As your total electric consumption approaches 500 KWh per month, it becomes increasingly more difficult to keep it balanced between the two meters. In my own case, I found myself swapping two circuit breakers every two months when I received the two CFE bills. (The two breakers must be in the same subpanel, one connected to the “red phase” meter and the second to the “black phase meter” and having significantly different values of load.) This is easier said than done because sometimes the loads change seasonally, especially if you are using an electric heater, for example.

As an example, here is a graph of the electrical usage for the two meters in our house over the last 2 years. The load balance looks pretty good, but remember that I was monitoring it fairly closely and making changes (circuit breaker swaps) from time to time. The two meters are marked as RED and BLK since these are the colors I used for connecting the two phases from the meters to the breaker box.

The average for the total of both meters is 912 KWh so if they were perfectly balanced, each meter would average 456 KWh and would be below the threshold for the much higher DAC rate shown as the heavy line at 500 above. The actual bills for both meters and the total (BLUE) is shown below.

Notice the two much higher bills for the bimonthly periods ending 1 Nov. 2019 and 1 Jan 2020. The usage for these two periods was not unusually high, but the DAC rate was triggered when the 12 month moving average exceeds 500 KWh. This is shown below. Just looking at the CFE bill, it is not that apparent when you are approaching this value.

The red meter almost hit this threshold in the 1-Oct-19 bill and did exceed it the following two periods. So now there are two questions: is there an easier and more accurate way to balance the loads between the two meters, and is it worth the effort? I believe the answer to both is yes.

The figure below shows the electrical wiring between the meters and the main entrance box at my house. It was not always this clean, but was rewired as discussed in the book you can order. Note that the following discussion only applies if you have two single phase meters as shown. They should be marked “1F2H” (1 fase, 2 hilos).

Instead of trying to balance the load (power) between the red and black phases, we can instead balance the energy measured by the two meters. To do this, we can install a transfer switch between the meters and the main circuit breakers that reverses the connection. Since the energy is equal to the power times the time, we can throw the switch once per month. Then, even if the power in the two phases is unequal, the energy measured through the two meters will be the same – assuming it remains the same throughout the two month billing period. Electrically, the switch is wired as shown below.

With the switch in the NORMAL position, the wiring is the same as without the switch, but with the switch in the REVERSE position, the black phase meter connects to the RED phase load and visa versa. So the scheme is pretty simple and should work well. I will post some actual results in about one year when I have them.

The finished installation is shown below. The transfer switch is on the left and the main circuit breaker box for the house is on the right. Installation is straightforward but must be done by a qualified electrician because the wiring requires removing the meters for safety. I used Juventino who can be reached at 415-109-5171 in San Miguel de Allende. Please do not attempt to do this yourself!

The second question is the cost effectiveness of this. The transfer switch is a “GE 100 amp 240 V non-fused emergency power transfer switch” sold be Home Depot (US) SKU# 548808. With tax and shipping to Laredo, it cost \$150. Shipping to San Miguel added another \$37. Installation cost was about 1000 pesos (\$40) so the total cost came in at \$227. My average total electric bill for all the months that were at the tarifa 1 rate for the past 3 years was 1950 pesos bi monthly. The average for the two DAC months was 3734 pesos, nearly double that. So the expected cost savings is 3734 – 1950 = 1784 pesos (US\$71) bimonthly. At that rate, the switch will pay for itself in a bit over 3 billing periods or about 6 months.

The only disadvantage I have found so far is that I need to reset the clock on the microwave after throwing the switch.

UPDATE March 2024 After 3-1/2 years of use (1-Jul-20 through 1-Mar-24), the following graph shows that with the transfer switch, the load remain very well balanced. The meters remain very well balanced during the period in which the transfer switch was used (shown by the horizontal arrow at the bottom of the graph). The only exceptions were in Jan 2021 and Jan 2024 when we used an electric space heater on one circuit.

The 12 month moving average is shown below. Again, the arrow at the bottom is the period when the switch was used. The vertical arrows indicate the billing periods during which this average exceeded the DAC threshold. Note that there was a 6 month delay between installing the switch and bringing the moving average below the DAC threshold.

## 7 thoughts on “Load Balancing Using a Transfer Switch”

1. K Frazier says:

I have had every imaginable (plus those un imaginable) with CFE that ocontinue on.
Solar might be the answer but I hope to sell this house…
Is their someone here in SMA who could do this balancing of the meters?
Thank you, Kathy Frazier

1. David Cunningham says:

Kathy yes there is but you need to determine first if it will make any difference, and whether you have the right sort of meters. If you send me a copy of your last CFE bill for the two meters you have (front and back sides) I will let you know. Send them to me at mexicodoppler(at)gmail(dot)com. Dave

2. Rod says:

Dave, one other consideration for going the transfer switch route. The line feeding one of our meters has some issues that CFE either can’t figure out or doesn’t care to try. Not a big deal however once or twice a year that line will go out of service for a few hours to a couple of days. With the transfer switch if that happens I just switch over to the other line and everything in the house especially the fridge and water pressure pump work, this obviously would be true for the other line as well. I’ve gotten pretty quick on flipping the switch so that our modems, clocks and most electrical devices don’t even reset. I guess it would be sort of a scheduled brown out.

1. David Cunningham says:

Rod,
Amazing! This feature never occurred to me, but seems like a good way to backup the most important half of your house.
Dave

2. Mark R says:

We are in a situation where we need to merge our existing two meters to one, because we are planning to purchase a solar system + lithium iron phosphate battery.

We can buy about 10 kilowatt-hours of LiFePo4 for under \$3000 USD (~29,000 pesos per 5kWh).

Our neighbour did this, and he claims he has a ROI of only 4 to 5 years, while paying only minimum monthly cost. Due to the large number of sunny days per year, and the ability for LiFePo4 to cycle abouit 5000-7000 times, it gives 15 years of nighttime solar.

We’re going to install solar this year, so we need to reverse the two-meter arrangement because it’s not practical with the addition of a grid-agnostic solar+battery system (newer inverters that no longer requires the grid to be live).

3. Roger Cowell says:

Am I correct in your illustration of the transfer switch both meters are 120v meters since they only have only one line besides the neutral? My house in Guadalajara has 220v coming in. Do I add a 2nd 220v meter and if so how do I wire the transfer switch or do I replace the 220v meter with 2 120v meters with a transfer switch.

We are all electric in my house and are getting killed.

1. David Cunningham says:

Yes you should have two 120 V meters instead of the single 240 meter to take advantage of the lower rate. But if your usage is really high, you might still be at the DAC rate even with two balanced load meters. Look at a typical bill and see what the total bimonthly energy is. If it is around 1000 KWH for the bimonthly billing period, you should be able to get it below the DAC rate by using two meters and dividing the load equally with the transfer switch. But if it is much higher, say 2000 KWH, you would need 4 meters to do this. You would be better off in that case changing your service to Tarifa 2 with your existing meter instead of the Tarifa 1 rate that I assume you are currently paying. Tarifa 2 has a higher base rate than Tarifa 1, but it does not punish you with the much higher DAC rate when you exceed 500 KWH bimonthly per meter.