Photovoltaic Generator and Heat Pump: Daily Power Generation and Consumption

You can generate electrical power at home but you cannot manufacture your own natural gas, oil, or wood. (I exempt the minority of people owning forestry). This is often an argument for the combination of heat pump and photovoltaic generator.

Last year I blogged in detail about economics of solar power and batteries and on typical power consumption and usage patterns – and my obsession with tracking down every sucker for electrical energy. Bottom line: Despite related tinkering with control and my own ‘user behaviour’ it is hard to raise self-consumption and self-sufficiency above statistical averages for homes without heat pumps.

In this post I will focus on load profiles and power generation during several selected days to illustrate these points, comparing…

  • electrical power provided by the PV generator (logged at Fronius Symo inverter).
  • input power needed by the heat pump (logged with energy meter connected to our control unit).
  • … power balanced provided by the smart meter: Power is considered positive when fed into the grid is counted  (This meter is installed directly behind the utility’s meter)

A non-modulating, typical brine-water heat pump is always operating at full rated power: We have a 7kW heat pump – 7kW is about the design heat load of the building, as worst case estimate for the coldest day in years. On the coldest day in the last winter the heat pump was on 75% of the time.

Given a typical performance factor of 4 kWh/kWh), the heat pump needs 1/4 of its rated power as input. Thus the PV generator needs to provide about 1-2 kW when the heat pump is on. The rated power of our 18 panels is about 5kW – this is the output under optimum conditions.

Best result near winter solstice

If it is perfectly sunny in winter, the generator can produce enough energy to power the heat pump between 10:00 and 14:00 in the best case.

2015-12-31: Photovoltaics and Power Consumption, Heat Pump's Compressor

But such cloudless days are rare, and in the cold and long nights considerable electrical energy is needed, too.

Too much energy in summer

On a perfect summer day hot water could even be heated twice a day by solar power:

2015-07-01: Photovoltaics and Power Consumption, Heat Pump's Compressor

These peaks look more impressive than they are compared to the base load: The heat pump needs only 1-2kWh per day compared to 10-11kWh total consumption.

Harvesting energy in spring

On a sunny day in spring the PV output is higher than in summer due to lower ambient temperatures. As we still need space heating energy this energy can also be utilized better:

2016-04-29: Photovoltaics and Power Consumption, Heat Pump's Compressor

The heat pump’s input power is similar to the power of a water heater or an electrical stoves. At noon on a perfect day both the heat pump and one appliance could be run on solar power only.

The typical day: Bad timing

On typical days clouds pass and power output changes quickly. This is an example of a day when sunshine and hot water cycle did not overlap much:

2016-03-29: Photovoltaics and Power Consumption, Heat Pump's Compressor

At noon the negative peak (power consumption, blue) was about 3,5kW. Obviously craving coffee or tea was string than the obsession with energy efficiency. Even the smartest control system would not be able to predict such peaks in both solar radiation and in erratic user behavior. Therefore I am also a bit sceptical when it comes to triggering the heat pump’s heating cycle by a signal from the PV generator, based on current and ‘expected’ sunshine and weather data from internet services (unless you track individual clouds).

Half a Year of Solar Power and Smart Metering

Our PV generator and new metering setup is now operational for half a year; this is my next wall of figures. For the first time I am combining data from all our loggers (PV inverter, smart meter for consumption, and heat pump system’s monitoring), and I give a summary on our scrutinizing the building’s electrical power base load.

For comparison: These are data for Eastern Austria (in sunny Burgenland). Our PV generator has 4.77kWp, 10 panels oriented south-east and 8 south-west. Typical yearly energy production in our place, about 48° latitude: ~ 5.300 kWh. In the first 6 months – May to November 2015 – we harvested about 4.000kWh.
Our house (private home and office) meets the statistical average of an Austrian private home, that is about 3.500 kWh/year for appliances (excl. heating, and cooling is negligible here). We heat with a heat pump and need about 7.200kWh electrical energy per year in total.

In the following plots daily and monthly energy balances are presented in three ways:

  1. Total consumption of the building as the sum of the PV energy used immediately, and the energy from the utility.
  2. The same total consumption as the sum of the heat pump compressor’s input energy and the remaining energy for appliances, computers, control etc.
  3. Total energy generated by PV panels as the sum of energy used (same amount as contributing to 1) and the energy sold to the utility.

Monthly energy balances: PV generation and consumption, May-Nov 2015

Monthly electrical energy consumption - heat pump and appliances, May-Nov 2015

In summer there is more PV  energy available than needed and – even with a battery – the rest would needed to be fed into the grid. In October, heating season starts and more energy is needed by the heat pump that can be provided by solar energy.

This is maybe demonstrated best by comparing the self-sufficiency quota (ratio of PV energy and energy consumed) and the self-consumption quota (ratio of PV energy consumed and PV production). Number ‘flip’ in October:


In November we had some unusually hot record-breaking days while the weather became more typical at the end of the month:


This is reflected in energy consumption: November 10 was nearly like a summer day, when the heat pump only had to heat hot water, but on the colder day it needed about 20kWh (resulting in 80-100kWh heating energy).

Daily energy balances: PV generation and consumption, Nov 2015

Daily electrical energy consumption - heat pump and appliances, Nov 2015

In July, we had the chance to measure what the building without life-forms needs per day – the absolute minimum baseline. On July 10, 11, and 12 we were away and about 4kWh were consumed per day160W on average.

Daily energy balances: PV generation and consumption, July 2015

Note that the 4kWh baseline is 2-3 times the energy the heat pump’s compressor needs for hot water heating every day:

Daily electrical energy consumption - heat pump and appliances, July 2015

We catalogued all devices, googled for data sheets and measured power consumption, flipped a lot of switches, and watched the smart meter tracking the current consumption of each device.


Consumption minus production: Current values when I started to write this post, the sun was about to set. In order to measure the consumption of individual devices they have been switched an of off one after the other, after sunset.

We abandoned some gadgets and re-considered usage. But in this post I want to focus on the base load only – on all devices that contribute to the 160W baseline.

As we know from quantum physics, the observing changes the result of the measurement. It was not a surprise that the devices used for measuring, monitoring and metering plus required IT infrastructure make up the main part of the base load.

Control & IT base load – 79W

  • Network infrastructure, telephone, and data loggers – 35W: Internet provider’s DSL modem / router, our router + WLAN access point, switch, ISDN phone network termination, data loggers / ethernet gateways for our control unit, Uninterruptible Power Supply (UPS).
  • Control and monitoring unit for the heat pump system, controlling various valves and pumps: 12W.
  • The heat pump’s internal control: 10W
  • Three different power meters: 22W: 1) Siemens smart meter of the utility, 2) our own smart meter with data logger and WLAN, 3) dumb meter for overall electrical input energy of the heat pump (compressor plus auxiliary energy). The latter needs 8W despite its dumbness.

Other household base load – 39W

Electrical toothbrush, at least no bluetooth.

  • Unobtrusive small gadgets – 12W: Electrical toothbrush, motion detectors, door bell, water softener, that obnoxious clock at the stove which is always wrong and can’t be turned off either, standby energy of microwave oven and of the PV generator’s inverter.
  • Refrigerator – 27W: 0,65 kWh per day.

Non-essential IT networking infrastructure – 10W

  • WLAN access point and router for the base floor – for connecting the PV inverter and the smart meter and providing WLAN to all rooms.

These are not required 24/7; you don’t lose data by turning them off. Remembering to turn off daily might be a challenge:

Boldly going where no one has gone before!

Non-24/7 office devices – 21W. Now turned off with a flip switch every evening, and only turned on when needed.

  • Phones and headsets: 9W.
  • Scanner/Printer/Fax: 8W. Surprisingly, there was no difference between ‘standby’ and ‘turned off’ using the soft button – it always needs 8W unless you really disconnect it.
  • Server in hibernated state 4W. Note that it took a small hack of the operating system already to hibernate the server operating system at all. Years ago the server was on 24/7 and its energy consumption amounted to 500kWh a year.

Stuff retired after this ‘project’ – 16W.

  • Radio alarm clock – 5W. Most useless consumption of energy ever. But this post is not meant as bragging about the smartest use of energy ever, but about providing a realistic account backed up by data.
  • Test and backup devices – 7W. Backup notebooks, charging all the time, backup router for playground subnet not really required 24/7, timer switch most likely needing more energy than it saved by switching something else.
  • Second old Uninterruptable Power Supply – 4W. used for one connected device only, in addition to the main one. It was purchased in the last century when peculiarities of the local power grid had rebooted  computers every day at 4:00 PM.

Historical UPS from last century

In total, we were able to reduce the base load by about 40W, 25% of the original value. This does not sound much – it is equivalent to a small light bulb. But on the other hand, it amounts to 350kWh / year, that is 10% of the yearly energy consumption!


Logging setup:

  • Temperature / compressor’s electrical power: Universal control UVR1611 and C.M.I. as data logger, logging interval 90 seconds. Temperature sensor: PT1000. Power meter:  CAN Energy Meter. Log files are exported daily to CSV files using Winsol. Logging interval: 90 seconds.
  • PV output power: Datamanager 2.0 included with PV inverter Fronius Symo 4.5-3-M, logging interval 5 minutes.
  • Consumed energy: Smart meter EM-210, logging interval 15 minutes.
  • CSV log files are imported into Microsoft SQL Server 2014 for analysis and consolidation. Plots are created with Microsoft Excel as front end to SQL Server, from daily and monthly views on joined UVR1611 / Fronius Symo / EM-210 tables.

Solar Power: Some Data for the First Month.

On May 4, 2015, we started up our photovoltaic generator. Here are some numbers and plots for the first month – and what I plan to do next.

Our generator has a rated power of 4,77 kWp (kilowatt peak), one module has 265 Wp. The generator would deliver 4,77 kW of electrical power under so-called standard testing conditions: An irradiance of 1000 W/m2 of light from the sun, a module temperature of 25%, and a standard spectrum of wavelengths determined by the thickness of the atmosphere light has to traverse (Air mass – AM 1,5, equivalent to sunlight hitting the earth at an angle of about 48° from the zenith).

Our 18 panels are mounted on two different roof areas, 10 of them (2,65 kWp) oriented south-east and 8 modules (2,12 kWp) south-west. The inclination relative to the surface of the earth is 30°, the optimum angle for PV at our latitude:

Plan of our house with PV modules.

Positions of our PV panels on the roof.

We aimed at using our 30° upper roof spaces most efficiently while staying below the ‘legal threshold’ of 5 kW, avoiding a more complicated procedure for obtaining a permit to install them.

The standard conditions are typically met in spring here – not in summer – as the efficiency of solar panels gets worse with increasing temperature: for our panels -0,44% of rated power per °C in temperature difference. If the temperature is 60°C, peak power (for otherwise same irradiance and spectrum) would drop by 15% . We can already see this effect, when comparing two nearly cloudless days in May and in June. The peak power is lower in the first days of June when maximum daily air temperatures were already about 30°C:

PV power over time, for a day in May versus a day in June

Total output power (AC) of the PV generator and input power (DC) for each string as a function of time for two days. 1) May 11 – maximum ambient air temperature 23°C. 2) June 5 – maximum ambient air temperature 30,5°C.

The temperature-dependence of performance might in part explain impressive spikes in power you see after clouds have passed: The modules have a chance to cool off, and immediately after the cloud has gone away the output power is then much higher than in case of constant irradiance. Here is a typical example of very volatile output:

PV power over time, data points taken every few seconds.

Output power of our PV generator when clouds are passing. The spikes (clear sky) show a peak power much higher than the constant value on a cloudless day in May; the troughs correspond to clouds shadowing the panels. The data logger included with the inverters only logs a data point every 5 minutes, so I parsed the inverter’s website instead to grab the current power displayed there every second (Using the inverter’s Modbus TCP interface would be the more professional solution, but parsing HTTP after reverse engineering the HTML structure is usually a quick and dirty ‘universal logging interface’.)

The maximum intermittent power here was about 4,4 kW!

Another explanation for the difference is local ‘focussing’ of radiation by specific configuration of clouds reflecting more radiation into one direction: Consider a cloudless region surrounded by clouds – a hole in the clouds so to speak. Then radiation from above might be reflected at the edges of that hole at a very shallow angle, so that at some place in the sunny spot below the power might be higher than if there were no clouds at all. Here is another article about this phenomenon.

A PV expert told me that awareness of this effect made recommendations for sizing the inverter change: From using one with a maximum power about 20% lower than the generator’s power a few years ago (as you hardly ever reach the rated power level with constant radiation) to one with matches the PV peak output better.

The figures from May 11 and June 5  also show that the total power is distributed more evenly throughout the day as if we would have had a ‘perfect’ roof oriented to the south. In the latter case the total energy output in a year would be higher, but we would not be able to consume as much power directly. But every kWh we can use immediately is worth 3 times a kWh we have to sell to the utility.

The next step is to monitor the power we consume in the house with the same time resolution, in order to shift more loads to the sunny hours or to identify some suckers for energy. We use more than 7000 kWh per year; more than half of that is the heat pump’s input energy. Our remaining usage is below the statistical average in Austria (3700 kWh per 2-person household) as we already did detective work with simpler devices.

Smart meters are to be rolled out in Austria in the next years, by 2020 95% of utilities’ clients should be equipped with them. These devices measure energy consumption in 15-minute intervals; they send the data to the utility daily (which runs a web portal where clients can access their data) but must also have a local interface for real-time logging given to clients on request. As a freshly minted owner of a PV generator I got a new ‘smart’ meter by the utility; but this device is just a temporary solution, not connected to the utility’s central system. It will be replaced by a meter from another vendor in a few years. Actually, in the past years we could read off the old analogue Ferraris meter and submit the number at the utility’s website. This new dumb smart meter, in contrast, requires somebody to visit us and read off the stored data once a year again, using its infrared interface.

I did some research on all possible options we have to measure the power we consume, the winner was another smart meter plus integrated data logger and WLAN and LAN interfaces. It has been installed yesterday ‘behind’ the official meter:

Our power meters in the distribution cabinet

Our power distribution cabinet. The official (Siemens) smart meter is the rather large box to the left; our own smart meter with integrated data logger is is the small black one above it – the one with the wireless LAN antenna.

We will combine its data with the logging of ‘PV energy harvested’ provided by the inverter of the PV panels – an inverter we picked also because of the wealth of options and protocols for accessing it [*]

For the first month we can just have a look at daily energy balances from two perspectives (reading off the display of the dumb smart meter manually every day):

  1. The energy needed by appliances in the house and for hot water heating by the heat pump – 11 kWh per day: On average 56,5% in the first month come from the solar panels (self-sufficiency quota), and the rest was provided by the grid.
  2. The daily energy output of the solar generator was 23 kWh per day on average – either consumed in the house – this is the same cyan bar as in (1) – or fed into the grid. In this month we consumed 27% of the PV power directly (self-consumption quota).
Daily energy balance: 1) The energy we consume in the house - partly from PV, partly from the grid and 2) The energy harvested by the PV generator - party used directly, partly fed into the grid.

Daily energy balance: 1) The energy we consume in the house – partly from PV, partly from the grid (left axis) and 2) The energy harvested by the PV generator – party used directly, partly fed into the grid (right axis).


[*] For German-speaking readers: I wrote a summary about different solutions for metering and logging in this case in this German article called ‘The Art of Metering’ – options are to use the official meter’s IR interface with yet another monitoring ‘server’, your own unrelated meter (as we did), a smart meter integrated with the inverter and using the inverter’s own data logging capabilities), or building and programming your own smart meter from scratch.

Two Weeks After Lift-Off

After a little delay our photovoltaic generator went online – we had been waiting for the delivery of this sophisticated addition to our office decoration:

Office Decoration

People on G+ had very cool suggestions, such as a rotating alien-fighting device throwing darts. Closest to the truth were: fuse box and fire alarm.

The box containing two knobs (actually the large box does not contain a lot):

Box with switches for PV DC cable

Two switches that are connected to that big red button downstairs, positioned next to the inverter for our PV panels:


We have two strings of modules, oriented perpendicular to each other; so irradiation on these is different. I add an overlay to a screenshot from Google Maps:

Plan of our house with PV modules.

Solar panels subject to different irradiance are connected in different strings – serial connections of modules; otherwise output power would suffer. The inverter has two inputs for two such strings and two MPP trackers that try to find the Maximum Power Point for each generator, by constantly probing each string’s current versus voltage curve.

Each strings is connected to one of the little red knobs, which are part of yet another safety mechanism. The inverter converts DC current from the panels to standard 3-phase AC output voltage (230 V each phase). It has surge protection (another grey boy, but downstairs) and can shut off power at its DC and AC connectors – but then there is still a voltage drop across the DC cable from the roof to the inverter.

DC voltages supplied by our PV generators are about 400V, but generally they can be close to 1000V. This is a risk for firefighters connecting themselves to the circuit via a jet of water. You ‘cannot turn the panels off’ as long as there is sunlight! In order to make sure that the voltage drops to zero as close as possible to the panels, those switches are installed.

That ‘firefighters’ switch is semi-mandatory here. Lightning protection is not mandatory too, but we decided we should finally have one. Since safety standards and costs of such protection have grown exponentially in recent years, we can brag with a Faraday cage with tighter meshes and taller antenna-style tips than all our neighbours.

Alien EMP Protection

I am sure it protects us not only from lightning but also from alien attacks (see image below) and EMP guns – and the wiring goes well with the surface-mounted aluminium tube for the DC and AC cables for the PV generator.

Alien EMP Protection

The big red button is in the tech gadget closet on the left side of the driveway.

Firefighters will pull or push the red button in case of a fire. We decided for the pull option as you are less likely to pull than push something accidentally.

What we did not know before installation: The switch will also be activated automatically in case of a power outage – this means: about every 2 years for a few minutes. but when the big red button has been activated you need to switch power on again upstairs in the roof, too!

Normally, the switch box would be tucked away in an attic, above a dropped ceiling. We have no attic anymore – this is all office space, 3,5 high in the center. We could have squeezed the box into the insulation. But then after every power outage we would have needed to climb up there, remove roof tiles and switch on power again. So we spontaneously decided to have it installed on the ceiling, above the Chief Engineer’s desktop:

Office Decoration

Last Monday The Metering Guy from the utility finally installed a smart meter, capable of metering both consumption and feed-in to the grid. He had to disconnect from the grid to do so. We switched on the inverter in bright daylight – and there was no power! Panic – what happened? I fetched the laptop and the inverter’s manual, ready for troubleshooting – until The Chief Engineer walked by, carrying a ladder, and grinning mischievously:

Have you perhaps triggered the firefighters’ switch when disconnecting from the grid?

I had forgotten about the switch only about 15 minutes after I putting big signs for firemen! But at least we knew it worked!

After one more controlled test of a power outage we were finally online. This is what power generation looks like on a nearly perfect sunny day now (2015-05-11).

PV Power over Time, 2015-05-11

Since May 5 we have consumed 11kWh / day on average; about 55% of this have been provided directly by the solar panels. Daily energy generation was about 23kWh; we used 27% of the power generated.

Greatest Innovation Ever

I like Top Something Lists, in particular the hilarious variety.

In a more serious state of mind I wondered what a list of the top inventions or top innovations of humankind might comprise. (Nitpickers, I don’t care about distinguishing ‘innovation’ from ‘invention’ here.)

Random googling yields list items such as The Internet, Money, Plumbing, and The Power of Story – here is a random list of lists:

The last list contains what my biased mind was searching for: Electricity, Water Power, The Light Bulb, The Steam Engine, The Electromagnet, The ElectronSemiconductors, The Transistor, and of course again The Internet. I argue the greatest innovation uses all these and is as important as Plumbing – actually our toilets and water supply would not work without it today: I nominate …

The Power Grid

… visualized like that in any news report on utilities or the grid I have ever seen on TV. So I adhere to the conventions:

You might say that I am cheating because the power grid is not a singular invention but rather a conglomerate of diverse inventions, held together by the glue of standardization, politics, and committees. I would argue I picked the grid for that very reason.

The more I learned about the power grid the more I wondered that it works at all – at that amazing level of availability. In Austria the average downtime per customer is about 45 minutes per year, that is electric power is available 99,99% of the time. Experts state that this even has a negative impact of our ability to cope with sudden blackouts. This is called the paradox of vulnerability: the less vulnerable you are as per statistics, the less you care about very improbably but disastrous events.

At every moment the consumption of electrical energy needs to be balanced with the demand. This sounds trivial but it means that if you turn on your oven, somewhere in your country (actually: in your control area) a gas turbine needs to spin a bit faster. In Austria the gates at a pumped-storage hydropower plant will open a bit more.

If you turn on your computer or other electronic device the compensation needs to be more sophisticated as modern devices distort the nice sine function that alternate current used to be in the old times.

Kölnbreinsperre from Arlhöhe

Storage lake, Malta power plants in Carinthia, southern Austria. Maximum power is 1,3 GW which is more than 10% of Austria’s peak power. (Wikimedia)

If consumption rises faster than demand the frequency of AC power decreases. All generators rotate in sync – most of continental Europe is one large synchronous area. The energy ‘stored’ in rotation is proportional to the square of the frequency. If the energy is not consumed the rotating masses can’t get rid of it. Since the factor of proportionality is the moment of inertia you can compensate for changes in demand by tweaking the generator, e.g. by controlling the flow of water. The grid codes agreed upon by all countries in a control area state that the operators of generators need to respond within seconds.

If something goes badly wrong the synchronous area would split into regions where generators spin with different frequencies – preventing to flow energy between these areas. This had happened in a blackout in 2006 in Europe, which was triggered by a – planned – disconnect of power lines in Germany: allowing for a ship to pass.

UCTE area split at 4 11 2006

Europe’s synchronous area split into three regions in November 2006 (Wikimedia)

What amazes me even more is that the system does still work so well, even after introducing feedback loops governed by a ‘capitalist’ market. I consider the power grid a combination of at least three networks: the network of electrical power, the communications network (stuff for cybersecurity nightmares), and a market of suppliers and customers. We can expect many new types of participants in this market as the producing consumer – the prosumer – and intermediaries aggregating demand and supply.

I am sometimes worried about the consequences of adding more smartness, intelligence and automation for technical and, above all, for commercial reasons. I am not that concerned about hackers changing the frequency of generators, but about perfectly well-controlled computers running mad at the electricity stock exchange (or by some harmless test command wreaking havoc – as described at the bottom of this post.).

In February 2012 is was really cold in middle Europe for about two weeks, and basically all power plants were up and running – not much reserve left for controlling frequency and power. There had been rumors on speculations impacting the stability of the power grid in Germany: Since the stock exchange prices of electricity were high, the balancing group representatives were said to have tweaked their forecasts. As a result the power needed was not standard power to be purchased on a market designed for that but precious energy that should have been dedicated to providing stability. The German regulator explained later that these alleged speculations would not have made sense in hindsight but it cannot be ruled out that representatives were tempted to do that beforehand.

I am aware of my very superficial description of how the market for electrical energy works – I have just tried to find anecdotal evidence of the entanglement of technical and economic feedback loops. The blackout in California in 2003 is often quoted as a textbook example of a software bug affecting infrastructure, as well as the market manipulations causing the Californian ‘electricity crisis’ have been considered an unintended side-effect of market liberalization.

This is all very interesting for the engineering, physics and IT geek (even including the geek who indulges in applying physics-style differential equations to economics). But the consumer of electrical power in me simply concludes that at all odds you should try to make yourself as self-sufficient as possible.

Advertisement for Windmill Electric Power Generating system 1897

Advertisement for Windmill Electric Power Generating system 1897. “Harper’s New Monthly Magazine” New York (Wikimedia)


For German readers – and actually in order to keep track of that myself – I add some sources only available in German:

Statistics of disruptions by the Austrian regulator, incl. exact definitions for calculating the minutes of disruption quoted in the post.

Malta hydropower plants in Wikipedia.

Stability of the German power grid in February 2012:
Austrian newspaper article – translating to ‘Gambling until Blackout’, a bit sensationalist.
Evaluation by the German regulator, see page 61. They really use the term temptation.