. For our purposes here, the goal is to determine how adequately your generator or inverter (or the one you plan to add) will cover your power needs.
Let’s start with the generator. Look at your power audit, determine which of your AC devices you will be using at the same time and arrive at a number, in watt-hours, for your overall needs.  
The generator should be sized so that your usage is about 50 to 75 percent of the generator’s rated output. If it falls below 50 percent, then the engine driving the generator isn’t operating at a fuel-efficient power level. If it falls above 75 percent, you risk not having enough excess capacity to allow for the start-up surge associated with certain types of devices (for example, air conditioning may require more power to start than it does to run). Operating below 75 percent of capacity will also allow you to add additional AC devices over time.
For those running diesel generators, these percentages become particularly important. Diesels must be run under load, in order to prevent carbon deposits from building up in the engine. So the load on a diesel generator must be adequate—without overloading it. Gas generators have no such requirement.  
When evaluating an inverter, the key concept to understand is that the power it provides comes from the boat’s DC battery bank. And that means the power it needs to run equipment must be reflected on the DC side of your energy audit. To do this, divide the watt-hour rating by 10 to determine the amp-hours  required to provide this power from the DC system and these amounts to the DC portion of the audit sheet. This factor accounts for the fact that inverters aren’t 100 percent efficient and consume power themselves.
Inverters normally have two ratings: maximum continuous watts and peak or surge watts. That surge or peak rating takes into account the startup loads on the device being powered. In most cases, it is about double the continuous watts but can run as high as 3 to 7 times continuous watts for induction motors such as those used in air conditioners of refrigerators. Like a generator, an inverter should not be run at maximum capacity. For example, if the anticipated load is 900 watts, then a 1500-watt inverter is probably needed. As a general rule, allow 10% to 20% over your estimated load. This provides a cushion for power surges, and for future growth.
On our sample audit, you can see that M/V Slo-Poke uses an inverter to power a limited number of AC devices, which add up to an additional 215 Ah required of the DC battery bank. Yet the installed inverter shows 60 Ah—clearly inadequate. The crew of M/V Slo-Poke would need to reduce inverter use or install a larger inverter and battery bank.

The last part of understanding power generation is something we all already know: batteries can be recharged. If you have systems to recharge the batteries on board—and almost all of us do—you’ll need to know how fast they can perform their task, in order to truly understand your capabilities.
You can rely on shore power. Running the main engine several hours a day to charge the batteries will also work, though it is an inefficient charging mechanism. If you anchor out or travel far from shore facilities, you should have other options—and there are many.
Solar panels or wind generators are common options for cruising sailboats, as are inboard generators or add-on, high-capacity alternators to provide charging power. All have plusses and minuses. The key is to understand how much energy these devices provide and under what conditions. For example, the amount of power provided by a solar array will depend on the size of the array, how efficient it is, how well it is oriented toward the sun and the amount of sunlight available. The panel manufacturers can provide power ratings. The same goes for wind generators; manufacturers can provide power output curves for different given wind speeds.
The best battery bank in the world won’t last long unless there is sufficient power to recharge it. There are two general scenarios; where AC power is available, as in shore power or a generator and where the sole charging source is the alternator(s) on the main propulsion engine.
In the case of AC power, there should be sufficient charging current available to keep up with the average DC load (from your audit) plus about 5% to 10% over to properly charge the batteries, Size the battery charger to provide this amount of current.
In the case where charging current is provided by the main propulsion engine alternator(s), the charging system will need to keep up with the average DC load as well as provide sufficient additional current to charge the batteries in the time frame available. This can be estimated by taking the number of amp-hours that need to be put back in the batteries and dividing by the available charging time. Then add in the average DC load to arrive at a required charging  capacity. Remember that alternator ratings are very optimistic, engine RPM and even ambient heat reduce the charging capacity.
Of course, with many different sources of energy coming into the system, juggling all the inputs can be confusing. Many boaters choose to install a battery monitor as part of their power system to help keep it all straight. A monitor, like those made by Xantrex or Magum Energy,  will track the charging amps coming into a battery bank from many sources (they can sense whether you are hooked up to shore power or have cranked up the generator). They can track the amp-hours used, providing sort of a “gas gauge” for your battery bank. Sophisticated models can control multiple banks and provide an estimate of the power remaining. Combined with a charger, they can regulate your alternator to deliver optimized charging current.
This is all very useful—but it is not a substitute for understanding your boat’s electrical capabilities. Running out of power away from the dock is at least embarrassing and potentially dangerous. Performing an energy audit


will give you the numbers you need to make better decisions, whether you are adding a piece of gear, replacing your existing equipment or simply want to cruise more efficiently and with more confidence. That’s something we could all use more of—and it’s something you just cannot buy.


Sidebar: Selecting an Inverter
As mentioned before, inverters are great for supplying 120-volt power for variable loads, up to about 2000-3000 watts. Inverters are easy to install, quiet and need no maintenance.
The first step in selecting an inverter is to determine whether you need a full sine wave or modified sine wave unit. The wave shape of each is shown on the diagram. Modified sine wave inverters are less expensive than full sine wave units. However, some devices don’t run well or even burn out on modified sine wave power.
AC devices don’t show whether they can run on modified sine wave power on their nameplates. It is up to the boat owner to discover whether they will or not, Boating user or owner groups are a great place to research this issue. Or ask your fellow boaters. You will soon find out who had bad experiences with what devices on a modified sine wave inverter. Some electronics seem to be more prone to problems than other devices.
The AC section of your worksheet will give the wattages of the devices you intend on operating off an inverter. Your inverter doesn’t need to be large enough to run all the devices at once. It’s up to you to determine what the maximum wattage you will be running at any one time and choose an inverter accordingly.
Don’t try to run the inverter at its maximum capacity. If your anticipated load is 900 watts, move up to a 1500 watt inverter. This will give you some cushion for both future growth and to handle the power surges encountered when some AC devices start up.
Most good sized inverters are designed to be fixed mounted, firmly bolted down. They should be as close to the DC battery bank as is practical to minimize electrical losses from the wiring. Allow adequate airflow around the inverter to provide cooling air, these units do produce heat. Inverters are not ignition protected so they can’t be installed in a gasoline engine compartment.
Smaller, portable inverters can be used to power electronic devices such as laptops or cell phone chargers. Many of these smaller units simply plug into a 12-volt accessory socket.
Larger inverters are often combined with battery chargers into inverter/charger units. These can be quite sophisticated, including battery monitors and battery temperature sensors to maximize charging rates. Newer models also include NEMA 2000 compatibility. The inverter, attached to your boat’s NEMA 2000 system, can even start the genset if the battery drops below a given point.
Always remember that this inverter power doesn’t come free, you need the battery power to back them up.


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