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Wind Turbine and Solar Panel Frequently Asked Questions

We answer your common alternative energy FAQ's below. Here, we cover questions about blocking diodes - when you need one and when you won't, charge controllers and their features, dump load types and sizes. Read on for quick, complete answers to some of your renewable energy questions.

      Blocking Diodes

      Charge Controllers

      Solar Panels

      In order to achieve the maximum performance from your solar panels, you should design your system such that the VOC (Voltage Open Circuit), of your solar panel(s) are between 1.4 and 1.8 times your nominal battery bank voltage. The Vmpp of your array (or single panel) should be 1.15 to 1.3 times the nominal battery voltage.

      To keep it simple, purchase panels for your system using the following simple guidelines

      • 12 volt system: with a Voc between 16.8 and 21.6, with 18 volts being about the best for colder areas, and 20-21.5 volts being better for hot conditions or long wire runs.

      • 24 volt system: with a Voc between 33.6 and 43.2, with 36 volts being about the best for colder areas, and 40-41 volts being better for hot conditions or long wire runs.

      • 48 volt system: with a Voc between 67.2 and 86.4, with 72 volts being about the best for colder areas, and 80-82 volts being better for hot conditions or long wire runs. (There is less power loss in the wires in a 48 volt system, then there is in a 12 volt system)

      Let's take a look at the specs of a 215 watt panel on the market today: The back plate tag reads:

      Vmpp: 29.2V

      Impp: 7.37A

      Voc: 36.5V

      Isc: 7.95A

      Voc = Voltage open circuit. This is the maximum amount of voltage a solar panel will produce, in bright sun, on a cold day, when measured with a volt meter, and the panel is not hooked up to anything (open circuit).

      Isc = Short circuit voltage, this is the maximum amount of current a solar panel can produce in bright sun, on a cold day, when measured with an amp meter, and the panel wires are shorted together (short circuit).


      Voc and Isc, can never occur at the same time, so we can not use these figures directly to determine how many watts a panel produces, but they are very important values in properly matching our solar panels and batteries.

      As we put a load on a solar panel, the voltage drops and the current goes up. There is a point in this load curve where the panel is producing its most power. This point is the Vmpp.


      Vmpp or MPV = Voltage Maximum power point, or Maximum power voltage (same thing.) This is the voltage that would be read by a meter, when the panel is in bright sun, on a cold day, and the panel is loaded (by hooking it up to a battery or D.C Appliance), such that the load causes a current draw (flow) of Impp.

      Impp = Current maximum power point  (the "I" stands for current in electrical jargon) . This is the maximum amount of current a solar panel can produce in bright sun, on a cold day, when it is producing its most amount of power (It maximum wattage, not maximum voltage)


      When Vmpp and Impp come together, then the panel is producing its maximum power. This should not be confused with "MPPT" which is a type of charge controller.

      All quality controllers work well when your batteries and panels are properly matched. So what is properly matched, or how do we achieve this match?

      Let's use a 24 volt system as an example.

      The best performance will be achieved if your solar panels produces a VOC of 1.4 to 1.8 times your nominal battery voltage. So in a 24 volt system, we want the VOC to be between 33.6v and 43.2 volts (a little higher or lower is fine here). The Voc of the panel name plate above shows 36.5, so this panel will work for us. This panel can be considered a "24 volt panel".


      But just how well will it work, how many watts can we actually expect? It's time to look at the Vmpp a little closer. The name tag shows a Vmpp of of 29.2v. Remember, this is the voltage we can expect when the panel is producing its best power. So, in our 24 volt system, our panels will produce their maximum power when our batteries are at 29.2 volts (not including loss in the wires). This is not bad, and in fact it may be just about perfect depending on how long your wire run is, how hot the panels are are etc. We might prefer to have a panel that produces it's maximum power a little lower in colder climates and if our wires are short, say 27.5 volts. Why?


      Let's do the math. The Vmpp of the panel is 29.2 volts, the Impp is 7.37 amps. Power (watts) = volts x amps. So, 29.2 x 7.37 = 215.2 watts. So this is where the panel manufacturer derives the 215 watts from. Now this figure is in a perfect world, where the sun is very bright and is it is a very cold day. This may occur a few times a year, but in reality, the brighter the sun, the hotter the day. The hotter the panels, the lower the voltage output, so the lower the wattage. Also, it does not matter how good your connections are and how short the wires are to your batteries, there is some loss between the panel and the battery. So let's take a more realistic approach to the possible power we can expect.


      Let's assume it's not quite as bright as we would like, and it's not anywhere close to cold enough to keep the panels at 25c, so we're going to say that our best power point (Vmpp) of this panel is probably closer 28 volts on a normal day (The panel is not changing, it's just the enviroment is not perfect). We might expect to see the VOC drop off to 33 volts or so in bright sun on a hot day. Now let us also say that we also have a 2% loss in our cable run. At 28 volts, we will lose about .6 volts. Now we have some loss in connections and the path through breakers and controllers. We need to add another .25 volts or so. This leaves us with a Vmpp voltage when measured at the battery of about 27.15 volts. What this means is this panel, in more real world conditions will produce its maximum power when the battery is at 27.15 volts. If we de-rate the Impp for the heat, less than perfect sun, and wire loss, we can hope for closer to 6.25 amps (this is possibly a bit optimistic).


      So, 27.15 volts, times 6.25 amps, we might see 169 (to 172) true charging watts, or about 80% of our panels nameplate. Hotter days, longer runs and this may be closer to 50%.


      The charge stage of the batteries also play into the the Vmpp.

      • If our batteries are very low, we would see something like 23.5 volts x 7 amps (the amperage would be a little higher as the voltage drops). Or 164 watts.

      • If our battery is very high (being equalized), we might see: 30v x 4.2 amps (the amperage will drop off as we approach the VOC), or 126 watts.


      What all this means is these panels (above) are a pretty darn good match for a 24 volt battery based charging system, in real world conditions, where most of us live. But let's just say you live in the north, and you have some very bright days and the wind has still got a cold bite to it. Then a panel with a little lower Vmpp might match your system a little better.


      There is another reason why you might want to use a panel with a lower Vmpp and that is our batteries are often lower than 27 volts because we constantly have a load on them, day and night. So in this case, if the batteries are likely to be at 26 volts most of the time, then a panel with a little lower Vmpp, might improve your total performance.


      When you follow these simple design rules, then you will achieve the real world, maximum power available from your panels. Any quality charge controller will pass the power from your panels to the batteries, no downshifting or up-shifting of voltage is necessary; and in fact, when properly matched, a PWM controller will work very efficiently and at a fraction of the cost of a MPPT controllers.

      Diversion Dump Loads

      No, if you will be using our charge controllers with solar panels (PV) only, you do not need a diversion load. If you will be using the controller with wind only (or together), you will need a diversion load.

      Yes. Diversion controllers work by diverting excess energy from the wind turbine to a diversion or “dummy load”. This diversion allows the turbine to remain under a load at all times.

      A solar panel may be safely disconnected from the batteries, but an active wind turbine should never be disconnected from its load (battery/diversion load). When a wind turbine is not loaded, it can easily speed out of control in high wind events, which can lead to catastrophic failure of the turbine as well as the possibility of damage and injury to other property and people.

      It is very important that your turbine has a very reliable load at all times.

      Yes and No. This question is asked of us nearly every day as it seems like the perfect load, but...

      The most common reason you need to divert the energy from your turbine is during high wind events (like thunderstorms) -- This is also the time the grid is most likely to fail. If the grid is down, so is your load, and the turbine spins out of control!

      If you have a 2nd, very reliable load, then a grid tie inverter can be used as the primary load. You will need two controllers, one for the grid tie, the 2nd for the fail safe load.

      Yes, but it really is not recommended unless you have two controllers. When a turbine is producing enough energy that it has fully charged the batteries and needs a diversion load, then that load must be there 100% of the time.

      Domestic hot water heaters cannot perform this task since once the water is hot, the heating element will be disconnected.

      Yes, but if the pump or light fails, so may your turbine. Heating elements and resistors are by far better choices for diversion loads.

      Your diversion load should be 10 to 20 percent larger than your wind/hydro energy output. You do not need to take into account any PV panels used if they are not routed through the diversion load. Please see the charge controller manuals for more information.

      This is a big question. A resistor rated at 500 watts means it can handle 500 watts of electricity without damage to itself, but this does not mean it will use 500 watts in all systems (at all voltages.)

      The actual amount of power used by a resistor depends on Ohms law (E=I*R), where E = volts, I = Amps and R = resistance in Ohms.

      So to determine the amps used by a resistor we will divide the volts by the ohms.

      Example: A 2 ohm resistor used in a 12 volt system (set to 14.5v), will yield 7.25A (14.5/2) or 105 watts (Volts x Amps), even if it says it's a 500 watt resistor. It would require a higher voltage (just under 32 volts) to bring this power dissipation up to 500 watts for a 2 ohm resistor! For a 12 volt system, you would need a 500 watt, 4/10 (.4) ohm resistor (good luck finding that dude), or five 100 watt, 2 ohm resistors in parallel to dissipate 500 watts.

      You can compare max 4 products.