A local solar company putting supports for PV panels on our roof. No that's not a tiny man on the ladder.

A local solar company putting supports for PV panels on our roof. No that's not a tiny man on the ladder.

We got solar panels installed on our home on 22 April 2016 - Earth Day, coincidentally. We now have a full year's worth of energy data since they were installed, so I thought I would look at how much energy they produced and how much we consumed during our first year of having solar. Fair warning: this post will inevitably be pretty wonky. I'm glad to delve into technical details further in comments or elsewhere if people are interested, but for the post itself, I've tried to keep the wonk in check.

One of the primary goals of our home renovation project is to reduce our environmental impact by reducing the carbon emissions associated with operating the house. In addition to reducing our energy consumption through robust efficiency measures, we decided that it also makes sense to generate our own renewable energy on-site to power the house. Since we're in the Pasadena, CA area, -- with 286 sunny days per year -- rooftop solar panels made sense for on-site renewable generation.

Usually it makes more sense to reduce consumption through efficiency measures (and conservation) before adding on-site renewables. Saving a kilowatt-hour (kWh) of energy through efficiency is often cheaper than generating that same kWh through a solar electric photovoltaic (PV) array. However, at some point after the "low-hanging fruit" efficiency measures have been completed, the price of reducing a kWh through efficiency matches the price of generating a kWh with a PV array, and at that point, it makes more financial sense to switch from reducing energy demand (efficiency) to changing the source of your energy supply (renewables). This break-even point is changing as the price of solar panels falls lower and lower.

In our case we knew from the beginning that we wanted a PV array, but we didn't want to wait until after the renovation was complete before installing it. We wanted to switch to renewably-generated energy as soon as possible, so we had the array installed before even beginning the renovation. This meant we had to size the array before knowing exactly how much energy our house would use. We intentionally oversized the array, allowing for excess generating capacity to power potential electric vehicles and to cover the energy consumption of a more typically-behaved household in the event that we sell the house later. 

This is Carlos, who runs the company that installed our panels. He's a badass.

This is Carlos, who runs the company that installed our panels. He's a badass.

As part of the PV installation, our old Zinsco electrical service panel got replaced with a new 200-amp one.

As part of the PV installation, our old Zinsco electrical service panel got replaced with a new 200-amp one.

Here's what the new inverter and service panel look like after installation.

Here's what the new inverter and service panel look like after installation.

We had a 4 kilowatt (kW) array installed in April 2016. It went online in May 2016. It was a magic moment when got the go-ahead from our utility and switched the inverter on, and saw the digital readout telling us that we were sending power back to the grid because we were generating more electricity than we were using. From May 2016 until May 2017, our house (and its occupants) consumed 913 kWh of electricity. During the same period, our PV array generated 5413 kWh, 593% of what we consumed.

Untitled drawing.jpg

But of course, electricity is usually not the only form of energy that a home consumes. In our case, we also consume natural gas, because we currently use it to heat our water. During the same period, we used 58 therms -- or 1699 kWh -- of natural gas. This brings our total household 12-month energy consumption to 2613 kWh. The 5413 kWh of generation represents 207% of our total consumption; we generated about twice as much as we consumed. There are more thorough and complex performance metrics that can be used (site vs. source energy, kilograms of carbon dioxide equivalent averted, adding water consumption, etc.), but for now, this is good enough.

Screen Shot 2017-09-04 at 7.14.05 PM.png

We generated more energy than we consumed for nine out of the twelve months, with Dec - Feb being the exceptions. The reduced generation during these months was expected, but the increased use was a bit of a surprise. We were doing a lot of cooking and construction work in these months, so I think that accounts for most of the difference.

These days, homes that generate as much energy as they consume -- so-called "Zero Net Energy" (ZNE) homes -- are all the rage. And not without reason. It's great to be able to tell people you live in a ZNE home, even if there's inevitably a tinge of smugness and superiority entailed in doing so. Another approach that's functionally equivalent, but not as easily bragged about, is to purchase grid-delivered electricity that is generated at a large-scale PV or wind-turbine facility far away. This is a good option for the large number of households for whom on-site generation doesn't make sense, and in fact it's likely more efficient (and cheaper) than on-site PV because it makes better use of economies of scale. However, from my perspective, interacting daily with the equipment and forces that generate the energy that you use makes all the difference. It removes some of the abstraction of the grid and the unseen far-off place, and makes energy -- an inherently abstract entity -- a little more relatable.

In time, I think ZNE homes will just be referred to as "homes" -- meaning that every, or nearly every, home will eventually become ZNE. California is already on this path. By 2020 all new homes built in California are required to be ZNE. The formulas that the state uses to evaluate whether or not a home is ZNE, as well as individual household behavior, will mean that every home built after 2020 will not necessarily generate as much energy as it consumes. But on aggregate, they should be close enough to net-zero that their overall impact on the electric grid will be negligible.

I am curious to see how our home's generation vs. consumption changes as the renovation gets closer to completion. Our first 12-month period of having solar is a little unusual because our house is a construction zone. We're running power tools for hours on end, but we also don't have any central heating or cooling system (we use a space heater and portable air conditioner), but also we don't have an oven, but also the house is not well insulated...all that to say, there are a lot of balls in the air right now, so it's hard to guess whether our overall consumption will be significantly different once the renovation is complete.


We're replacing the gas water heater with a heat pump model that should use significantly less energy, but we're also adding a whole house space conditioning system, new kitchen appliances, etc. Who knows which way it will all wash out. I suspect that after everything is done we'll use about as much energy as we do now, or slightly less, but we'll be much more comfortable and have a lot more services.

The more important thing is that after the renovation, whoever lives in this house will likely use a lot less energy than they would living in a typical house. Frankly, it doesn't really matter that much where we live -- we will always be low energy users. It's just the way we are. The real energy savings and carbon reduction potential comes from the (normal) people who don't think about energy or climate constantly -- they are the people who use the energy and whose reduced consumption will make a big difference.

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  • Hi Chris -

    It would be great to get a one0year later update on your home and systems performance.


  • @Leo Wow it sounds like you have quite the PV array. Can't quite discern, but sounds like you're close to (or at?) ZNE?

    As for the 76 kWh/month, I feel the same way when I read that the average US household consumes over 22,000 kWh a year: how is that possible? Granted California has some of the most mild climates in the country, but even so? Collectively, we have a lot of energy out there just begging to be saved.

  • @ Jonathan

    1. Cost of PV array was about $15k before 30% federal tax credit.

    2. The financial payback for our system is not very compelling right now. Our utility bills gas + electricity averaged less than $30/month before we did anything on the house, because of our -- as you suggest -- atypical behavior. It's really hard to justify much at all when the total potential monthly savings is only $30. 
    3. We looked a bit at storage at the time. But it wasn't really ready yet, and there were regulatory hurdles from the utility disallowing "islanding" your house during grid outages, for fear of electrifying the lines while they're being repaired. Having storage but not being able to run your house off of it during an outage defeats much of the purpose. My understanding is that since then this has changed and islanding is possible. If we were doing it now I would consider storage. This is especially true given the recent availability of time of use (TOU) rate structure for residential customers, which could allow you to save energy during periods of excess generation and sell it back to the grid during a period of high use when prices are high (e.g., extremely hot summer days here in southern California).

  • @ Brennan

    Hey thanks! Yeah, it's time for an update on the energy consumption now that I've installed the mini-split but have yet to completely air seal the house. Let's just say that there's been some rebound effect. :)

    I'm jealous of your area's community choice aggregation (?) options. I hope that comes here to SoCal soon.

  • @Grey re: estimated vs. actual production

    Good question! The estimated first-year annual production was 5654 kWh, actual production was 5413 kWh.

  • WOW! 913kwh/12= 76kwh/month. How is that possible?

    I live in central Minnesota and started with manual adjust ground mount of 7kw PV  a few years ago.

    Then added 9kw with that feeding to the local Rual Electric Coop distribution system Oct. 017.

    If I do a little extrapolation to include 12 months it produced 21,200 kWh/year=1767/month average.

    I did ground mount because of shading issues on the house and I had a full sun 24/365 location 

    450 feet from the service so ran as high a DC voltage (to limit wire size and loss) into the garage mounted SMA inverters with "secure outlet" for back up power at least when the Solar World modules are producing. On average we get 196 sunny days. So your getting (5413kwh/4)/286=

    4.732 kwh/kw/sunny day. I would be at (21,200 kWh/16)/196= 6.760 kWh/kw/sunny day. A difference like this does not seem possible at a percent increase per sunny day of

    6.760-4.732/4.732= 43%. My extrapolation of 9 months data to 12 months (based on previous 7kw data) of the 16kw can't be that off, maybe 2 or 3 percent.

    Please check my math if you care to.

    The boost in sunny day production could have to do with:

       -Single axis manual seasonal adjust done 2 to 6 times a year

       -moudule shading/dust/dirt/snow etc

       -north/south orientation

       -DC and AC loss from module to production meter including line/connection and inverter loss.

       -meter inaccuacy?

       - module conversion efficiency including module rating, module temperature

       - reflective snow boost in winter 

       -average actual radiance per sunny day due to locations atmosphere.


    I even-though ,because off a net meter change a year ago, I pay extra for the privilege sending

    "Reverse Power" back to distribution lines of 7kw grandfathered, 9kw under new rules of 9kw-3.5kw = 5.5kw x $2.40/kw = $13.20/month, I chose to add 9kw for similar reasons such as 2 EV's

    at 6000 to 8000kwh/year. Also the house is all electric with off peak DHW, ground loop geothermal heating and cooling ,which in Minnesota could give me a heat/cool/DHW 

    yearly load of 10,000kwh.

  • Hi Jim,

    Thanks for your well thought-out and written piece.  I agree with your comments on doing efficiency first before (on-site) renewables.  We have a fully weatherized, nicely efficient home but, for various reasons, went with (grid-delivered) green electricity.  A few questions if you will:

    1. What was the "out the door" price for the solar install? 

    2. Based on a year's worth of (perhaps atypical) use and PV generation data, have you calculated a rough payback period ? 

    3. It sounds like you don't have battery storage, but just feed excess power back to the grid.  Did you consider doing on-site storage?  It's pricey, but did you look at systems like Tesla's Power Wall?  Consider PV roof shingles rather than rack mounted?  I'd be interested in your thinking on them.  Thank you.  Jonathan

  • Hey Chris, y'all are kicking butt!! I'm sure your new HVAC won't use any energy, because you won't turn it on, right? ;) What a cool, cool project. 

    On a related note, our whole region just created a community power agency (Monterey Bay Community Power), which provides 100% renewable electricity. Every customer of PG&E in the region is automatically enrolled, billing/distribution remains through PG&E, same or lower cost per kWh. This is making me really happy that we installed a mini-split heat pump for heating and cooling our sieve/house; high-efficiency natural gas isn't looking so pretty when its faced against 100% renewable electricity (of course we still have gas tankless DHW and cooking...). 


  • Curious to know how the kWh production turned out relative to what the installer indicated you should expect. 

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