Over a year ago we installed a 10.4 kW grid tie solar system.   You can read about the shakedown period here.   This post will cover all of 2019, the first full year the solar system was operational. [Technically, it covers the period between Dec 12th 2018 through Dec 11th 2019, as Duke Energy bills us mid-month.]

In this 12 month period, we consumed 16,695 kWh of power at our house (a 3 bed 2 bath ranch with all electric utilities + two electric vehicles). [We used 16,796 kWh the prior 12 months, so our usage did not appreciably change due to installing the solar system.]

Of this total electrical usage, our solar system produced 15,252 kWh or 91.4% of our total electrical usage, while we purchased 1,443 kWh from Duke Energy and the electrical grid. [There are no economic benefits to producing more than we use, so the ideal system would hit 99.9% of actual usage. We were aiming for 90% when we designed our system.]

Over the year, we paid Duke Energy $314.49 ($130.80 for required connection charges, and $183.69 for the electricity we imported from the grid, averaging 12.7 cents per kWh.)   This compares to our previous yearly cost for power of $2,211.13, giving a yearly cost savings of $1,896.64.   After the EIC tax credit, our solar system cost us $17,439.20, which gives a payback period of 9.19 years. (I’m deliberately ignoring the interest we could have earned by investing the money we paid for the solar system in the stock market, which counteracts the fact that I’m also ignoring the fact that Duke energy raises their rates every so often.)

As the solar system is expected to have a working lifespan of 15-25 years, any energy it produces after the payoff period will be pure profit. So yes, a solar system does make economic sense, in addition to the environmental, social and political benefits.

Question: “We’re getting whole house solar soon. We are wondering if we should get an electric or solar pool heater?”

To answer this question, I want to keep my terminology clear. I will use “solar-thermal panels” to refer to the rubber roof-mounted panels that have pool water pumped through them for traditional solar pool heaters. I will use “PV solar panels” to refer to photovoltaic panels that generate electricity directly from sunlight. Any pool heater powered by electricity must be a heat pump pool heater (a electric resistance heating element is incredibly inefficient when compared to a heat pump).

There are a lot of trade-offs involved in this question.

First efficiency: Using the sun’s heat to directly heat pool water (as with pumping pool water through a rubber solar-thermal collector on your roof) is about 80% efficient (80% of sunlight is converted to heat in the water). Commercial PV solar panels are only 18-22% efficient converting sunlight to electricity. If you use a restive water heater, this gives you 18-22% efficiency overall, which is bad. HOWEVER, if you use a heat pump electric pool heater, the heat pump uses that 18-22% electricity to move heat from the air into your pool with a 3-4X advantage, so your overall heating efficiency is actually directly comparable to a solar-thermal water heater! [The same math applies to domestic electric hot water heating…hybrid heat pump water heaters are good, traditional resistance elements are not a very efficient use of electricity.]

Second, cost: A heat pump pool heater costs money. So do the extra PV solar panels to power it (plus the up-sized inverter as your PV solar system is larger). I suspect the extra money will be slightly more than a $5,000 solar-thermal water heater arrangement, but it may not be TOO much more. It all comes down to how many kWh you plan on spending to heat your pool. (A heat pump pool heater takes as much power as a whole house AC, it’s basically an AC unit in reverse…pulling heat from the outside air and putting it into your pool.) So this is not an inconsiderable cost.

A 100,000 BTU heat pump pool heater uses 5,000 watts (5kW) when running, or 5kWh per hour of heating. If you wanted to run your pool heater for 5-6 hours every day, you would need to add 5kWh of PV panels to your roof to offset this electric usage. At $2.40 per watt installed, an extra 5,000 watts of PV panels would cost you $12,000!

However, if you only run the pool heater for a few months out of the year and are on a net-metering arrangement, you need to install fewer PV panels, as PV panels produce power all year round. So if you plan on running your pool heater for 5 hours a day only 4 months out of the year, you could get away with installing only 1/3 as many PV panels, or 1.6kW at a cost of $4,000. [You may also have to buy a $2,500 heat pump pool heater if you don’t have one already.]

Third, complexity: A solar-thermal hot water heater requires extra pipes from your pool pump up to the panels on the roof of your house and has a lot of potential leak points. Extra PV solar panels on your roof and a larger inverter don’t add much to the complexity of a whole house PV solar system, just makes it larger, and PV panels have very minimal maintenance issues in the future. An electric heat pump pool heater does require plumbing into your pool pump, but is physically located with the rest of the pool pump equipment, and doesn’t require water pipes up to the roof. Keeping the “roof system” separated from the pool system by electric wires as opposed to water pipes makes “plumbing” easier.

Fourth, roof space: Do you have enough roof space to support all the energy you want to use? Solar-thermal pool heating panels and PV panels used with an electric heat pump pool heater use roughly the same roof surface area for the same amount of heat in the pool. The PV solar panels cost more than rubber pool solar-thermal heat collectors, but have to be replaced less frequently. Heating a pool takes a lot of energy regardless of if you are doing it via photovoltaic or solar-thermal collection. Check with your solar installer to see if you have enough sun facing roof space for everything you want to do.

Fifth, ease of use / flexibility: An electric pool heater allows you to heat the pool whenever you want (cloudy weather, nights). You may need to pay the utility company for your extra use of electricity in bad solar weather, but you have the option to do that if you want. If you install extra PV solar panels to support an electric pool heater, you have the option to NOT run the pool heater and bank extra electricity, which can be used by any electric appliance in the house or an electric vehicle.

[A solar-thermal pool heater prevents you from spending extra money heating your pool at nights or in cloudy weather, but may slightly increase the power needed to run your pool pump unless you actively switch the roof mounted solar heat collectors out of the system when not needed.]

Of course, the cheapest option is to not heat your pool at all.

I’ve had a Moto X4 phone since November of 2018 (I bought it new from Google Fi) and recently the power button started failing intermittently.   Reading forums on the internet, having power buttons fail on phones is now a “thing”. Apparently phone manufacturers are cheeping out on the physical buttons to save money? Seems like a bad part to have fail.

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I recently purchased a WorkZone Wall Mount Power Board (with detachable tool holder) as an impulse buy at my local Aldi (WorkZone is the Aldi brand for household tools). I needed more light, I could use a power strip, and I liked the fact that each outlet had its own switch so that I could turn on/off battery chargers depending upon if I was using them or not.

The lights worked well, it included a nice template for placing the screws to match the slots in the back and everything was going well until I tried plugging in one of my battery chargers. The outlets were super stiff on the first plug in (not unusual for new outlets), but then they just started to feel dangerously loose. They didn’t appear to grip the plugs well or make good electrical contact. For a device rated at 1875 watts, this was concerning to me.

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Five months ago we turned on our new grid tie solar system and started to produce power.

Component Failures & Warranty Service
In the first five months, we have had two components replaced under warranty. The first was one of the 36 SolarEdge DC optimizer units (A $75 component with a 25 year warranty) that are mounted under each of the solar panels. The Solar Edge monitoring system had flagged that one of the optimizers was not reporting (and presumably also not producing power) and sent a message to our installer (3Guys Solar). They called me (before I even knew anything was wrong) and let me know they would be sending a crew out to climb up on the roof to replace it the next day, so that particular panel was down for only 2 days. The other 35 panels continued to produce power.   The crew said that they sometimes have to replace several optimizers on a house, and some houses never have any fail.

The second component to fail was the system’s main DC->AC inverter, a 10kW Solar Edge grid tie inverter (a $2,000 component with a 12 year warranty). On October 23rd I noticed on the phone monitoring app that we had no power produced, so that night I went out and rebooted the inverter.   The next day, 3Guys solar called me to report that they had received a fault code from the Inverter via the Solar Edge monitoring system and were working with Solar Edge to try and resolve the fault. The following day they called again to tell me that they would need to replace the inverter under warranty. Unfortunately, it took close to two weeks to receive the replacement unit from Solar Edge, so we were not producing solar power again until the 7th of November.

It was concerning to have the inverter   fail within the first three months, but it is covered by a 12 year warranty, was replaced within two weeks and we haven’t had any problems since. 3 Guys Solar also sent us a check for $95 to cover the cost of the energy generation lost during this period. This is a limited time program, and was not part of their original install contract, so the check was not expected but appreciated. (By my estimates they overpaid us by 20-30$ for the energy the system wasn’t producing while the inverter was down.) The warranty coverage by 3Guys Solar (the installer) and Solar Edge (equipment manufacturer) left nothing to be desired.   My expectation is that with solid state electrical equipment like this, most of the failures that are going to happen will happen early in the life-cycle (or very late in the lifecycle), and I hope the equipment will be stable now that we’ve gotten the early failures out of the way.

Power Generated & Usage

In this 5 month period, we generated 5096 kWh of solar power, and used 7405 kWh of electricity (paying for 2309 kWh from Duke, at a cost of $337.44 (-$95 credit from 3GS lowers this to $242.44)).

In the same 5 month period last year, we used and paid for 7378 kWh of electricity from Duke, (costing $977.81) so our usage appears to be closely correlated to before we had the solar system.

By averaging production between the week before and the week after, I estimate that in the 15 day period our inverter was down we should have produced an additional 540 kWh of power ( $70 of electricity at 13 cents per kWh). So without the inverter failure, we would have produced around 5636 of the 7405 kWh we used, or 76% of our total electricity usage via solar. [With the inverter failure, we were only at 69% of our power from the sun.] Our goal was to produce 80-90% of our electricity from the sun, so these numbers are close to our goal, and I hope that the sunny spring (and no more inverter failures) will raise our percentage.

From a cost perspective, because Duke Energy has a fixed customer charge, our solar cost savings is lower than percentage of power generated, and was a savings of between 65% to 75% off our electric bill.

Seasonal Effects on Future Predictions

Estimates made with less than a full year of solar data are going to be wildly inaccurate. The Aug->January time period is cooler than other months, so our AC usage will be lower, but there is also less solar production in the winter, meaning that our generation will be lower as well.   The following numbers are a guess, and are much less accurate than those I hope to calculate after the system has been working for a full year.

Over the same 5 month period last year, the solar system has saved us between $640 and $735 (depending upon if you add in the non-contractually obligated 95$ check that 3Guys Solar sent us).   This equates to an estimated payback period of   11 to 12.75 years.   (The 1.75 years, or 14% difference in payback time is driven by the 13% difference in cost savings that $95 check equates to over the 5 month period.)

If it turns out that our solar system produces a much higher percentage of our usage in the sunny spring months, or if Duke Energy raises their rates, this payback period could drop.   If we use too much AC in the summer, or Duke lowers their rates (?!?!) the payback period could increase. I’ll feel much more confident about the estimate after collecting a full year of usage and generation data. (And even then, solar generation and electrical usage can still vary year to year with the weather.)

Update: Read the 1 year report for a better set of annual numbers.