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That is a safety issue required by the utility. The utility want to assure that if they open a fuse or breaker in the supply that the distribution system is deenergized. It's easy to make inverters that would work without external voltage.
It's also required by fire code, so that when the fire department shuts off power it is off. Doesn't mean that it can't be designed around, such that the inverter actually looks PAST the master breaker, out into the system, and decides if the power has been shut off or if the grid is off. If the master power switch is off, then off goes the inverter as well. Give them another 5-10 years and it'll be common to have a "grid down" inverter.
You have good logical questions and I should have given more details. Sorry!
I do live in Nevada, Las Vegas to be exact. I want the PV system because I want to be cool in the summer and not worry about the electric bill. NVEnergy was recently bought by Warren Buffett and friends and historically his utilities go up 10% per year and I know I can't afford a 50% increase in 5 years. I am quite tired of sweating when we have no shortage of free sunshine.
My house is 1 story, 1500sf, and I have a 5ton ac unit on the roof. I already have solar hot water up there too. I will be grid connected so there are no batteries needed. NVEnergy will not pay money for power I return to them but I do get a bill credit. I may not generate enough power during the hottest part of the year but I should have enough of a credit built up over the winter to make up the difference. At least that's the plan.
The quotes I've gotten so far range from 30K to 21K for an 8KW system. The system will be on a S facing roof with no obstructions. It will get a lot of sun year round. I am supposed to get back 30% of whatever I spend on my taxes.
We're in the process of trying to sell the house with grid-tied solar - grid tied solar that covers all of our yearly electrical AND generates around $2,000 a year in payments - and solar thermal that provides all domestic hot water from about April through October, and the realtor says a lot of potential customers are scared off by this "new fangled thing."
We have the same problem with renting out our vacation home. Lots of people think solar is cool, but its not for them!! We have friends that also rent out vacation homes and they have quit telling people that they are off the grid. On both our homes the solar system is completely transparent to the renter.....we even had renters that did NOT realize we were off grid.
As a side note, the worst energy users are those that profess a "environmental" vision. I am not sure why, but I suspect most think solar energy is free and abundant.
They have to draw down the battery bank and then the generator starts automatically to recharge the batteries and the generator clock tells me exactly how long it ran!!
Once that generator starts running solar is no longer free and abundant!!! And generator run times are not cheap!! It is getting to the point that once somebody states that they are environmentally oriented I feel like cancelling the reservation!!
I have thought about putting a surcharge on energy hogs.
It's also required by fire code, so that when the fire department shuts off power it is off. Doesn't mean that it can't be designed around, such that the inverter actually looks PAST the master breaker, out into the system, and decides if the power has been shut off or if the grid is off. If the master power switch is off, then off goes the inverter as well. Give them another 5-10 years and it'll be common to have a "grid down" inverter.
It may be a fire code issue too, but utilities require a house to automatically isolate itself from the grid if it generates power. The easy way to comply is with an inverter that shuts down if there is no external voltage from the utility.
There's so much good info from people who know and I certainly don't. I wanted to mention from my very limited experience, be sure you read your warranty. One of the buildings on a base where I worked had PV panels that were under warranty but going on top of the roof to fix them wasn't covered. So the base had to remove the panels and pull them down in order to get warranty work done... or pay the contractor to get up there.
There's so much good info from people who know and I certainly don't. I wanted to mention from my very limited experience, be sure you read your warranty. One of the buildings on a base where I worked had PV panels that were under warranty but going on top of the roof to fix them wasn't covered. So the base had to remove the panels and pull them down in order to get warranty work done... or pay the contractor to get up there.
Thanks! Good point! The warranty is supposed to be the PV equivalent of bumper to bumper for 10 years. And the panels are 25.
It may be a fire code issue too, but utilities require a house to automatically isolate itself from the grid if it generates power. The easy way to comply is with an inverter that shuts down if there is no external voltage from the utility.
It's called an automatic transfer switch...Any EE could tell you that
Off the shelf automatic transfer switches won't work as they transfer load between two power sources when power is lost to the primary supply. What is needed in this case is a switch that will open circuit the utility supply and then parallel (not transfer) when utility voltage is restored. That is more complex because the closure must occur when the two systems are in phase. ..Any EE could tell you that
Transfer switches isn't the approach most utilities and PV suppliers have taken. I don't think PEPCO has approved automatic transfer switches for interconnected PV systems. Anyone else have information on specific utilities?
BTW it is actually more complex to design an inverter that shuts down when utility voltage is dropped than one that keeps generating. Many pleasure boats use such independent inverters, as will off-grid PV systems.
DC - Yes, a more complicated transfer switch, but it has to have a fail safe to not allow power to be transferred back to the grid if the grid is off - but allow power to still be transferred to the house. Be UL and power company approved to. AND be able to do this without a battery backup system to be "affordable". I believe that this is possible, I just have not seen one YET.
Fire code states there must be a way to completely turn off the system from the outside. So a main that turns the whole system off, which should be part of any installation regardless. Even if just for your own safety - as in a totally off grid, non-code cabin in the middle of nowhere, I would want a want to turn off the entire system. (Not everyone will do this or want this... But it just makes sense to me)
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