Solar Equipment Ratings
In general, the ratings on solar electric components are reliable and useful. However, they do require some interpretation. Let's start with the panels.
Ignoring the differences in Voltage and size, all panels are rated in Watts. As in our load analysis, to find the total ouput we multiply Watts X Hours.
In the example of my 900 Watt system, in the summer I may AVERAGE 7 hours of sun per day. In the winter I may average 4 hours of sun per day. I chose to design my system based on winter conditions, so I have extra power in the summer. This is useful for pumping water. So our theoretical output is 900 Watts X 4 Hours/Day = 3600 WattHours/Day. In reality the output is about 20% less due to lower output in the morning and evening, wire loss and the optimistic way panels are rated. 3600 - 20% gives me an AVERAGE of 2880 WattHours per day to work with. This is a conservative number, but good enough for design purposes. Comparing this number with the load analysis will let you know if you are in the ballpark with the size of your array. The array output also should be balanced with the size of the battery bank for optimum battery charging. We'll examine this on the next page.
WHY ARE PANEL RATINGS SO OPTIMISTIC?
In the specs for all modules are: Wp = peak watts = nameplate rating = Pmax = the max power they could get at STC (standard test conds). Depending on the load applied, the panel operates along the "IV curve." (I=current vs. V=voltage). Generally as a load is applied the voltage drops, while the current stays more or less constant after rising to a certain point. The relevant point is that there is a certain load which results in the maximum POWER out of the panel. Typically it is at around 17V. Whatever point it is determines the Wp. That point is designated as Vmaxpower or Vmp or "rated voltage," and Imp or "rated current." If we apply a short circuit across the terminals of a panel, the current will reach its highest and the Voltage will be zero. This is one endpoint of the IV curve. It is described by Isc or "short circuit current." This is the number used to size wire and overcurrent devices with some additional safety factors added. If we have an infinitesimal or zero load, the voltage is at its highest and current is Zero. This is the other endpoint of the IV curve and is described as Voc or "open circuit voltage." This is the number used to determine if an electronic device will blow up when connected to the panel.
SO, the S.T.C. rating is a laboratory rating based on a certain number of photons at a certain temperature and also based on the panel being loaded just so, such that it is operating at Vmp and Imp. In real life, heat, dirt, haze, and a system operating at a different voltage than Vmp all can reduce the output as measured in Watts. "Maximum Power Point Tracking" devices (SMA brand inverters, RVPP brand charge controllers) regulate the load on the panels so they operate at Vmp and may be worth the extra expense in many situations.
For smaller systems, it can be helpful to look at rated amps instead of Pmax to get a more realistic idea of output.
What if the sun doesn't shine?
This gets a little tougher since a battery capacity changes with the rate of discharge. Different manufacturers use different discharge rates when they publish specs. In the solar-powered home, the "100-hour" rate is closest to what the batteries actually experience. If you use the "20-hour" you will just have a more conservative number, which is fine.
OK. Let's take the example of an L-16 type battery, a favorite in solar homes. Let's say it is rated at 375 AmpHours at the 100 Hour Rate. Remember from load analysis Amps X Volts = Watts. An L-16 type battery is a 6 Volt battery, so 6V X 375ah = 2250 WattHours. Quite a lot of energy. BUT remember this is the TOTAL amount of energy in that battery, at that discharge rate. If we were to use it all, we would basically ruin the battery after one use. What we need to do is cycle these batteries somewhere around 25% Depth-Of-Discharge (D.O.D.) so they last for years instead of one week.
What we can actually get out of a fully charged L-16, for design purposes, is 2250 WattHours X 25% = 562.5 WattHours. Multiply this by the number of batteries to figure the battery bank total capacity.
I am not aware of any inverters which are inaccurately rated. The problem is accurately rating surge loads. Motor surge is a complicated subject beyond the scope of this page, and is very different than a simple "resistive" load. This is because maximum current draw cycles may not coincide with the voltage sine wave cycles of the AC power. It is often stated that motor surge loads are twice the running load. It may be more conservative to figure three or four times as a conservative simplification. This can also greatly affect inverter efficiency and may not be reflected in the specifications.
Another issue is the inverter's "idling" load. Many inverters have a standby mode which will let the inverter sleep if there are no loads on it. Unfortunately many loads are too small to wake up the inverter, so many users leave the inverter on all the time and waste a few watts. The inverters we sell have small enough idling loads that it is not a problem in a house-sized system.
If it is possible to run small 24/7 loads directly off the batteries; then the inverter can sleep until needed for a larger load. This would be practical for a 12V system and may be worth the extra complexity. In very small systems it may be advisable to manually turn the inverter off when not needed.
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