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Tag: grid-tie

Making Solar Pay

Posted by tomwiles at 8:54 PM on May 8, 2013

I have always been fascinated by the idea of generating my own electric power. Back in late 1998 I installed a solar power system that has sixteen 75 watt solar panels, along with a 4,000 watt power inverter/charger and a bank of expensive deep-cycle batteries.  Mention solar power, and most people think that all of these elements are necessary, including the expensive bank of batteries.

It turns out there is a much better way to think of home solar energy – use solar energy equipment strictly to push power back into the electric company utility grid. Batteries should never be considered to be part of a solar installation unless utility power just isn’t available, say in a remote location. Battery technology is an albatross when it comes to being able to store enough power to meet real-world needs.

If electric grid power is available, there are only two elements necessary – the arrays of solar panels, and what are called grid-tie inverters. In this battery-free scenario, the math of pushing power back to the utility to offset electrical use becomes much more interesting.

Power companies in the United States are required by law to “buy back” consumer-generated power. A grid-tie inverter takes the DC power being generated by the solar panels, inverts it into AC power, and then sends it back directly into the grid via a standard AC power plug plugged in to a regular 110 volt outlet. It is possible to have more than one grid-tie inverters, which also come in different sizes.

The relatively high-end inverter that I have is capable of producing 4,000 watts sustained output. So, if I wanted to push 4,000 watts back into the electric company utility grid, I would need at least two more arrays of solar panels feeding DC current into the inverter.

In my case, the batteries died within about the first three to four years. I simply turned the equipment off and my youngest brother sold the battery carcasses to a battery recycler. The equipment sat dormant until yesterday. A friend that does solar as a hobby helped me check the inverter and get it up and running again. I contacted my electric company and they sent a man out this afternoon to look over and approve my system, an absolutely necessary step. So the net effect is that now whenever there is daylight, the inverter is pushing power back into the grid. Obviously the maximum amount of power is generated when the solar panels are in direct sunlight.

The electric company performed a test of the inverter to make sure that if there is a grid power failure that the inverter automatically cuts off its own output. This is quite critical to the power company, because they want to be absolutely certain that in case of a grid power failure, no user-generated AC current is being fed back into the downed power lines.

I was able to verify that my inverter was pushing power back into the grid by turning off all internal breakers in my house so that no power was being used. At that point I looked at the power meter out on the utility pole and it was actually running backwards! Of course, in normal operation with different things consuming electricity in the house it is unlikely it will run backwards much, but it will be slowed somewhat.

My local electric company is a rural electric cooperative and they actually encourage customers to set up these types of “selling” consumer-generating power systems. It helps them reduce peak demand, thus reducing the need for more electrical generating capacity on the utility’s side. Solar panels are generating electricity at peak capacity when peak demand is likely to occur when air conditioning demands are at their highest.

Can a system like this ever pay for itself? It depends on the initial cost of the equipment, installation expenses, and how long of a payback period you are able to live with. If you can do most of the installation work yourself, then obviously the math works better. Eliminating the batteries really helps the cost come down.

An HQRP 1,000 watt grid-tie inverter sells for $287.95 on Amazon.Com. Aleko brand 75-watt solar panels sell for  $149 dollars each. Sixteen of these solar panels multiplies out to $2,384 dollars. With brackets, wiring and installation let’s estimate a total package price of $4,000, which may or may not be wildly off one way or the other. The 1,000 watt electrical output of the inverter would have to offset $4,000 dollars worth of electricity over a period of years before it would pay for itself, which is likely a long period of years. If the price of the equipment and installation can be brought down, then the payback period shortens.

My electric company will only allow this type of setup to function as an offset. So, let’s say that someone was putting more power back into the grid than they were actually consuming. My power company will never issue a check for the power, so it’s really just an offset for how much I consume. With enough equipment feeding power back into the grid, it would be possible to bring electrical grid usage down to zero.

Many local and state governments offer tax rebates for new solar equipment installations, which could also help mitigate the cost.

The beauty of a battery-free grid-tie solar user-generated power system feeding into the electrical grid is that once it is initially set up, everything happens automatically. Since I already have the equipment and it is long since paid for, I might as well be utilizing it to offset a portion of my power usage.