If you are thinking about solar panels for the first time the following information may help you. The details of putting together any electrical systems can get pretty complicated. To keep life simple when it comes to solar power you need to answer two basic questions - How much power do I need? and how do I set up a system to capture what I need?
There are different ways to configure solar power systems, however, on a boat what is called a standalone, or battery backup system is by far the most common. These systems allow you to capture energy from the sun, store it in your vessels' battery bank, and use it as necessary.
The battery bank is the center of every standalone system. The batteries act as a storage tank, holding a certain amount of electrons until they are needed to run an electrical device. The size of your reservoir will determine how many electrons you can store. At the same time, the rate at which you use electrons will also determine how long the batteries will last before needing to be recharged. For optimal battery life and reasonable cost most systems are designed to hold from 3 to 5 days worth of electrons depending on your power needs. If your system stores less than 3 days of power your battery will be draining and refilling electrons, or cycling, more often than it is designed to handle. If you want to store more than 5 days of power the cost of such a battery bank can get pretty high, as can the space requirements it will need.
Batteries don't care where the electrons they store come from, as long as the incoming energy roughly matches the outgoing energy. Once a battery gets about 80% full of electrons you must have a system in place to prevent the battery from becoming too full or severe damage can occur.
Charge controllers are designed to keep your batteries from overcharging. Controllers keep an eye on how many electrons have filled the reservoir, otherwise known as the battery voltage, and if the voltage gets too high the controller will either open the charging circuit to prevent any more electrons from flowing in, or dump excess electrons into a heater element.
In addition to a controller an effective system needs a monitor to let you know how full or empty the batteries are. The simplest monitors have a series of colored lights that display the battery voltage, while more complex monitors can show these details along with the current rate that electrons are flowing into and out of your system. Monitors are often integrated into controllers, although they are also sold as stand alone units. With space being at a premium on most boats, integrated units tend to be quite popular, while separate monitors are usually found on larger vessels, or those with complex and/or sensitive systems.
You can run a system without a monitor, but it is like driving a car without any gauges, not a very good idea.
Batteries deliver direct current, or DC, when you draw power from them. Most electrical devices on boats are designed to run on DC power. Many of the modern conveniences found onboard these days, like microwaves and TV's, however, are engineered to run on alternating current, or AC. Unlike DC, AC cannot be stored in a battery, AC has to produced as needed. To meet this demand you need a device known as an inverter. Inverters convert DC from the battery into AC and send it to those devices needing AC power. Numerous inverter sizes exist, but you want one that will deliver enough power to start and run all the AC devices you might turn on at the same time, but nothing more.
Common sense dictates that any time a wire is connected to a power source you should have a fuse or circuit breaker in place just in case something goes wrong and too much power flows through your wires. The analogy of a bursting water hose is an apt comparison, with the big difference being that bursting wires often lead to fires and explosions. The fuses or circuit breakers you choose for your vessel should be rated for DC use, and appropriately sized for the wire's they are protecting. Since it is designed to handle bigger loads, DC rated equipment is usually much more stout and often more expensive than AC gear, but this is not an area where you want to get cheap. Fires on board are a nightmare you never want to encounter.
Before determining the specific components your system will need you must decide the system's voltage. Renewable energy systems can be built around battery banks running at 12, 24, or 48 volts. The higher your energy needs, the higher your system voltage should be. Like most things on boats, there is no one system that works for everyone, but here are some general guidelines. Systems using up to 2,000 watt-hours per day (we'll get to these calculations in a bit) are usually fine at 12 volts. For 2,000 to 7,000 watt-hours per day required, most systems will work better at 24 volts. If you are drawing more than 7,000 watt-hours per day shoot for a 48 volt system.
Now for the fun part, determining your power requirements.
We get questions all the time that go something like this, I have a 30 foot boat, what size system do I need?
Since everyone's power needs are different, there is no simple answer to this question, but getting to the answer is a straight forward process that a little time, some sixth grade math and the following steps can help you solve.
To calculate your power requirements you need to list every electrical device you use, how much wattage it uses, how many hours per day it runs, and how many days per week.
Electrical devices usually disclose their power requirements on the nameplate, in the owners manual, or, of course, online. Some devices may only tell you amperage and voltage, but you need to determine wattage. Wattage = amperage x voltage. For example, a gizmo's nameplate says "3.0A 120V 60Hz". This gizmo is rated for a maximum of 3.0 amps at 120 volts/60 cycles per second. 3.0 amps times 120 volts equals 360 watts.
A summary of the steps required to build your system is shown below. Feel free to print this page.
For every AC electronic device list the following
(a) Device Wattage Requirements __________
(b) Hours Used/Day __________
(c) Days Used/Week __________
Multiply: (a) x (b) x (c) = (d) __________
Divide: (d) ÷ 7 = (e) __________
(e) = Average Watt-Hours/Day
Add: (e) for each AC device = (f)
(f) = Total AC Watt-Hours/Day ___________
Multiply: (f) x 1.1 = (g) __________
(g) = Total Corrected DC Watt-Hours/Day
This step accounts for the inefficiency of most inverters. The calculation gives the actual DC Watt-Hours/Day that the AC devices will draw from the batteries.
Repeat Step 1 for all DC devices to determine Total DC Watt-Hours/Day (h) __________
DC device power requirements are totalled without any corrections for inefficiency.
Add: Total Corrected DC Watt-Hours/Day (g) + Total DC Watt-Hours/Day (h) = Total System DC Watt-Hours/Day (i) __________
At this point you can email or fax your Total System DC Watt-Hours/Day to us along with the typical area where you will be operating and we will get back to you with our component recommendations. If you prefer to see the results for yourself, keep going.
Divide: (i) ÷ System Nominal Voltage (12, 24, or 48) = Total DC Amp-Hours/Day (j) __________
Multiply: (j) x 1.2 = Total Daily Amp-Hour Requirement (k) __________
This calculation accounts for losses in wiring and batteries.
Divide: (k) ÷ Estimated Hours of Sun/Day (l) __________ = Total PhotoVoltaic Array Current in Amps (m) __________
This calculation can be tricky since the Estimated Hours of Sun/Day is subject to change. There are detailed Solar Insolation Maps all over the Internet that can help with this number
Decide what PV Module (Solar Panel) you want to use for your system. You will probably want to try the following calculations with several models depending on size and $ constraints.
Find the Peak Power Current in Amps (n) __________ for the PV Module you want for your system.
Divide: Total PhotoVoltaic Array Current in Amps (m) __________ ÷ Peak Power Current in Amps (n) = Number of PV Modules Required in Parallel (o) __________
You will probably end up with a fractional number of modules required from this calculation. Since module don't come in fractional sizes, you will need to either round up or round down to a whole number. As a rule of thumb, it is usually a good idea to round up if your fractional requirement is .30 or higher.
If you have decided on a 12 volt system you are all done calculating the modules you need. If you need a higher voltage system keep going.
Enter the System Nominal Voltage you are using from Step 6 above. This is usually 12 or 24.
Find the Module Nominal Voltage for the panels you want to use. This is usually 12 unless you are using a special order module.
Divide: System Nominal Voltage ÷ Module Nominal Voltage = Number of Modules Required to Be Wired In Series (p)__________
Multiply: Number of Modules Required to Be Wired In Series (p)__________ x Number of PV Modules Required in Parallel (o) __________ = Total Number of PV Modules to Satisfy System Requirements(q) __________
If, after all this work the number of modules you need is too high or too expensive your options are simple. Reduce your power consumption, add a secondary charging system such as a wind generator, etc... or fire up the engine/generator.