This is the third post in my series on our photovoltaic generator. It had been a part of previous post with the data for the first month, but I cut and saved it as the other post was so long already.
I am now also able to present data for two months of operations: Below is an updated plot of daily energy balances in the second month. Again, I am combining data from our photovoltaic inverter’s logging and from our meters that track the difference of consumption and fed-in energy.
There are two ways to judge the system’s performance in relation to your ‘autonomy’ and economics:
- Self-sufficiency quota: Which part of energy consumption in the house is harvested from the solar panels?
- Self-consumption quota: Which percentage of the PV power created can be consumed immediately.
June was sunny and hot: We harvested more energy due to the higher number of sunny days, but we did not reach May’s daily high score of 32,88 kWh again: Longer days cannot compensate for the reduction of the efficiency of the panels due to higher ambient air temperatures (as discussed in the previous post).
Comparing May and June I noticed:
- We need less energy per day on average: 10 kWh instead of 11 kWh. Either we spend more time outside than sitting in front of a computer, need less light, or (most likely) we are too obsessed with playing with our new metering gadgets and optimizing our energy consumption.
- As a consequence – especially in combination with a higher average PV energy output of 25,5 kWh versus 22,1 kWh – our self-consumption quota slightly decreased: We could use only 25,8% of PV power directly, versus 27,5% in the month before.
- For the same reason, we are more ‘autonomous’, self-sufficiency quota increased: On average 66% of the energy needed in the house was directly delivered by the solar panels (Before: 55%).
Now the tantalizing questions are:
Which quotas will be reach for a full year? How would those numbers change if you had a battery? Would this justify the investment?
The following is my personal assessment, based on our (Austrian) market of electricity and our way of using electrical energy as home owners.
My assessment is based on the following:
- I pay three times more for electrical power than I will earn when feeding in to the grid (about € 0,18 versus € 0,06).
- Thus the goal is to maximize the amount of energy used in house and only sell to the utility if you cannot use your power.
- A battery can store the surplus of energy harvested during the day, so that you can consume it in the night or when clouds pass.
- The ‘profit’ from having it battery is thus the difference in costs of power and feed-in tariffs (€ 0,12), multiplied by the energy you are able to ‘shift’ to the darker hours.
To determined estimated profits I need to compare our self-sufficiency or self-consumption quotas with and without a battery. In June 2016 I will have data for the latter – energy generation and consumption for every 15-minute time slot. A battery can then be added in a simulation using this logic:
- Energy not consumed in the house is used for charging the battery unless it if fully charged already.
- If the solar power is not sufficient to cover current demands, energy is delivered by the battery – unless it has reached its minimum allowed capacity (80% of nominal capacity for Li-in batteries, 50% for lead-acid ones).
- The conversion of energy is not perfect and battery losses need to be accounted for.
Before I will be able to do this, I play with ballpark numbers, see e.g. inverter vendor SMA’s planning guide (diagrams and examples p.32), or somebody else’s load profile. This tool by a German developer allows for playing with the size of the battery and PV generator interactively (the legend of the plot is in German, but the monthly balances and input fields are titled in English).
Those tools and diagrams need the following input data:
- Yearly energy consumption: About 7500 kWh in our case, more than 4000 kWh is for the heat pump.
- Yearly energy harvest from solar panel: Using one of the free PV simulation tools that take into account weather data (e.g. PVGIS provided by the European union) we should end up with 5300 kWh.
- Size of the battery and percentage of usable capacity. I play with a hypothetical Li-ion battery with 10 kWh, thus 8 kWh maximum energy.
At the German website you can pick different load profiles, I compare different ones that might be similar to our usage pattern.
The results indicate that …
- the estimated self-sufficiency quota for a full year would be about 30% for our system; and it would increase from about 30% to 55% on adding a battery.
- we might be able to shift less than 2000 kWh per year by the battery. Note that calculations based on self-consumption quota also include the battery losses – about 500 kWh per year – so data based on the self-sufficiency quota should be used to see the real usable energy.
Multiplying this with the difference in costs and profits (€ 0,12 per kWh) would result in a profit added by the battery of about € 240 per year. BTW this is less than 50% of the profit gained by using the PV generator without the battery.
I researched prices of Li-ion batteries before we purchased our generator and the cheapest one I found was a 8 kWh battery (nominal capacity) for € 6500. Tesla has recently announced its Li-ion Powerwall home battery with 10 kWh, to be sold to $ 3500 to contractors. Prices for installations and price difference between a ‘classical’ inverter and one that can manage a battery not included (less than € 500 here). If you have the battery installed together with the panels, there should not be much additional installation efforts though, compared to the time needed for working on the roof. But I think a 10 kWh battery will not be cheaper than something like € 4000 unless one more ‘disruption’ hits the industry.
So the estimated profits of € 240 are considerably less than 10% of the costs of the battery. Since the estimated life-time of the battery is 10-20 years it is likely not to pay itself off until it has to be replaced again. (No interest rates and realistic net present value considered.) The solar panels will last for decades and still deliver power – though at a slowly deteriorating efficiency – when the payback period has passed.
Those 2000 kWh might even have been too optimistic: The more energy you can use immediately, the less economic is the battery – as there is less energy left to be shifted at all:
- We use the office during the day: I had always noticed that electricity bills from the past clearly show if we were commuting to a workplace far away or using our own office.
- We use a heat pump, and we schedule the hot water cycle to match the sunny hours.
I think that you should try to use energy more efficiently first – before adding a battery to shift unnecessary loads. The decision for a battery should be based on an optimized load profile.
For example, Tesla presents some data for a typical energy consumption of devices on their Powerwall website – see bottom of this page. A refrigerator should use 4,8 kWh a day? If I’d have such a refrigerator I would rather replace this than investing into a battery. A modern fridge should not use more more than a kWh per day.
Using a heat pump in middle Europe I might have an unusual view of autonomy. But I believe heating energy in northern latitudes is often neglected in discussions about becoming more self-sufficient by using solar and wind power. If you don’t want to depend on a utility, why would you want to depend on the vendors of fossil fuel or wood – and related financial markets and volatile prices? Here, we have a very reliable power infrastructure, access to 100% green power harvested locally (wind or hydro). But oil and gas have to be imported, and the energy for heating in winter is several times the electrical energy consumed by appliances.
A heat pump the best way to utilize solar and wind power for heating, but you have to face solar energy’s obvious disadvantage: You cannot store summer’s surplus of PV energy for winter (not until a commercially feasible combined fuel cell plus electrolyzer will be available). PV energy from generators that fit onto typical roofs are too small to cover heating energies in winter. Our expected PV output in the coldest months will be less than 10 kWh per day, whereas the heat pump might need up to 35 kWh (one quarter of heating energy). And lest we not forget power versus energy: The heat pump always operates at full power, with its rated power being determined by a worst case minimum ambient temperature. You will need to input something like 2-3 kW for a rather short period. The more average power you need, the more often the heat pump is turned on. So you also need 2-3 kW of solar power or battery output power – a challenge given irradiance in winter and typical battery inverter’s output powers.
In summer a 10 kWh battery the capacity of Tesla’s will be fully charged quickly: You can expect half a year of autonomy, but have to feed in your surplus to the grid. In winter, the battery will hardly be charged as all power is consumed immediately. So the estimated self-sufficiency quota will be well below the absolute theoretical maximum of yearly PV energy harvest over yearly energy consumption (about 70% in our case).
Perhaps you wonder why I am only pontificating on the economics of batteries. I haven’t mentioned the battery’s function as a backup system yet for these reasons:
- Expected downtime per utility client here is less than an hour per year.
- In winter a backup system would certainly be more interesting if you depend on a heat pump – but, as mentioned above, the battery will hardly every be charged. The storage and latency built in in heat pump systems will cover the typical downtimes easily. You use a storage of heat rather than one of electrical energy.
- You might need to plan carefully which devices are allowed to run during a blackout, as the output power of inverters is limited, and typically of the same order as an electric stove or a water cooker (2 – 3 kW – as high as the heat pump’s input power!). If I only want to be sure that my computer does not crash or want to have a chance to look up the utility’s website, then I’d rather go for a UPS (Uninterruptable Power Supply). A UPS is a battery you connect your important devices to directly, whereas a battery-powered backup system plus inverter needs to maintain a small AC network, controlling frequency and voltage like The Big Grid.
- A full-featured backup system compliant with our local safety regulations will need some components not included in vendors’ turn-key system. At least a few months ago it was hard to obtain a definitive and unambiguous statement about what exactly the full backup solution would entail.
Another thing I am still waiting for is the option to use a car battery (easily) also as a home storage battery. AFAIK car batteries as the Tesla’s do not allow for bidirectional charging and consumption. (Edit Dec. 2015: Nissan’s LEAF can do that but this solution had not been available in Europe yet).
But fossil fuel burnt by cars are much tougher to get rid of in Austria than fossil fuel burnt in homes. Statistics show it is seems very hard to reduce the former, whereas the latter is gradually decreasing as people prefer heat pumps or pellet stoves today.
I have reduced my carbon footprint drastically in recent years: by driving much less and turning to ‘remote work’. Now we would be ready to replace one of our two cars by an electrical one, as we can plan better when and how it will be used. But why buy the battery twice – one for the car that seldom uses it and one for the home?
That said, one should always keep in mind seemingly unrelated investments and not apply double standards. For example, a lightning protection system for a house like ours costs about € 4000. So if I pay that for mitigating an event due every 1000 years (estimated by a tool provided by our national weather agencies – probability of a lightning strike) – I sort of nearly feel obliged or entitled to buy a battery.
I am also aware of the fragility, if not absurdity, of financial forecasts in times like this – I don’t even need to factor in the collapse of the eurozone or something. I have just heard rumours about drastic changes to be made to utilities’ pricing models. Utilities here start to lose money when serving PV system owners: In those € 0,18 per kWh fees for transmission via the grid are included. But if you deliver energy to the grid you are not charged for kWh transmitted. Transmission system operators have to swallow your energy and are burdened with managing the volatility of renewable energy. Currently running costs of electricity are high whereas the fixed costs like metering (which I factored in in that € 0,18) are negligible. An envisaged model might comprise a rather high base fee instead – no matter if you are a consumer and/or run a power plant – plus low running costs of about € 0,10 / kWh. If those rumours come true, it would become more interesting to go off-grid completely which is not an option given the constraints I outlined above (summer/winter, heat pump, high peak power of appliances).