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Real True Solar Panel Output

June 4th, 2012


After living off the grid now for the last 4 weeks full time, I am getting a better idea of the true solar panel output during any given time of the day. I can also see how rain or cloudy weather affects the solar panel output. When you buy a solar panel, you see a wattage rating on it. But the real solar panel output is a totally different story.

I have two homemade solar panels, each rated at 65 Watts. That means I should have about 130 Watts total energy output during every hour of the day. At 12 Volts DC, that means about 10.8 Amps of power being put into the batteries every sunny hour. But the reality is much different.

First of all, your solar panels (or cells) were tested at the factory under a bright light simulating full direct sunlight with no clouds. This would be the optimum condition for maximum output. Of course, we rarely have that condition except for a couple hours each day when the sun is directly overhead and there are no clouds.

I just installed a high precision Amp meter in my charging system and was totally shocked to find that I am only getting about 3 Amps output with the sun directly overhead and no clouds. At any other time of day there are about 1 to 2.5 Amps output. If it is raining or cloudy, I get about 1 to 1.5 Amps. That means I get between 10% to 30% of my solar panels rated energy output at any given time!

I have checked and re-checked all of the wires and connections. There is nothing wrong. And I am not alone in this. Most off grid power systems do not produce anywhere near their rated energy output.

But why then do we not get the full power output to the batteries?

There are a few different factors that influence total solar panel output.

First of all, the sun is moving (well, we are). The solar panels are only getting direct full sunlight for a couple hours each day. That is the easy one to figure out. To remedy this you need a solar tracking controller which moves the complete solar panel array so that it is always facing directly into the sun at any given time of the day. But these cost money or are complicated to make yourself. They need to be rugged in order to survive extreme winds and storms.

The next problem is that solar panels are made to produce about 18 Volts. This is so that when the sun is not directly overhead, the panels are producing at least enough voltage to charge your batteries a bit. Lead acid 12 Volt batteries need about 14 Volts to charge them. If your panels are producing 18 Volts, but your batteries require 14 Volts, then you are just dumping all the excess energy. It is wasted. Your solar panels are forced down to 14 Volts. It is even worse when your batteries are lower. At 12 Volts you loose even more potential energy because you force the solar panels down to the battery voltage of 12 Volts. Anything above 12 Volts is lost. Of course, any voltage below your battery voltage is lost as well. This means on cloudy or rainy days, mornings and evenings, you may get nothing.

The solution is a MPPT solar charge controller. This is a Maximum Power Point Tracking charge controller. This device monitors your solar panel output voltage and the battery voltage. When the solar panels are producing less than the battery voltage, they boost the voltage, while reducing the current and charge the batteries. When the solar panel output is higher than your battery voltage, the MPPT controller reduces the solar panel output voltage while increasing the current into your batteries. What is does in essence is matches the solar panel output voltage to your battery output voltage to get maximum charging current at any given solar panel voltage. These are also very expensive.

Due to the costs many solar energy systems do not include either solar tracking controllers or MPPT controllers. But at $5 to $10 per Watt, that is a lot of wasted energy. Take my example numbers and extend them out to a 1,000 Watt solar array. That means that for an initial investment of as much as $10,000 I may only get about 100 Watts to 300 Watts of total solar energy output at any given time. These numbers are averaged for a northern climate and throughout a variety of weather conditions. Sunny states will get much better output.

Now the cost of a solar tracking controller and a MPPT controller seem more reasonable.

I will be building both of these in the coming months. Living off the grid on 1 – 3 Amps of power is a joke. Ask any auto mechanic and he will confirm that this in nothing for a battery. I get about 300 Watts of total usable energy a day. A common desktop computer uses 300 Watts (per hour minimum). Your light bulb uses 100 Watts. Run a light for three hours and that is all I have.

Well, anyway, this blog post is being typed on a laptop that is charged by off grid power. It is a clean, challenging and fun way of life to say the least.

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Troy Reid


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