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By Oct 23, 2010

Welcome back to GreenergyStar Report. We have investigated the disadvantages of a battery PV system last time, so this week we will zoom-in to a batteryless system with Grid Tie Inverters.


So, what are Grid Tie Inverters?

Before we look into a Grid Tie Inverter, we should learn what an inverter is. An inverter is a device that will convert Direct Current (DC) (such as solar, wind and battery power) to Alternating Current (AC). The reason we have to do this conversion is because almost all home appliances (such as TV, computers, fridges and etc.) are running on AC. For those who are eager to learn, DC is a single-direction flow or electric charge from negative to positive while AC alternates between positive and negative 50 to 60 times a second. The brilliant minds out there have determined that alternating current can be transmitted over longer distance more effectively than direct current and therefore, home appliances are designed to adapt AC.

Knowing what an inverter does, a Grid Tie Inverter – in addition to converting DC to AC – will also synchronize the AC output so it is in phase with the utility network. This allows excess renewable energy to be sent back to the grid. By doing this the grid is acting as an infinitely large battery bank to store green power. We no longer have to spend thousands to build our own battery system, no longer need to worry about replace / dispose dead cells, and no longer need to waste precious space to store these chemical / toxic in our basement. Best of all, this new “battery bank” is maintenance-free and is readily available to every household. You are connected 24/7, rain or shine. It doesn’t matter if its overcast for the next two weeks ’cause “The Grid” will have you covered. Got problems? Dial a 1-800 number and a lineman from your local electric company will show up and fix all your troubles, free of charge.

… I am not convinced. With batteries, everything is under my control. With Grid Tie Inverter, it feels like I have no control what the utility does.

Good point. How do we know if the utility will hold up to their end of the bargain on the green energy we generated? Simple. Generally, you will need to see if they have programs called “Net Metering”. What net metering does is after registered and approved by the utility , they will (literally) have 2 meters (or a bi-directional meter) installed to monitor the power you generated and consumed and calculate the difference at the end of the month. You get billed if you consumed more than you generated, and credited if vice versa. Some utilities do have a premium buy back rate for green energy generated with qualified grid tie equipments which will significantly cut down the break even period. Think of this in terms of banking. For those people who use batteries, it is like burying money in the backyard. Yes, there is absolute control over the buried piggy bank. But no, it is not safe. And yes, your money (battery) will depreciate (get discharged) over time. However, for those who use Grid Tie Inverter and register for Net Metering, it is like opening up a high rate saving account in the bank. You deposit when there are excess cash, and withdraw when you are in need. Best of all, green energies are qualified for premium buy back rate so that can be considered as the “interest”.

What happens if there is a power failure?  Will the Grid Tie Inverter stay online and provide power? Will this create a risk for the linemen working outside?

According to the national safety standard (UL1741), Grid Tie Inverters are required to have an “anti-islanding” function. Basically, what this means is the inverter will shut down during a black out to ensure no line workers will be at risk of being shocked. Unfortunately, this means there will be no back up power when there is a power failure.

What are the overheads of owning a grid tie system?

Frankly speaking, it is quite costly to build a standard grid tie system. First, there will be the price of the Grid Tie Inverter. They usually range from US$1000 to US$4000. These inverters will require a high DC voltage to work so a lot of solar panels will need to be put in series …which equates a lot of money spent in purchasing solar panels. Since we are dealing with several hundred volts, a qualified electrician will need to be hired to do the wiring and connections. This type of project is for dedicated homeowners who are determined to go green as the upfront cost is quite a financial burden.


I want to do my part for the environment, but this is way over my budget… is there any other way that I can do this at a …smaller pace? …Preferably DIY?

Of course! The systems mentioned above are standards in the last decade. It has the advantage of eliminating the cumbersome batteries. Yet at the same time it poses the disadvantage of high upfront cost which has turn off many property owners from going green. If we can scale down by 10 times… and have a Micro Grid Tie System… perhaps such projects would be much more manageable.


Stay tuned for Part 3 – Micro Grid Tie System

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