Have a curiosity about solar power that you’ve never pursued? Now’s your chance to use this fairly simple do it yourself project as a stepping stone to even greater applications while learning the basic calculations and considerations required!

Cliff Johnson

Materials

• solar panel
• hardware for panel mounting
• outdoor wire (cable)
• lightning arrestor
• grounding wire/stake
• charge controller
• batteries
• disconnect, fuse, switchgear
• wall switch and related wiring
• cabling for inverter
• inverter
• AC wiring from inverter to light
• light fixture and fluorescent bulb

* Note - Instructions will be provided to help you select the appropriate capacity of these components, assess the duty cycle of light, and choose an ideal location for solar-powered light.

Overview
What follows is a brief overview on installing a solar-powered fluorescent light in your home or garage. Each topic touched on is the subject of many articles unto itself, so this should be viewed as a starting point. The project will likely require further research into those aspects where you feel uncertain as to how to proceed.

This project requires basic electrical and wiring skills or assistance from a qualified electrician. It involves the following steps: mounting a solar panel on your roof, grounding it, running cabling from the panel inside the home or garage, connecting it to a charge controller, wiring the charge controller to a battery, installing fusing and switchgear, wiring in an inverter, then wiring a light fixture to the inverter.

Selecting a Battery
The first thing you need to decide is how long you want the light to be on each day - this will determine the size of your solar panel(s), battery or battery bank and charge controller. The longer you want the light to operate each day, the larger the capacities you will require, the more complex your project will be and the more expensive it will be.

For simplicity, let’s assume you want to install a light in a closet that may be on about half an hour each day at most. If you use a 13W spiral compact fluorescent bulb, it will use 13W x 0.5Hrs of power, or 6.5WHrs (Watt-hours). Your inverter will, let’s say, have an efficiency of around 80% (assuming you choose a relatively small inverter - recommended when powering a small load to improve efficiency). This means your batteries will need to supply 6.5WHrs / 80% = 8.125WHrs. At 12V, this means 8.125WHrs / 12V = 0.677AHrs (Amp-hours) of battery power.

So, 0.677 AHrs is the amount of energy that will be used each day from the battery that will need to be replaced by the solar panels. Now, sadly, not every day is gloriously sunny - in fact, not even every month can be counted on to be sunny. Last October, where I live, we received very little direct sunlight the entire month, and it became quite apparent that my solar yard light was woefully lacking in reserve solar panel and battery capacity.

However, to plan for an entire month without sunlight is a bit much - we’ll plan for one week of solid uninterrupted greyness. That means our battery’s capacity must provide 0.677 AHrs for 7 days (0.677 AHrs/day x 7 days) which is 4.74 AHrs. However, since batteries only supply their rated capacities when they are new and in perfect condition, it is always wise to choose a battery that is of greater capacity than what is required to accommodate gradual degradation. I would recommend a deep cycle battery that has around 8 to 10 AHr capacity. This shouldn’t be overly expensive - check for these at battery re-builders that deal in RV batteries.

Choosing Your Solar Module
Now to choose the solar module (panel) we will use to recharge your battery. Where you live, and the number of bright sunlight hours you can count on each week in December will guide your panel choice. The reason you want to consider December is that in the Northern Hemisphere (June for the Southern), this is typically (but not always) the month which provides the least amount of sunlight. Certainly, this is the month that provides the shortest periods of sunlight and the lowest angle on the horizon. Solar panel output ratings assume bright and direct sunlight conditions, so you need to derate this output for winter conditions.

In our example, after a period of one week of virtually no charging, the battery needs 4.74 AHrs replaced. Our panel would need to provide 4.74 Amps for 1 hour, or more realistically, 0.474 Amps for 10 hours. Now, sadly, yet again we run into further inefficiencies - a battery only charges at about 60% efficiency. A solar panel’s wattage is usually based on its output current multiplied by its peak output voltage, which is usually 16.5V. Given that when fully charged our battery is at around 13.8V or so, this means that we have to drop our solar charge voltage via the charge controller and waste some more power in the process.

So, based on a sunlight table (eg. http://aa.usno.navy.mil/data/docs/RS_OneDay.php), find out how many hours of sunlight (from sunrise to sunset) you get on the first day of winter, multiply that by 60% for battery charging inefficiency, by 80% for charge controller power dissipation, and by about 80% to account for the reduced strength of the sunlight received in winter and you will have the number of effective hours at full sunlight you can expect.

In our example, let’s say you live somewhere that gets 8 hours of sunlight on the first day of winter. 8 Hrs. x 60% x 80% x 80% = 3.07 Hrs. My 10W panel puts out 0.61Amps at 16.3V at its rated output, so in those 3.07 Hrs. of sunlight you would replace 3.07 Hrs. x 0.61 Amps = 1.87 AHrs of energy per day of charging (not counting the energy used that day). Based on this rate, it would take just over 2.5 days worth of winter sunlight to replace the week of cloudy skies, which is probably manageable, especially since there may be a tiny amount of charging on cloudy days. However, you can decide what criteria you want to use based on your needs and your typical climactic conditions.

Other Equipment
Given that this project uses a relatively low power panel, a small charge controller should work fine. These are available at some retail outlets like Wal-Mart and Canadian Tire, and aren’t overly expensive. As for cabling, I would recommend 14 to 16 Gauge wire be used from the solar panel to the charge controller and then to the battery, as this provides fairly low voltage drop and good current handling. However, in wiring the battery to the inverter, you will want to use at least 14 Gauge wire (ie. Gauge number no larger than 14), since inverters use higher current than what will be in the charge circuit.

Assuming that this inverter will only be powering a single fluorescent light, I recommend a small one - around 100 Watts continuous rated output. Smaller inverters tend to be more efficient if left running without a load attached, and minimizing parasitic current draw is a good thing for solar applications. Also, I recommend switching the inverter on the DC side, so that it draws no power when the light is off. Switching it on the AC side involves lower current, but also means that it will continuously drain a small amount of power from your battery. Some inverters can tolerate starting up with a load attached, although it may take a little longer for it to start under those conditions. If your inverter cannot do this, you will need to leave it running continuously and switch the AC side.

As for fusing, the inverter instructions should specify its maximum current draw - size the fuse about 20% higher than that. Install the required size of fuse inline from the battery to the inverter. However, if your inverter has fairly high capacity (say, 450W) and you use 14 gauge wire, the wire current limit is what you will fuse for rather than the inverter capacity, since the inverter could draw more current than what the wire could handle. NEVER exceed the rated current capacity of your wire - that is a great way to burn down your house, and since you did the installation, your insurance is not likely to cover it.

Mounting
When installing the solar panel on the roof, remember that a solar panel is like a sail - the bigger it is, the more wind it will catch on a windy day. The points I’m trying to make are, bolt that thing in securely, use solid materials for building your brackets, and choose appropriately sized screws and bolts for fastening it all together. You don’t want it ripping off and taking a piece of your roof with it on a really windy day. If you live in a place that experiences hurricane-force winds, consult with a carpenter or roofing contractor to ensure your mounting meets regional requirements and laws.

I recommend using angle aluminum for the solar panel mounting bracket - its easy to cut and fairly strong and rigid if sized appropriately. Face the panel South if you live in the Northern hemisphere, and North if you live in the Southern hemisphere. If you live on the equator, you’ve got it easy - just lie it flat facing straight up. The angle to the horizontal axis (tilt angle) at which you mount your panel is very important. A rule of thumb is to add 10 degrees to your latitude, but I suggest looking up a table that specifies the optimal angle for your location in winter
http://www.macslab.com/optsolar.html or http://www.windsun.com/PDF/pvangles.pdf (page 2) or http://www.theenergygrid.com/grid/articles/paneltilt.html. If your panel is optimally installed for winter, it will likely receive more than enough light for summer. However, I do caution that you not install it where anything will shade it during daylight hours, winter or summer. Shade = lost output, and a little shade can trim your output immensely, so avoid shade at all costs.

Grounding
Another strong recommendation is to ground your panel properly, and depending on your local code requirements, there may be no choice in the matter. Surge suppressors are available that mount at the panel and connect to both the positive and negative panel outputs. They have gas-discharge tubes that will discharge any large transient voltages to ground. If your house gets struck by lightning, the surge suppressor will direct the current from the panel wiring to your outdoor grounding stake rather than inside your house. Also, this is another handy thing about aluminum for the bracket - it gives you plenty of choice as to where you want to attach your grounding wire. I suggest you read up a bit on proper grounding so that you use an appropriate gauge of wire and install your ground stake deep enough.

Wiring & Wall Penetration
In making your wiring connections, you have three choices: you can use crimp terminals for wires that attach to terminals, get crimp joiners for attaching wires to other wires, or solder the wires together and use heat shrink to cover the joint. The latter is my preferred method, but not everyone chooses to solder. Whatever connection method you use, wrap the joint (even crimp joiners) with black electrical tape, or rubber tape which is even

better. Moisture from the elements corrodes electrical connections and causes them to fail surprisingly quickly, so ensure that precautions are taken to keep all connections away from moisture.
After installing and wiring up your panel (make note of which conductor in your cable is positive and which is negative) route the cable(s) to the edge of your roof and along the side of your house to a convenient point of entry into the house to where your batteries are, and to a convenient place for your grounding stake. Try to keep the cable length to a minimum to avoid line loss. Wire dissipates a little bit of energy per unit length - more wire = more loss.

As an alternative, you could run the cabling (not the ground wire though) through your roof, down through your attic or even mount your battery in the attic if that works. Just be sure to seal any holes in your roof really well using a proper roofing sealant (this includes sealing around the bolts used to mount the panel). Also, seal the point of entry of the cables into the house if not mounting the batteries in the attic. Expanding foam can come in handy here, just be careful - that stuff can get out of hand (it expands slowly to three times the volume that you initially apply).

Use either a terminal strip or a junction box for connections to your charge controller, or if you are lucky your charge controller will have screw terminals mounted on it to make your connections. Sadly, most cheaper models don’t, so you’ll probably have to use a terminal strip. Fortunately, a lot of this stuff is available at your local Home Depot or similar - no need to go to exotic and expensive specialty stores for sealants, rubber tape or terminal strips.

Depending on the type of fuse-holder you choose, it is probably a good idea to mount it in the same enclosure as the rest of your electrical connections, just be sure that all exposed wire joints are properly taped (heat shrink is better) and contained so that wires can’t rub against each other and possibly short. Short circuits are bad - if you have a short on the wiring from the battery before the fuse, it could easily lead to a house fire. Another thing t consider at this point is venting your battery. If you have a small battery mounted somewhere where you have a large space or good ventilation, you probably don’t need to vent to the outdoors. However, large battery banks do need outdoor venting to prevent explosive hydrogen gas (formed when charging batteries) from building up.

Install your wall switch and run the wiring. If you are switching DC power before the inverter, remember to use heavy wire gauge and a switch that can handle the current. AC wiring is fairly trivial - use household 14 gage electrical wire, and a standard household switch - both of which are cheap and readily available. If your inverter is small, you can probably get away with using regular 14 gage electrical wire and wall switch (that’s what I used) but be advised that the switch may wear out a little quicker if arcing is occurring when switching the inverter on and off.

If you are handy with electronics, I strongly recommend using DC power to switch on the inverter, then use a time-delay relay to turn on the connection to the AC load about one second after the inverter starts - this should give the inverter time to start developing smooth AC power before exposure to the load (as I said, some inverters don’t like having a load connected at startup). This way you don’t have any parasitic current draw, however, a little expertise is required to design and build the delay circuit. Commercial time-delay relays are available, but from what I’ve seen, they are obscenely expensive.

Now all that remains is to run the wiring from the inverter to the light fixture. This follows the normal household light fixture installation procedure with one exception: when attaching to the inverter, if you are using one with a plug output, you will need to connect a standard wall plug to the inverter end of the wiring. Use wire staples to hold the wire in place and observe electrical codes for this. When in doubt, consult an electrician in your area. Be sure to check with local electrical codes and regulations to ensure compliance, and if possible get a qualified electrician to inspect the installation and sign off on it so that if anything does go wrong, you can prove that your system was installed properly. That’s it - your solar-powered fluorescent light installation is done!

© Cliff Johnson 2007