Grid-Tie Inverter – How Big?

So, what is the right size grid-tie inverter for your energy system? Let’s go through this with a quick exercise using data from my home to give you an idea of how to tackle this problem.

Here is a chart of the power that I used today.

Google PowerMeter - July 11, 2011

The peaks shown on on Google PowerMeter are averaged so I have to say that the actual peak demand measured over a 15 minute period was 10.415 VA. So, what size of grid-tie inverter would I need to be able to handle this peak demand from either grid or generated power?

(By the way, I am going to use VA (Volts*Amperes=Power) here instead of kW (killowatts) to notate power. There is another post that describes the difference between the two, but in ideal situations they are equal.)

One of the inverter companies, Outback Power, specifies that their GTFX3648 3600VA inverter will handle power surges of 4000 VA for up to 30 minutes. This assumes that you have a battery bank with sufficient capacity to feed the inverter at this rate for 30 minutes, but battery bank sizing is for another post. Lets configure two of these as a 240Volt system. That would be continuous power rating of 7200 VA and a 30 minute peak rating of 8000 VA. My peak usage of 10.415 kW lasted 15 minutes. But that is still too much for this inverter pair.

If I had a system that could supply the power, then the inverter would have to be rated at a minimum of 10,000 VA and would have to sustain my overloaded condition of 10.415VA for 15 minutes. Not outside of the possibilities offered from existing products.

Also, the GTFX3548 datasheet shows a max AC input current of 60 amps. Multiply by 120 VAC and that limit is 7200 X 2 = 14,400 VA. Good. So, I get two Outback GTFX3648 inverters configured for 240V to power my house. Hold on. This example is not finished yet. Too soon to draw a conclusion.

What is the rated power limit? In other words, what is the absolute highest power draw that these inverters will stand up to without burning up or faulting? Two configured as a 240V system (split phase I call it) would provide up to 3,600 X 2 = 7,200 VA of AC power. They would also provide 4,000 X 2 = 8,000 VA if overloaded for a maximum of 30 minutes. And they can provide up to 5,000 X 2 = 10,000 VA for 5 second surges which is enough for most well pumps, refrigerators, and other motorized appliances that have a startup current surge. Then there is the 6,000 X 2 = 12,000 VA for very short current surges (less than on second I think). None of those values is big enough for my 15 minute overload. A bigger system is needed.

So, let’s double up the inverters with two on each power phase. Now we have 14,000 VA max rated power supplied with an overload capacity of 16,000 VA, etc. This is definitly big enough with room to spare. You have to understand that I have not been monitoring the power use long enough to know what to expect in the Winter season, so a large safety factor like this, an additional 4400 VA, is a good place for me to start.

What have I learned from this exercise? I may have an average power use of 1,950 VA, but a 2,000 VA inverter system is far too small to handle the real load fluctuations that I will be experiencing. I have described a very expensive inverter system to try to handle the peak loads from my home. But there are other lower cost ways to deal with sharing grid power that I will cover in another post.

More to come …

*NOTE: These calculations assume that the inverter is not responsible for supplying the current to re-charge the battery bank and that this recharge power comes solely from a renewable energy source such as wind, solar, hydro, biomass generation, etc. If the Outback also had to re-charge the batteries, then the 60 Amps AC max supplied from the grid would be shared between the AC loads and the internal battery charger. I have not asked the guys at Outback how this would be divided up or which load would get the higher priority. That will have to wait for another post.

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