How Is Photovoltaic Output Power Used: Onsite Consumption vs Grid Export and Billing?

When a photovoltaic (PV) system is added to a home or business where does the output power go? Does is get consumed immediately at the home or business, assuming the new power is lower than the currents loads. Or does the PV power back-feed immediately into the utility system? If it feeds into the utility system, then is this generated power subtracted from the end user’s bill?

Electricity follows the easiest path. If it's needed locally, it will be used locally, offsetting what you would have pulled from the power company. If it's not needed, then it flows out to the power company, to be subtracted from your bill, usually at a lower rate than you pay. The moral of the story is to use it, it's cheaper than selling it back.

Feeding power back to the power company is complex and the equipment needed is far too expensive to make sense unless you produce bunches of power. Photocells produce DC power. This must be converted to AC, which you may do anyway, but it must be an accurate sin wave, which is less common. More important, it must be synchronized with the incoming AC, which requires fancy switch gear that is specially approved and so expensive. If the AC weren't synchronied it could result in a virtual short. Generally only major electricity suppliers feed back into the grid. I believe a special, and expensive, meter would also be needed. I did some PLC work for a company with 1 megawatt turbine generators and the synchronization was a big deal.

Synchronization WAS a big deal for small producers, it’s not any more. Fully synchronized inverters are now common and they regulate voltage as well, commonly referred to as “grid tied inverters”. The current standards are as follows UL standard 1741 and IEEE standard 1547 and 1547.1. There are many features that these standards require but basicly the inverter will not start providing power unless there is a input power available for the inverter to sync to. Like you say it used to be a big deal as it was on large aircraft with multiple engines/generators feeding a common electrical system. It is now simpler to generate DC and let the inverter worry about sine wave creation, sync and regulation. On a 747 the weight saving per generator would probably be in the 350-400 lb range for each gen as there was a transmission to control speed in between the engine and generator let alone the electrical equipment rendered obsolete.

well, if you have a home or business solar cell array that's DC it probably still is a big deal with a very poor payback. You are talking about an inverter/synchronier and switch gear. They're not giving those things away. When I was involved with a small 4 Megawatt turbine generator in the old days, almost 5 years ago, when they redid their power line connection the new gear was expensive, it had to be installed by a incensed electrician and inspected, also not free. I may be wrong, but I think it would take a VERY large array for it to be cost effective.

Note that at current electricity rates of $0.10 or so a kW-hr (the utilities pay less than they charge) a 100 KW array (a pretty big solar cell array) brings $10 /hr, nothing to write home about.

Actually the inverters range from under 200 watts to 250 kW and there are some utility scale "megawatt" versions. Price on solar modules are currently under $1 usd per watt down from about $8 per watt about 8 years ago. The inverters have not seen such drastic price decreases but have gotten more reasonable as well. The inverters and modules generally have a ten year and 20-25 year warranties respectively. The technology has matured and is in the grasp of most homeowners. Many people still have the idea that it is to complex or expensive and it’s simply not true anymore. Literally a person could install one 215 watt module and micro inverter and spend less than $500. These can be ganged together and very large arrays made. Whether or not an electrician has to get involved depends on where you live. Where I live there is a $2 per watt rebate so it doesn’t make sense NOT to install it when it is warranted for 10-25 years and in reality someone else pays for most the installation. Here’s a link to a micro inverter: http://enphase.com/products/m215/ I’m not promoting this brand over another but you may notice they have a 25 year limited warranty. Payback in most case studies I’ve done recently have paybacks under 7 years. I have 5 kW installed at my home and this time of year I actually produce more than I use. I also have a wind turbine that feeds through a grid tied inverter and spring is very good to me. My worst time is August with the AC running and I still get 50% of my power from solar.

A $2500 grid tie inverter isn't very expensive. It's the same price as a off grid inverter, and allows off grid as well as grid tie operation, so you don't have a roof full of useless panels when the grid is out. No special meter is needed, the meter you already have is usually sufficient, though some power companies require a second one. Most power systems are producing sine wave power, it's not uncommon at all. Synchronization is now easy, and has been for some years.

I'm a off grid and grid tie installer.