Grid-Tie – Maximum benefit – Minimum cost

Building a grid-tie system can be very expensive if done without thought. If you simply define the highest peak power that you will ever use and build a GT system that will handle that amount, then you will be wasting a lot of your time and money on unused capacity. This article will help you to understand how to get the maximum benefit from your grid-tie dollars.

In the article, Grid-Tie How Big? we described an expensive alternate energy system that covered the worst case peak power used in our home. Now we are going to get practical and separate the needs from the wants to lower the cost of the energy system while we continue to lower the monthly electric bill. How far down can we take it? 50% lower? 75% lower. 90% lower? Let’s find out.

In a nutshell, you want to determine the energy uses that meet your baseline needs and add any critical appliances such as well pump, freezer and refrigerator to that list. I am going to abbreviate that as “Base/Crit” from here on. Install two circuit distribution panels; one for your high wattage appliances and general household circuits and a sub-panel for the Base/Crit appliances. You tie the main panel to the grid as in a standard installation. Use a properly sized circuit breaker to feed power to your grid-tie inverters and then feed their power to the Base/Crit panel. Let me explain why this is a desirable configuration.

The first thing we must do is determine what our Base/Crit needs are. What do we want to have powered up when the grid goes down? (and it certainly will, from time to time). A 1200 VA well pump with a 2100 VA startup power. A 150 VA refrigerator with a 250 VA startup surge (the freezer is similar) and 1530 VA of general lighting, computer equipment, and other appliances. The baseline use for my home office with several computers and lights is 1,530 VA. I also have a 1,200 VA heat pump keeping the office warm (or cool). Can we build a smaller grid-tie system to handle this configuration?

1530 VA is not necessarily the lowest level of power consumption in the sub-panel but it is a good place to start this example. The well-pump comes on and the power draw increases to 1530 + 2100 = 3630VA for a few seconds then settles out at 1530 + 1200 = 2730VA for the next 45 seconds as the well-pump restores the water pressure. If we turn on the heat pump we have 1530 + 1200 = 2730 VA. The heat-pump generally operates for 15 to 20 minutes per cycle, so there is a chance that the well-pump will turn on again during that time. 2730 + 2100 = 4830VA surge then 2730 + 1200 = 3930VA for the next 45 seconds. Add the freezer and refrigerator. So far the worst case is all devices on with a short surge of 5130 VA and a peak load of 4230 VA for 45 seconds.

Lets look at a pair of Outback GTFX3048 inverters configured for 240VAC (split phase US and Canada). It supplies 6000VA continuous with 6400VA for 30 minutes and 9600VA for 5 second peaks. The well and the heat pump are both 240VAC appliances. We are well under the full load capacity of these inverters so we could actually move some more of the household circuits from the main panel to this baseline/critical panel if we wanted to. But let’s not get too far ahead in our thoughts.

If the Base/Crit list is sufficient, then we may want to look for a less expensive lower power rated grid-tie inverter pair, but Muphy’s law would suggest that the power required will grow to fill the available capacity. So, lets stay with the Outback for this example. We add the rest of the house outlets and lights for a total of another 1000 VA. We choose to hand wash the dishes (Yes. I help in the kitchen every day) when the grid is down leaving that appliance tied to the main panel. So the power used is now 3030VA and the worst case surge is 6130 VA with a peak load of 5230 VA for 45 seconds. Still within the limits of the GTFX3048. We still need to estimate (or measure) the average amount of power used in this sub-panel. So, why not take a guess and use 2000VA. Not much power used at night, office space used 10 hours a day, turn down the heat pump after hours, etc.

So, what is this going to do to our electric bill?

Since the power company has been measuring our monthly usage for many years, we can get a very good idea of the yearly average power we use from our electric statements. Take the meter reading at the beginning of the year and subtract it from the meter reading at the end of the year and divide by 12 months. I get 53.8 kWH per month as an average.

What power will my new grid-tie system provide? Well, the design limit is 6000VA continuous power which is more than we buy from the grid, on the average (72kWH per day!). But because of the way we have wired in the sub-panel for the house and home office circuits we recognize that the sub-panel power is not available to all of the appliances in the house. Some are still connected to the grid through the main panel. Let’s use our best guess average value of 2000VA or roughly 2.000 kiloWatts. Multiply that times 24 hours a day and we have 48kW.

So what is the result here? We consume an average of 53.8kWH per month. We draw maybe 48kWH per month from our grid-tie system. In dollars and cents? At 10 cents per kWH (our current rate) we are buying the difference; 53kWH – 48kWH = 5kWH or $0.50 per day. The 100% bill would have been $5.30 per day for 53kWH times 10 cents per kWH. This also assumes that our grid-tied system suppements the power used in the main panel. We may not be selling power to the grid, but we are selling it to ourselves by backfeeding it to the main panel to power the kitchen lights and outlets for example. (This also depends upon the state of the batteries and other conditions that will be discussed in another post.)

Translated to monthly charges we would have spent $159 per month on the average if the grid supplied 100% of our 53kWH of average power. (plus the base monthly charges and taxes, ughh). But with our grid-tie subpanel we generate a lot of our own electricity from a renewable energy source and pay only $39.75 to the power company (plus base monthly charges and taxes). We have cut our electric bill to 25% of $159.00. Does that mean we have cut it by 75%?

We have diverted $144 each month (average) into our own power generation station. If it cost us $20,000 to install it we would recover the cost of the principle investment in 11.5 years years. That doesn’t sound all that good, but …

1. We have power when the grid is down.
2. We don’t lose the contents of our freezer when it is down for a three weeks, as it was when the last ice storm blew through here.
3. We are helping the environment by reducing the amount of energy that the nasty dirty old grid has to produce.
4. We don’t need to buy a fossil fuel backup generator. (A good 2000 Watt generator costs over $2000!)
5. Five years from now when our bill would have risen $40 (24%) to $199 per month it only rises $10 per month to $49.
6. The power that we generate is cleaner than grid power. The voltage is stable with low distortion (THD). Our electronic appliances love it.
7. __________________________________ (your justification)

Independence carries a cost, even if you buy only partial independence from grid power.

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