First Year of Rooftop Solar Power and Heat Pump: Re-Visiting Economics

After I presented details for selected days, I am going to review overall performance in the first year. From June 2015 to May 2016 …

  • … we needed 6.600 kWh of electrical energy in total.
  • The heat pump consumed about 3.600 kWh of that …
  • … in order to ‘pump it up to’ 16.800 kWh of heating energy (incl. hot tap water heating). This was a mild season! .
  • The remaining 3.000kWh were used by household and office appliances, control, and circulation pumps.

Disclaimer: I am from Austria –> decimal commas and dot thousands separator :-)

The photovoltaic generator …

  • … harvested about 5.600kWh / year – not too bad for our 4,8kW system with panels oriented partly south-east and partly south-west.
  • 2.000 kWh of that were used directly and the rest was fed into the grid.
  • So 30% of our consumption was provided directly by the PV generator (self-sufficiency quota) and
  • 35% of PV energy produced was utilized immediately (self-consumption quota).

Monthly energy balances show the striking difference between summer and winter: In summer the small energy needed to heat hot water can easily be provided by solar power. But in winter only a fraction of the energy needed can be harvested, even on perfectly sunny days.

Figures below show…

  • … the total energy consumed in the house as the sum of the energy for the heat pump and the rest used by appliances …
  • … and as the sum of energy consumed immediately and the rest provided by the utility.
  • The total energy ‘generated’ by the solar panels, as a sum of the energy consumed directly (same aqua bar as in the sum of consumption) and the rest fed into the grid.

Monthly energy balances for photovoltaic generator: Energy used directly versus fed into grid

Monthly energy balances: Electrical energy used in total and energy used by the heat pump.

In June we needed only 300kWh (10kWh per day). The PV total output was more then 700kWh, and 200kW of that was directly delivered by the PV system – so the PV generator covered 65%. It would be rather easy to become autonomous by using a small, <10kWh battery and ‘shifting’ the missing 3,3kWh per day from sunny to dark hours.

But in January we needed 1100kWh and PV provided less than 200kWh in total. So a battery would not help as there is no energy left to be ‘shifted’.

Daily PV energy balances show that this is true for every single day in January:

Monthly energy balances for photovoltaic generator in January 2016: Energy used directly versus fed into grid.

We harvest typically less than 10 kWh per day, but we need  more than 30kWh. On the coldest days in January, the heat pump needed about 33kWh – thus heating energy was about 130kWh:

Monthly energy balances in January 2016: Electrical energy used in total and energy used by the heat pump.

Our house’s heat consumption is typical for a well-renovated old building. If we would re-build our house from scratch, according to low energy standards, we might need only 50-60% energy at best. Then heat pump’s input energy could be cut in half (violet bar). But even then, daily total energy consumption would exceed PV production.


I have covered economics of the system without battery here and our system has lived up to the expectations: Profits were € 575, the sum of energy sales at market price  (€ 0,06 / kWh) and by not having to pay € 0,18 for power consumed directly.

In Austria turn-key PV systems (without batteries) cost about € 2.000 / kW rated power – so we earned about 6% of the costs. Not bad – given political discussions about negative interest rates. (All numbers are market prices, no subsidies included.)

But it is also interesting to compare profits to heating costs: In this season electrical energy needed for the heat pump translates to € 650. So our profits from the PV generator nearly amounts to the total heating costs.

Economics of batteries

Last year’s assessment of the economics of a system with battery is still valid: We could increase self-sufficiency from 30% to 55% using a battery and ‘shift’ additional 2.000 kWh to the dark hours. This would result in additional € 240 profits of per year.

If a battery has a life time of 20 years (optimistic estimate!) it must not cost more than € 5.000 to ever pay itself off. This is less than prices I have seen in quotes so far.

Off-grid living and autonomy

Energy autonomy might be valued more than economical profits. Some things to consider:

Planning a true off-grid system is planning for a few days in a row without sunshine. Increasing the size of the battery would not help: The larger the battery the larger the losses, and in winter the battery would never be full. It is hard to store thermal energy for another season, but it is even harder to store electrical energy. Theoretically, the area of panels could be massively oversized (by a factor – not a small investment), but then even more surplus has to be ‘wasted’ in summer.

The system has to provide enough energy per day and required peak load in every moment (see spikes in the previous post), but power needs also to be distributed to the 3 phases of electrical power in the right proportion: In Austria energy meters calculate a sum over 3 phases: A system might seem ‘autonomous’ when connected to the grid, but it would not be able to operate off-grid. Example: The PV generator produces 1kW per phase = 3kW in total, while 2kW are used by a water cooker on phase 1. The meter says you feed in 1kW to the grid, but technically you need 1kW extra from the grid for the water cooker and feed in 1kW on phase 2 and 3 each; so there is a surplus of 1kW in total. Disconnected from the grid, the water cooker would not work as 1kW is missing.

A battery does not provide off-grid capabilities automatically, nor do PV panels provide backup power when the sun is shining but the grid is down: During a power outage the PV system’s inverter has to turn off the whole system – otherwise people working on the power lines outside could be hurt by the power fed into the grid. True backup systems need to disconnect from the power grid safely first. Backup capabilities need to be compliant with local safety regulations and come with additional (potentially clunky / expensive) gadgets.


14 Comments Add yours

  1. Here’s the one big thought I currently have: knowing what I know now about (1) what you’re doing and (2) the stupid direction my province is heading with energy creation, if I could redo my career it would likely be as an electrical engineer and the focus of my work would be on teh Research, development and marketing of smarter energy efficiency products, a thing that my place is sorely lacking in. At the moment we do a great job here on the passive stuff, particularly in regards to minimizing heat loss. Where we lack is in the obtaining of inputs. Typically we are still stuck in direct heat either through combustion of furnace oil or direct electrical heat. We could do so vert much better and I sense a ready market given the appropriate marketing plan.

    1. elkement says:

      While I think that would be a great alternative career fitting you perfectly, I suppose you do a great job already in your current career – also in that particular respect ;-) … training the future teachers that will instill fascination for STEM and possibly also renewable energy in children! One of my colleagues in the degree program in sustainable energy systems I completed three years ago was a physics teacher – and he told me later that he put a lot more energy-related stuff on the agenda in class now.
      Things that make me hope are reports about changing attitudes in the younger generation – like car sharing and just having access to a car instead of owning it (and enyoing the iconic symbol of freedom) getting the new normal.

      I think what we (a country, a community..) needs is a bottom-up movement of genuinely interested people … who are motivated by something beyond low heating costs and who actively want to do something instead of waiting for somebody else’s initiative. People with skin in the game. Austria had been one of Europe’s solar thermal energy pioneers (by far the largest area of solar collectors per capita, together with Cyprus, in the 2000s) – thanks to a grass roots movement by DIY enthusiasts and tinkerers building their own systems. The same went for early pioneers in ground-source heat pumps during the 1970s oil crisis. Funded research projects by universities and companies came later (exactly like Nassim Taleb describes true innovation – tinkerers drive innovation and Soviet-Havard planned R&D follows suit…)

      We – as a company – would not exist, were it not for people that tinker with their heaters and control units in a way people were fond of playing with their cars or their home cinema systems before.

  2. Your report is right in line with everything my husband and I have been reading over the past ten years for off-grid energy production here in Canada. There seems to be no big way to shift off grid, so we look more and more at the small steps we can take to reduce our heating needs. After spending almost one year in the house after the upgraded insulation, we can say that even in this mild winter we had cost savings. We also noted that we didn’t begin to use the air conditioner unit until most of our neighbours had been 6 to 8 weeks using, and even then we’ve had little need to run it but rarely. Another thing we experimented with is a 1/2 inch thick slate tile in the two small bathrooms. The flooring effectively holds the heat from the forced air furnace in the winter and the coolness from the air conditioner, affecting the temperature of the entire room for long after the furnace fan stops. We’ve read about flooring and stone fireplaces and walls being used as a thermal mass for collecting and holding heat, but we didn’t imagine the impact it could make. (Some off-grid houses built in Canada have boasted that their stone fireplaces have maintained indoor temperatures for as long as four days in winter without a fire.) When we renovate the main floor of the house, much of the new flooring will be stone tile.

    1. elkement says:

      I can imagine how frustrating it would be to heat with solar power in Canada: Austria is at the latitude of the US-CA border – so with less sunshine but a colder climate than we have results can only be more chilling :-)
      Here, air heat pumps + PV are sometimes marketed as if you could really heat a typical home easily … although it should be obvious that you cannot, just by comparing publicly availably numbers for monthly solar energy harvests and your power or gas bill. It might work for a small passive home but I feel there is the typical rebound effect: The better insulation gets the larger the houses people want to build – heat load stays the same.
      As for flooring – very intresting! Here, screed used with floor heating loops is the main thermal mass in place in modern homes (and there are lots of research projects using additional blocks of concrete, masses that could be ‘switched on’ when heat should be released…). Thermal mass is a must with heat pumps as a (brine/water) heat pump always powers at full load, like: 2 hours on and then 2 hours off if half of its rated power is required, and temperature variations are then evened out by the thermal mas. We prefer water’s thermal mass – a water buffer tank for the heating floor loops, that can also be used for passive cooling. It also gives you also more versatility re control logic. Its only downside is size and space in the ‘boiler’ room – and home owners don’t want to sacrifice too much of their space in their modern homes for the ‘technical room’ … that has to accomodate to much gadgetry, like solar power inverters, central ventilation, IT network or home automation distribution cabinets…
      Of course, from aesthetics and ‘user experience’ perspective, floor tiles trump every other thermal mass I guess :-) From what I read about heating systems in North America I also concluded that heat distribution in the house is done rather with air (like air/air heatpumps instead of our air/water or brine/water heat pumps) than with water pipes, so that a hot (or cool) water buffer tank is not an option (or at least you could lose energy as one more water/air heat exchanger is required).

      1. One of the most vexing issues of increasing insulation and securing a vapour seal on exterior walls is humidity control in the building. We have to monitor the moisture content in the home fairly closely, use a heat recovery ventilation unit, fans and direct venting, as well as a dehumidifier, even after increasing air ventilation via the attic. It feels like an on-going experiment, as there is little definitive information available in the residential construction sector to provide guidance. You had once commented that it wasn’t a common problem in your area to have moisture and condensate inside the home and in the attic; I wonder what the impact would be if we changed from a forced air system to a boiler system, and installing it as you describe (such as, within the flooring). The heating distribution could be quite different.

        1. elkement says:

          I think air versus water distribution should not matter in this respect as in both cases the air inside the house is separated from the air outside … it is air/air heat exchange versus air/water heat exchange. Modern buildings have heat recovery and ventilation here, too. One thing I like about floor heating is reduced convection and more heating by radiation – and thus less distribution of dust.

          Humidity is an issue with the old buildings here: The small farm former houses built by poor people ~100 years ago. Moisture enters the walls from the ground as houses have no cellar (or only a small one beneath a single room), and they had not been insulated properly against water (and cold) from below. If you heat-insulate such a building’s facade ‘too much’ then the moisture might not be able to leave the house. I have seen even building experts advising against state-of-the-art heat insulation because of that, even if houses are renovated according to regulations.
          We had our house heavily insulated nonetheless (but only made changes to the the facade, not the floor) and insulated against moisture from below using an injection technique (injected liquid should spread out and form a thin plastic film, an insulating layer intersecting with the wall, stopping rising moisture). In summer we use a small dehumidifier occasionally (we call R2D2 because of its shape) since last year – since we can solar-power it :-)

          Some small remaining humidity can be ‘window-vented’ away easily, and I accept this as a trade off for lowered heating demands: We need considerably less energy now although the usable area of the house has more than doubled, compared with the state before renovation. In our worst year, after we had removed ‘natural insulation’ – decades old dusty straw – from the old attic, we needed more than 30000kWh for heating only 75m2. Now we need only 20000kWh for 185m2. (I know I should convert to other units ;-)) And our facade is not water-tight, it need not be in our place … we use a so-called ‘open’ facade with special varieties of styrofoam (with tiny ‘holes’) and plaster that lets moisture diffuse out of the building … material developed especially for renovated buildings like ours.

          1. Very interesting. In new home builds it is required to place thick plastic sheeting below concrete slabs at or below grade, to prevent water penetration into the building. The issue in older homes, like ours, is that ground water is wicked into the home. We also used an insulation that allows the water to leave the structure, but because of its increased cost and newness on the market, we usually see residential upgrades that apply a non-permeable foam insulation to the outside of the building. It’s unfortunate, but some practices here can cause more harm than had nothing been done to ‘improve’ the house.

            I had a glimpse at non-commutative geometry this year–something that is over my head at this time, but I keep wondering how nc algebra could be used to evaluate the many variables that interact in heat transfer through air and solid materials. Maybe I’m just being hopeful that there’s some work for me to do in this area. ;)

    2. Hi there Michelle, thought I’d co-opt Elke’s space for a bit to check in with you. I am assuming that the foray into the world of science and math is going well and am always anxious to hear about it because of the growing realization of the need to add the letter “a” to the old STEM group. Given that the thinking associated with the arts and humanities is already strong with you it’s exciting to see how the science/math thing is augmenting it.
      If you’re interested, check out STEAM education next time you’re connected to your university library system. There’s some interesting work happening here and there in our country’s schools and I think it’s becoming a real movement, jut just a phrase.

      1. elkement says:

        (I am happy to host a Canadian chat ;-))

      2. Thanks Maurice! I will check it out. I also am trying to get another blog up and going, but oh my… life is so busy right now! But yes, it is going well. I feel right at home in the math classes, especially as I have mostly filled in what was lacking and what I forgot from basic high school.:) I will check out STEAM (how intriguing).

        1. I know how you feel. My poor old blogs are languishing too. It’s lack of time, not lack of ideas. Still, work is fantastic these days😀

          1. Great to hear about work and life.

  3. So, what’s the conclusion here? Your PV system is supplying 30% or your needs and you couldn’t (directly) use 70% of what you generated, for lack of storage? [Did I miss something? Were you able to run your meter back during times of surplus?] If that’s the case, what do you estimate to be the time-to-payback on the investment? I’m sorry to cut-to-the-chase on this but I think it’s important to know what the real short and longer-term future holds for this technology. Would you say that it’s not overly cost-effective now but that once Tesla gets storage technology where it needs to be, the future might be a bit brighter?

    1. elkement says:

      70% were sold to the utility at the low market price, the fed-in energy is metered separately; meters don’t run backward (this would be much more attractive :-))
      It’s not only for ‘lack of storage’: You cannot save electrical energy generated in summer for winter. A battery would increase those 30% to 55% at best.
      My personal conclusion is that the system is economical without batteries as 6% ROI is fine for me. The payback period is about 17 years, but I rather compare the investment to lending money to banks that might charge negative interest rates (in Europe).
      The battery would not yet be economical, as it hardly pays itself off until it has to be replaced. Systems have to become much cheaper or have to live much longer. The latter would also be better in terms of ecological footprint as the battery is the worst part of the system in that respect. As many companies try to follow suit and manufacture batteries now like Tesla, I hope that we will see innovations and drops in costs.

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