Prepping

It’s been two weeks since the hurricane passed by and screwed up my analysis of the new solar system. Let’s look at the numbers from the past two weeks’ production:

Overview

The house used 621.4 kwh during the period, or an average of 44 kwh per day. The lowest demand day was 36.2 kwh, and the highest was 60.9 kwh. The system generated a total of 663 kwh, which works out to an average of 47 kwh per day. The low was 36.6 kwh, the high was 57.6 kwh.

Overall, it appears as though my system is properly sized as far as generating capacity. How well will we do for the main intended use? That depends on storage capacity. As I said, I have been using the power company as a battery of sorts, in that I send power to the grid during the day, then take power from the grid at night. That begs the question: What if the grid is down? To answer that, we have to dive a bit deeper into the numbers.

A Closer Look

Our highest use days are when we are both home, and when it is hottest outside. This isn’t surprising, since the air conditioning runs more on hot days, especially when we are home. On top of that, the clothes dryer uses a HUGE amount of power, and laundry days are also heavy draw days (the dryer uses more power than both air conditioners combined).

The average solar intensity at my location for any 24 hour period is 200 watts/square meter for an average production of 47 kwh. The best day for production was 60.9 kwh, with an average solar intensity of 232 watts/ square meter. No surprises there- 16% more sun gets you 30% more power.

The good news here is that the hottest days also tend to be the sunniest days, so heavy use days caused by the cooling system are also the highest production days, with the result being the largest daily deficit was only 7.4 kwh. The largest surplus was 13.4 kwh.

What this means is that, should the grid go down, this surplus is likely wasted power. In our case, we would be wasting about 3 kwh per day. Saturday, August 10, and Sunday, August11 were our worst two days:

Saturday, August 10

39.9 kwh produced, 45.9 kwh used, 6.0 kwh deficit

The lowest outdoor temperature that day was 77 degF, with the high being 92 degF. Average solar intensity was 166 watts/square meter.

The air conditioner compressors ran for 6.5 hours

During the day on Saturday, we did 2 loads of laundry.

Sunday, August 11:

44.2 kwh produced, 51.6 kwh used, 7.4 kwh deficit

The lowest outdoor temperature that day was 77 degF, with the high being 93 degF. Average solar intensity was 190 watts/square meter.

The air conditioner compressors ran for 6.6 hours

During the day on Sunday, we did 3 loads of laundry. One of them was bedding, which takes more time and power to dry.

What about the batteries?

The day that saw us import the most from the grid was Sunday, August 18. We imported 25.9 kwh from the grid on that day, with most of that being the hours of 6 pm to 10 pm. This was while we were doing laundry.

My Analysis

We are producing plenty of power for our needs. We are keeping the batteries charged at 85% so that there is plenty of power available for a grid outage, and we are using the grid like an extra battery, storing our excess production during the day in the grid for use at night. If the grid were to go down, we would likely have enough battery power to run the house as normal, but we would likely have to restrict the activities that use the most power (laundry, cooking) for daylight hours. Additionally, there would have to be a restriction on the amount of laundry- no more than one load a day.

Other than that, we appear to have sufficient total capacity, and sufficient battery storage to run the household without interruption.

Interestingly, as a side note: The area near my house has a lot of solar power. Doing the area survey with my drone, I noted that 23% of the homes within a mile of my house have solar systems installed.

I was trying to get some solid data on the performance of my solar array, but we were interrupted by Hurricane Debbie. Still, negative data is still data.

• When it’s a typical Florida sunny day with sunshine until the arrival of our typical afternoon thunderstorms, my 10 kWDC array generates about 58 kwhAC per day.
• Two days before Debbie arrived, it was getting quite cloudy. We only generated 32 kwh.
• As the rains began to arrive on Sunday, our output dropped to 18 kwh.
• It rained all day on Monday, and we only generated 17 kwh. Still, generating that much power while simultaneously getting more than 3 inches of rain is a feat.

It’s sunny today, so we are back to full power. I want a week of solid, typical weather before I lay it all out.

We lost power once for about 15 minutes, in addition to a handful of blips, where the power was out for less than a second. The Powerwalls transferred us to battery power in about 1 or 2 seconds. Just long enough that we had to reset the clocks.

More data to come.

I previously recommended the use of a controller for your water heater as a way of controlling power consumption. Now that I have the solar up and running, the Tesla app is a great way to monitor and check up on your electrical loads.

Now that I have the ability to do this, I am convinced that the Aquanta water heater controller doesn’t actually DO anything. Whether I am home or not, using hot water or not, the amount of electrical power that the Aquanta is claiming I am using to heat water doesn’t change, at about 15kwh per day. The only time the amount of power the Aquanta was claiming went down is when we put the system in “Away” mode- telling it to turn off the hot water while we were out of town. At that point, the Aquanta reported that we were using about 1 kwh per day. The only problem? The amount of power we were using for the entire household didn’t change, according to the Tesla app.

At this point, I began suspecting that the Aquanta is a random number generator. It reports to you how much money it is saving you, but isn’t actually doing anything. I decided to test it and see what was going on. I am going to make a serious accusation here, so let me state that I did all voltage readings using a FLUKE 115 True RMS multimeter.

I measured the voltage across the output of the Aquanta while it was in “efficient” mode with a green LED showing and got a reading of 249 VAC. The water heater uses power consumed of 4500 watts (4.5kw), so a little application of Ohm’s law results in 18 amps and 14 Ohms of resistance. We can directly use Ohm’s law because the water heater has no capacitance or inductance, it is a purely resistive load.

Then I put it into “Away” mode. The LED on the unit turned red, so I measured the output voltage and got 70 VAC. Another application of Ohm’s law using the same 14 Ohms we calculated before tells us that the water heater is now using 5 amps and 50 watts of power. Take that 50 watts of power and multiply it by 24 hours in a day, and you get about 1 kWH per day.

So it doesn’t shut off the water heater, it merely lowers the voltage from line voltage down to about 70 VAC. I am not sure when or how it does this, other than in vacation mode, but I don’t think I am saving any power.

I just don’t think that this is a good unit to use if you want to save power, so I am withdrawing my recommendation of the Aquanta unit. My recommendation is to either use an electro-mechanical water heater timer, or a smart pool pump relay.

So we are four days into operation of the solar equipment. Right now, we are operating on a limited basis, more on that in a moment. I am not home because we are on the road, but both air conditioners, all vampire loads, and the water heater are running.

Output

We chose a system that is 10 kilowatts DC. That translates to about 8600 watts AC. The system begins generating electricity at about 9am, and reaches a peak of 8kw at around 10:30. By 1pm, the Powerwalls are fully recharged from the night before. At that point, there is no place else for the generated power to go, so system output drops to match whatever the house is using. More on that later.

We haven’t used any electricity from the grid since the sun came up on Friday morning. That indicates that I have enough panels for my house.

Storage

There are two places that I can store the power I generate. One of them is in the Powerwall, which has a total capacity of 27 KWh. Right now, I am maintaining a minimum of 30% as emergency backup, and using the rest to compensate for lower output at night or overcast conditions, or to make up for transient high loads, like when both air conditioners and the water heater are running at the same time. The advantages of using the battery are that the power stays within my home, and losing the grid means that I can still access it. The disadvantage is that the upfront cost of batteries is high.

The second place that I can store generated power is in the grid. The electric company buys my excess power in the form of credits that I can redeem when my system can’t keep up with the loads that I am placing on it- nighttime, stormy weather, or when loads simply exceed what I am producing. The advantage of this is that the upfront cost is low, but the disadvantage is that it relies on the electrical grid for redemption.

I can’t use the grid as storage because I don’t yet have permission to operate (PTO) from the power company. I should get it within two weeks after our final electrical inspection, which is supposed to be this week. So we should be fully operational by August 9.

Results So Far

Each day, we are generating between 35 and 45 kWh before panel output is reduced when the batteries are full. The solar energy being generated is directly running the house during the day, with the rest charging the Powerwalls, which run the house at night.

The water heater is using 4 kw when running, the upstairs AC is using 1.5kw, the downstairs AC uses 2.7kw, and the rest of the house uses 0.3kw. Since the ACs and water heater don’t run all of the time, the panels are more than capable at this point of keeping up by charging the batteries during the periods when the large appliances aren’t running.

Once we get our PTO, I will know more.

The solar power system was installed this week, We turned it on this morning. 24 panels, each capable of supplying 420 watts, for a total capacity of 10 kW. We can’t yet sell the power back to the power company, because they haven’t yet approved our application. Until then, we will run off of batteries and solar, with the excess being given to the power company free of charge. Hopefully, that will change within a week or two.

The install took two days, even though it was supposed to only take one. On day one, the team got the Powerwalls mounted, and 21 of the 24 panels on the roof before an incoming afternoon thunderstorm stopped work for the day. On day two, they got the final three panels up, ran all of the conduit and wiring, then shut power off to the house for about an hour so they could make all of the connections. They turned the system on, but that was at 1700, after it began raining again, so we didn’t generate any solar at all yesterday.

At 1000 this morning, we were generating 5 kW from solar while only using 0.5 kW, with the 4.5kW of excess going into the batteries. Our Powerwalls are already charged to 50% of capacity, and we have already generated 6.5 kWh purely from solar.

I will revisit the numbers within a week or so. It’s still to early to talk about how well the system is going to meet our needs.

All permits for the solar project have been issued and approved. Now we need to get on the installation schedule.

IcyReaper wants to know more about the panels that we selected. I checked through the previous posts and realized that I hadn’t talked about them at all. The PV panels are the heart of any solar system, so let’s review them.

When considering which manufacturer I wanted to go with for our PV panels, we wanted to go with a large, reputable panel manufacturer. SunPower, REC and Panasonic are three manufacturers widely known for producing some of the highest quality solar panels with low degradation and good warranties. For that reason, they cost up to 30% more, but I wanted reliability.

Although REC was originally a Norwegian company with a good reputation in the industry, their panels are actually made in Singapore. The company has been bought and sold several times, and is now owned by another company headquartered in India. The company makes panels with higher efficiency and more output at up to 470 watts per panel, but they are more expensive than some of their slightly less efficient models. It winds up costing more to get the extra output than it would to simply add more panels, and roof space isn’t an issue for me, as I have a lot of sun facing southern roof to work with.

We decided on the REC Alpha Pure 2 panel for our PV panel. The spec sheet can be found here (pdf alert). The panel is 1.8 meters by 1 meter, and has an output of 420 watts with an efficiency of 22 percent. It has a 20 year warranty and has been tested as retaining 92% of its rated output at 25 years. If the REC panels are installed by a certified installer, the warranty is extended out to 25 years. You can read a review on REC panels here.

REC solar panels operate at high efficiency, have a low 0.25% annual degradation rate and come with an excellent 25-year performance, product and labor warranty. As I mentioned in earlier posts, the panels are designed for 140 mile per hour winds and hailstones of up to 35mm. In my book, that made them durable enough to withstand some serious weather.

The panels look nice, because they are pure black with no light colored lines. They simply look better than the older panels.

The disclaimer: I don’t advertise, and receive nothing for my reviews or articles. I have no relationship with any products, companies, or vendors that I review here, other than being a customer. If I ever *DO* have a financial interest, I will disclose it. Otherwise, I pay what you would pay. No discounts or other incentives here. I only post these things because I think that my readers would be interested.