Volt-Amps vs Watts
You often see inverters rated in volt-amps (VA or va) instead of watts. For a perfect resistance, such as a heating element, watts and VA are essentially the same. However, many appliances, especially motors, are not a "perfect" load. Thus, the watts actually used may be less than the VA - but the inverter sees the VA. The motor or appliance may be using only 100 watts, but the inverter sees an "apparent" power of 150 VA.
The difference is due to the fact that in a perfect resistance, the voltage and current are perfectly in sync, but in a load with capacitance or inductance, the current may lag (or lead) the voltage waveform. This results in a higher apparent load to the inverter. In the first few milliseconds of a motor starting, the two may be nearly 90 degrees out of phase - this is the reason for the very heavy startup currents for AC motors. It is also the reason why inverters must be oversized if using large AC motors (but not DC motors).
Blocking and Bypass Diodes
Many older books and articles recommend using blocking diodes to prevent reverse current flow back through the panel at night ("dark current"), many others do not (including us, mostly). It actually depends on the situation, but as a general rule in 12 volt systems, you will lose more power from diode losses than you will from leakage back into the panel at night. The situation gets much worse at higher temperatures with crystalline panels. All regulators (charge controls) have built-in blocking circuits. About the only case where a blocking diode might be needed is with small thin-film panels, such as the Unisolar US-5 or Siemens ST5, where the panel is connected directly to the battery. Diode losses are much less in higher voltage systems, such as 48 volts.
Blocking diodes and bypass diodes are NOT the same thing. Often the actual diode type is the same but serves different purposes. See the illustration below for examples of each.
In this diagram, the two diodes at the END are the blocking diodes, which prevent reverse current flow at night. These are seldom needed in systems with controllers, as nearly all charge controllers have a built-in method of preventing reverse current.
The diodes ACROSS each panel are the bypass diodes. Most medium and larger panels come with these already installed. The purpose of bypass diodes is to shunt the current around a shaded, weak, or damaged panel. If the full current passes through a shaded or weak panel, overheating and damage may occur. Bypass diodes are not needed on 12-volt systems (optional on 24-volt systems), and should always be used on 36 volt or higher systems. See also our tip below on shading.
Charge Control and Battery Charger Settings for Batteries
Nearly all batteries used in solar and backup systems are Lead-Acid. There are some differences that you should be aware of to get optimum performance and life from your batteries. Some charge controls have settings to choose between "flooded" and "sealed" batteries. This has caused quite a bit of confusion with the newer AGM (absorbed glass mat) batteries. Since gelled need to be treated more gently than the flooded or AGM (such as Concorde PVX/LifeLine), most controllers make the assumption that all sealed batteries are GELLED. Not so! - All of the AGM batteries that we sell should use the "Flooded" setting if there is no setting for AGM (unless recommended specifically otherwise by the manufacturer), which is typically about 2/10ths of a volt higher than the voltage setting for gelled. Also, if your charge controller has a choice, AGM batteries do not need nearly as much equalization as flooded, and most only need equalization about 2 or 3 times a year. Since AGM batteries do not have liquid, there is no stratification in the cells.
Heat vs Solar Panel Output
All solar panels lose power at higher cell temperatures. This is why most panels are designed for 16.5 to 17.5 volts output at room temperature - when the panel cell temperatures get up to 150 degrees F or so, voltage output can drop as much as 20%. It is for this reason that we do NOT recommend the so-called "self-regulating" panels, such as the Siemens SM46 or Kyocera KC35 for hot climates. These are lower voltage panels, and at hot desert temperatures, output can fall below the 13+ volts needed to charge the battery. In a typical solar panel, about 14% of the sunlight is converted into electricity, 7% is reflected, and the rest is turned into heat - this is why solar panels MUST have space under them for air circulation. It is also one of the reasons why we do not recommend the "solar roofing tiles" - since they are actually part of the roof, they can lead to tremendous heat buildup in the house.
Most amorphous, or thin-film, such as Unisolar and the Siemens ST series, have less power loss at high temperatures. Some recent measurements done here in Phoenix in the summer showed that at an ambient air temp of 109F, that the Unisolar 64 put out about the same amps into the load as the Siemens SP-75, despite the 11 watt higher rating of the Siemens. If you plan on using solar where the cell temperatures will be high, such as the Southwest deserts, we recommend considering the Unisolar over the crystalline panels. The one problem with the Unisolar panels is that due to their overall lower efficiency, the physical size per watt is 50-60% more than a crystalline panel - however, if measured on an "area per delivered amp" basis, there is less difference.
Theft and Vandalism
Because solar panels are somewhat expensive, they are a target for theft in some areas. There is no sure cure for this, but using theft-proof hardware, such as bolts that require special wrenches can help. Often the best prevention measure for both theft and vandalism is to make the solar panels less obvious, especially if they are visible from a major road or highway. It is also a good idea not to "advertise" them - such as one county in California did, by placing large signs up at a roadside rest stop bragging about their use. The panels were gone in a few weeks.
It's not like there are hordes of panel thief rings roaming the countryside, but it is something you should be aware of.
Shade losses in PV panels
All panels, regardless of what the advertising says, will lose considerable output even if only partly shaded. Some, such as the Unisolar panels, lose less in partial shade, but the output is still reduced. In crystalline panels, when a cell is shaded, it essentially "turns off" that cell, turning it into a high resistance. If a single cell in a panel is shaded, it can reduce or even completely cut off the output of the panel. In some cases, it can also result in overheating of the cell as the unshaded cells try to force current through the high resistance cell. Unisolar panels are less susceptible due to built-in bypass diodes on the cells, but will still lose up to most or all of their power with 15-20% shading due to the voltage dropping below the battery voltage.