My household has a base power consumption of approx. 200W-300W. I am considering if it was possible to get a reasonably sized solar panel to cover its needs when the sun is up, with a seamless transition to the grid for spikes / low light times.

As I am a complete newbie to solar energy, is a solution like the above feasible? What kind of device would I need to interface, let's say a 300W solar panel to the rest of my system? I am not interested in energy storage / delivery back to the grid, I am just after supplementing my current supply with a bit from the sun.

  • " I am not interested in energy storage / delivery back to the grid, I am just after supplementing my current supply with a bit from the sun." - Why are those mutually exclusive? – mbeckish Nov 14 '14 at 15:31
  • Sorry if I was not clear - what I want to say is that I am not looking for a solar panel that would feed back energy to the grid. All I need is for my equipment to use the "solar" energy when available. No batteries to store it etc. – petr Nov 14 '14 at 15:36
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    This is a topic you probably want to involve a knowledgeable professional. Handling power consumption is something most DIYs can learn pretty easy. Power generation is an entirely different animal with an entirely different set of concerns. If you've never dealt with it before I STRONGLY recommend you get help from someone who has for at least your first project. (There are resources out there that cover the basics, but expect it to take some time to properly research this) – Eric J Fisher Nov 14 '14 at 21:40

Typical grid connected installs of solar panels on homes need a couple of items to get them to work (it's not just lets slap some panels up there and wire it into the house). Solar panels are DC (direct current), your house is AC (alternating current), so a inverter is needed to create the AC current. Also that AC current needs to be synced to the same wave form that is being feed from the grid, another device (or sometimes packaged in the inverter). Then the new solar AC is then tied into your main panel, typically requiring another box. Also depending on the install, the panels might be electrically separated from each other (a good thing) requiring another device. Once connected, excess power from the panels will feed back into the grid and in some areas this will be credited back to you on your bill.

All said, this work needs to be done with a permit, typically with buy off from the power company on size of panels. Also for most DIY'ers, this is out of their scope of skill. Get some quotes from solar panel companies and electricians in your area to get an idea of what you are looking at.

Also look at the cost of the system and how much it is going to save you each month. Solar is popular here because power goes up to .30 a kWh depending on usage. But for my energy usage, I'm only sitting at .11 on average making solar unreasonable. My last bill was under $2 after energy program usage credits where applied.

  • It's also worth noting you REALLY need to know electricity for this type of work. Most DIY projects related to power around the house are based on consumptions and related limitations. When you're generating power while the fundamentals are the same, what you need to consider changes significantly. Depending on the specific needs and available hardware how you wire your circuit to your panels could be a lot different. Incorrect installation could range anywhere from useless (a waste of money) to a SERIOUS fire hazard. – Eric J Fisher Nov 14 '14 at 21:30

What you propose is technically possible, but probably not worth it. The major is problem is that generation must always equal load. Let's say you're using 300 watts, but your panels are generating 1kW. Where is that excess power going to go? You can't just turn off the sun. You would need something like a "dump load"--basically a giant resistor to turn the excess electricity into heat. You could turn this into a water pre-heater by immersing the resistor in a waterproof coating in a tank where incoming water can be pre-heated before it reaches your actual water heater. The electronics needed to do this kind of thing are called a charge controller, commonly used in off-grid installs.

Charge controllers typically work in conjunction with batteries, so you'll need one specifically for a battery-less install, if such a thing exists, or need to modify it accordingly.

Next, you need to modify the board to switch back to grid power when the panels aren't producing enough power (nighttime, cloudy day, high load, etc). Again, since charge controllers don't typically do this, you'll be looking for a very specialized piece of hardware, or need to make it yourself, with all the attendant dangers and liability issues involved with diy high-voltage electronics.

This is all going to be very complicated and probably not worth it compared to selling electricity back to the grid with a legit, aboveboard, permitted system. If you want to avoid that, the past of least resistance is to to use batteries to get through the night, not grid power.

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    -1 since solar panels do not need a dump load at all. Solar panels are perfectly able to supply no power at all. To see this is the case, just consider a solar panel sitting in the sun connected to nothing (or even short circuited!). Where is that "excess" power going? Nowhere, because there isn't any power being produced! You need a dump load for typical permanent magnet wind or hydro turbines that cannot run no load or it will spin out of control. But otherwise agree, not worth switching the grid power on and off... just get a grid tie inverter. – bobfandango Nov 14 '14 at 21:42
  • Okay, I learned something, thanks! But if you don't physically disconnect it, is there still a way to get it to not generate power short of throwing a blanket over it? – iLikeDirt Nov 14 '14 at 21:57
  • Yes, if you short circuit a solar panel (the opposite of disconnecting it), it will stop generating power. Counterintuitive, I grant you. But at short circuit, a current will flow but with no voltage. P=IV. But since V is zero, so is power. Search google images for 'solar panel power curve'. Consider, e.g., imgur.com/R5MICfe. At 1000 W/m^2, that panel can produce about 40 watts at the max power point (I~=2.3A, V~=17.5V). For the same insolation and a load of only 20 watts, the panel will operate at I~=2.4A, V~=8.1. – bobfandango Nov 14 '14 at 22:17

Everyone seems to have misunderstood the question. If his house consistently has a base load of 300 watts and he attaches a 200 watt panel with embedded microinverter, there would never be an issue about dumping the load onto the grid or turning it into heat. Shouldn't this situation be easy? Just connect it to the house after the meter but before the panel, right?

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