Browse Categories
Mailing Lists

Frequently Asked Questions



How large a solar panel (solar module) is necessary to power my load?
The sizing of solar modules depends upon many factors. You begin by determining the power load you desire to support and how many days you need to be able to sustain without sun and how many hours of sun you have per day at your latitude (basically determines your battery bank size).

[Back To Top]

What do I need to power my backcountry applications?
During remote operation several issues arise. You need relatively rugged solar panels that are lightweight and compact.  You will also need a charge controller and a deep cycle battery large enough to power your devices. Usually most people pack an 8 amp-hour battery and use 10.4W panels.  Larger loads can be supported by a 20.8W or 32W panel or alternatively by two 20.8W or (for larger loads) 32W  panels in parallel and as many batteries as are needed.  Backpacking requires attention to weight. Batteries are your heaviest issue.  Having ability to monitor voltage is crucial.  Our charge controller kit (can be purchased assembled) includes a voltmeter.  Connectors are key.  You need connectors that prevent reversal of polarity and that are durable. We recommend the Anderson Powerpole and sell pairs of connectors for $6.50 a pair (two connectors and pins/terminals).

Some smaller loads (iPods, some PDAs and many cell phones) will charge directly from a 10.4 or 20.8W panel.  Small loads that won't run directly from a panel can be run using a DC-DC converter (8A for large loads, 3A for smaller loads). Using a DC-DC converter avoids the weight of a Power Center.

[Back To Top]

What is a blocking diode?
A blocking diode is found on some solar panels to prevent current from flowing backward through the solar cells when the solar panel is in the dark or shade. Blocking diodes however involve voltage drop (usually about one volt or so) that does little for you overall.  If you are using a charge controller you don't usually need blocking diodes. A charge controller is certainly a minimum requirement for a solar installation.

[Back To Top]

How are most solar modules constructed?
Most solar modules are constructed by encapsulation. The most common encapsulating material is EVA (a vinyl acetate similar to hot melt glue).  The module is constructed as a sandwich on a glass front with the cells embedded in the EVA between the glass and a teflon sheet behind (usually Tefzel).  Glass encapsulated solar modules work well but must be handled with reasonable caution to prevent breakage. The other common encapsulation technique involves UV-transparent epoxy encapsulation.  This is actually the best option for portable/survival applications as epoxy encapsulated solar modules are very rugged and are lighter weight. Our rugged folding solar panel uses epoxy-encapsulated solar cells.

[Back To Top]

Are there any installation standards?
Yes, for home installation you need to follow the National Electrical Code (NEC).  The NEC stipulates minimum installation standards for safety and reliability.  You're also well advised to install U-L rated inverters. If you're uncertain of what you're doing you probably will want help from an installer.

[Back To Top]

How are solar modules priced?
Pricing of solar modules depends upon their construction method, the efficiency of the cells used and their size. Generally larger panels cost less (usually between 5 and 6 dollars per watt).  Smaller panels cost more per watt related to construction cost of the laminate (often 13 to 15 dollars per watt). Cells with very high efficiency and small size/epoxy encapsulation cost as much as 18 dollars per watt and are intended for specialty applications (survival, expedition, etc.).

[Back To Top]

What kind of batteries do I need?
Your choices generally include flooded or sealed (AGM) batteries. Sealed batteries require limited maintenance and no watering.  This is not however achieved without some compromises.  Flooded batteries are more common in truly serious photovoltaic installations.  Flooded batteries require periodic water addition (hydrocaps help here, they have a catalyst that recombines the hydrogen and oxygen produced during charging to produce water, which returns to the battery cells).  Flooded batteries require periodic overcharging to equalize cell voltages (called equalization).  One never equalizes sealed batteries (which is harmful to them).

[Back To Top]

What is a battery meter?
Battery meters help monitor voltage and charge state of your batteries. They are truly helpful, however knowing your battery charge state/voltage relationship you can simply look at the nighttime equilibrium voltage and simply determine proportionately what the state of charge (50% between minimum and maximum charge voltage and you're 50% charged, etc.  It's linear in most cases).  Battery meters give you more precise and real-time current flow information (amp-hours in and out as well as instantaneous current flow).

[Back To Top]

What is grid-connection?
Grid connection permits your solar installation to feed power back to the grid.  This can be purely altruistic in intent (so-called "e;guerilla solar"e;) or it can be used to defray your energy costs.

We can quote pricing for any component of a solar power system and grid-connected systems. We offer SunWize grid-interconnected system kits.

[Back To Top]

How do I keep my batteries healthy?
First, start by a proper maintenance (PM) cycle.  Regularly monitor SG and electrolyte levels in flooded batteries.  Avoid cell contamination or electrolyte depletion.  Running batteries flat is also an excellent way to kill batteries.  Avoid freezing.  Use a desulfator (prevents electrode sulfation).

[Back To Top]

How do I keep myself safe around my batteries?
Keep a large quantity of sodium bicarbonate (you can buy large boxes of Arm and Hammer for this intent).  This can be used to neutralize sulfuric acid in case of a spill or worse in case you get it on yourself.  Follow code to prevent inadvertent short circuits and the catastrophic results.  Battery enclosures are also helpful with ventilation to remove hydrogen that builds up during charging.  Many inverters have outputs for ventilation fans that activate above the so-called "e;degassing voltage."e;  This is a voltage at which hydrogen starts to build up (usually above 12.8VDC or so).  There are several solutions to ventilation.  A pre-fabricated solution is the Power Vent fan.  This adapts to conventional PVC tubing.

[Back To Top]

What is a Desuflator?
A desulfator serves to limit sulfation of battery electrodes.  Batteries have a resonant frequency around 3Mhz and the desulfator pulses the battery at this resonant frequency and helps dislodge microscopic crystals on the battery electrodes (really works).

[Back To Top]

What kinds of charge controllers are there?
The two key types of charge controllers are the pulse width modulated (most common) and the diversion load controller.  The PWM controller basically constantly pulses the battery with current.  The width of the pulse is proportionate to the degree of charge depletion of the battery.  The less charged the battery is and the pulses end up very long (such that you are charging nearly continuously with brief interruptions).  The more charged the battery is and the pulses end up very brief with long intervals without current flow.  Diversion charge controllers divert charge when the battery reaches a desired charge set point.  This method is popular for instance where simple designs are desired and/or when controlling a wind turbine (where constant load is necessary to help control blade speed). Chargers generally seek to charge the battery at 1/20th of the amp-hour capacity (i.e. 5A charge for a 100A battery) and the charge rate is kept constant throughout the initial bulk of charging.

Maximum Power Point Tracking (MPPT) charge controllers convert solar panel open-circuit voltage (20 to 22V for a 12 volt system) to the battery voltage (13.8VDC, matches the point on the solar panel voltage-current curve where power is maximum). Using a MPPT controller you can economize somewhat on the number of solar panels one has to purchase for your system.

[Back To Top]

What is temperature compensation?
Battery float and bulk charge voltages are proportionately increased as temperature drops.  After all the battery is basically a re-dox chemical reaction and as temperature drops the equilibrium voltage changes requiring higher voltage.  Temperature compensated charge controllers increase voltages continuously as temperature drops. Most lead-acid batteries have a temperature coefficient of about -4mV/degree Celsius per 2V cell (away from 70F)  which means 24 mV/degree Celsius for a 12V battery.  Drop temperature by 20 degrees centigrade and you will need to increase the float charge point by 0.5V or so.

[Back To Top]