Snorri's Solar System

As the cost of diesel fuel rose to over $6 on the islands off New England’s southern coast, the thoughts of a green cruising boat using solar power becomes more appealing.  Although the technology to provide motive power from the sun is limited and not yet cost effective, you can easily provide all of your boat’s electrical needs from the sun while cruising.

My wife and I recently downsized from a Nordic Tug 37 to a Nordic Tug 32.  The pre-owned Nordic Tug 32 that we purchased had most of the cruising accommodations we required including air conditioning, diesel forced-air heat, propane stove/oven/broiler, but it did not have a generator or an inverter. We viewed this as an opportunity to design and install a solar system to meet our tug’s DC (12 volt) and AC (120 volt) needs.

Snorri's four solar panels are almost invisible on the pilothouse roof.

Since Snorri (32-225) spends all summer either on her mooring in Stage Harbor at Cape Cod’s elbow or at anchor in a scenic cove while cruising, we never rely on shore power between spring commissioning and winter hauling.  We also like to stay put in each cruising destination for several days before moving on.  Our previous Nordic Tug (Snorri, 37-049) required the generator to be run daily to charge the battery banks when moored for more than overnight.  Several of our favorite harbors have significant eel grass problems, so cleaning the seawater strainer every time the generator was operated was a necessity.  Our design goals were to eliminate the hassle with a generator, keep the refrigerator/freezer running even when we were away from the boat, not burn any fossil fuel, and implement it for less cost than installing a generator.

There were several questions that needed to be answered in order to determine whether a solar system would work for us:  How much power did we use on a typical day?  How much power could be generated by a single solar array in our cruising area?  How many solar arrays would it take to generate our power needs?  Was there room on the pilothouse roof for the appropriate number of solar arrays?  How much battery capacity did we need to allow for cloudy days?

Power Usage

The first step was to determine our power usage.  We made a table listing all of the 12-volt and 120-volt devices we used on a typical day (see Table 1). We looked up the amperage or wattage for each electronic device, converting amps to watts by multiplying by the voltage when necessary.  This information can be found on manufacturer’s websites or on the device itself.

In the third column, we entered the number of devices in service, like the typical number of lights we have turned on at night. AC devices powered by an inverter need to be multiplied by 1.15 to adjust for the energy loss in the inverter circuit.  This factor is entered in column four for the 120-volt devices and 1 is entered for the 12-volt devices.  The fifth column contains the number of hours we used each device per day. For appliances that we didn’t use every day, such as the television or stereo, we prorated the weekly usage.

Table 1. Daily Utilization

Although considered to be an efficient 12-volt refrigerator, our Nova Koolside-by-side refrigerator/freezer consumes two-thirds of our power requirements.  The usage in the table came from Nova Kool whose tests estimate the power consumption of the refrigerator/freezer at 43.2 amp-hours per day.  This power was necessary to maintain the refrigerator’s temperature at 40° with an ambient temperature of 75°.  Clearly on warmer days, the refrigerator will draw more. 

Not included in the table is the hot water heater.  On a cruising day, the engine coolant circulates through the hot water heater providing adequate hot water for showers and dish washing.  But that heat is usually lost usually over night.  Adding an insulating blanket around the water heater would help reduce the heat loss.  But we would need to run the water heater for 30 minutes per day if we wanted hot water.  Since the water heater draws 1500 watts AC, it will double our daily power use (174 amp-hours/day) on the days it used.

Power Generating Capacity

Sources on the Internet provide tools to calculate the average daily output for a solar array based on your latitude ().  For example, at 41.669°, the latitude of Chatham, Massachusetts, a 130-watt solar panel will generate 67.7 amp-hours per day in mid-June (see Table 2). 

Government databases, also available on the Internet, provide the kilowatt-hours of solar energy absorbed per square meter per day for the past thirty years.  These figures are based on actual observations regionally throughout the U.S.   If you do the math, a 130-watt solar panel on Cape Cod generates an average of 52 amp-hours in June according to the observed data. 

Table 2: Daily Amp Hours Output for a Single 130 Watt Panel in Summer
(Predicted at 41.67° N Latitude, Observed on Cape Cod)

Solar Panels

Solar panels come in all sizes and shapes but the typical panel used to charge 12-volt batteries is made up of 36 photovoltaic (PV) cells sandwiched between glass and enclosed in an aluminum frame.  Multiple solar panels are connected in series to create a solar array.  Recent advances in cell processing technology have increased the wattage per square foot of a solar panel.  Two of the leading companies, Kyocera and Sunsei manufacture 130-watt panels that measure approximately 2’ by 5’ and weight between 25 and 30 pounds.  Sunsei solar panels are available from West Marine for $1,259.  We purchased our Kyocera panels from a supplier of home energy products for $629 each. 

The pilothouse roof on our Nordic Tug 32 easily accommodates two 130-watt panels.  Since there was room, we installed two 130-watt panels and two 65-watt panels to provide us with  .  In addition to the solar panels, a solar charging system needs a solar charge controller to regulate the charge and protect the batteries.  A battery temperature sensor connecting to the solar charge controller to the battery bank adjusts the charge settings providing additional regulation and protection.  We installed a Xantrex 60 amp pulse width modulated charge controller with battery temperature sensor and a remote digital display that was located in the pilothouse.  The total cost of the panels and charge controller was $2,400.

Remaining Components

Snorri was originally outfitted with four 6-volt golf cart flooded (wet) batteries.  We decided to replace the five-year old batteries with deep-cycle AGM (Absorbed Glass Mat) batteries for multiple reasons: AGM batteries are sealed and cannot leak acid or fumes, they require no maintenance, they are immune from freezing damage, they can sit in storage and only lose from 1% to 3% charge per month, and can be discharged to 80% of capacity. AGM batteries have one disadvantage—they cost two to three times more than flooded batteries.  We chose to install six Life Line AGM 6-volt batteries ($2,000).  Each pair is connected in series and the pairs are connected in parallel to produce 12-volts with a total capacity of 880 amp-hours.  This functionally gives us 700 amp-hours of stored power for cloudy days.

Completing the system is a Xantrex 2000-watt pure sine wave inverter (Prosine 2.0 ) and an 150-amp alternator (Powerline  23-88) with a Balmar voltage regulator (Maxicharge 612).  There are opinions about the need for a pure sine wave inverter on a cruising boat.  If you plan to run appliances that demand a clean pure source of AC power, then a pure sine wave inverter would be your choice.  According to industry pundits, a television and computer will run without buzzing or humming, AC motors will start more easily, and appliance such as microwaves will run more efficiently. 

The high-output alternator, replacing the original equipment alternator on Snorri’s Cummins 220 engine, combined with the Balmar voltage regulator not only provides high amperage but also battery specific smart charging voltage control to achieve a full charge of the AGM batteries quickly and safely.  (See Diagram 1 for a schematic of the completed solar system.)

Installation of the solar panels, solar charge controller, AGM battery bank, inverter, battery temperature sensors, remote digital displays for the inverter and solar charge controller, and the necessary cable connections required 32 hours of labor by an experienced marine systems integrator.  An unanticipated cost was that of the materials to build the cables.  Thirty feet of black and red No. 4 AWG twisted copper cable was needed to connect the solar panels on the pilothouse roof to the charge controller and batteries in the engine room.  Fourteen feet of black and red No. 0000 (4/0) AWG cable was needed to make up connections between the batteries in the bank and between the bank and the inverter.  Due to the high cost of copper, this expense was around $800. 

At the time of this writing, Snorri has operated exclusively on solar power for over ninety consecutive days.  The refrigerator has been running the entire time (even when we were away from the boat) and its temperature has ranged between 34° and 39° F.  Solar has powered hot water for showers and all of our other electrical needs without a single problem.  On a typical New England summer day, the battery bank is fully charged before noon and has never been discharged more than 50 percent.  Despite what many people think, solar panels even generate power on cloudy days but we have observed very little generated power in fog.  Nevertheless, we couldn’t be more pleased with the results.

Diagram 1
click to enlarge

List of Components

Click for Photos of Completed System