Friday, November 21, 2014

Grid independence - so far, so good

Ever since we bought our own house in 2002, it was a dream and goal of mine to become as independent from conventional external energy sources as possible. The late Hermann Scheer's concept of decentralized energy production and consumption at all levels (home, community, county, state and country) just made so much sense to me that I wanted to try to make realize it for our family home. This is how far we got...

Step 1: Produce your own power

In 2012, the German feed in tariffs as part of the Renewable Energy Law (EEG) were about to drastically change, so for economic reasons it was time to act fast. My experience with local banks and promotional loan systems like KfW was not entirely positive, so I was lucky enough to receive bridge financing from friends. In time, we built a 27.26 kW peak PV-array on both sides of our roof (facing East and West). This installation consists of 116 Sanyo 235W panels and 2 SMA TriPower 10000 inverters that manage 4 strings of panels. Given the east/west layout, the maximum yield would be about 20 kW, so no additional provisions for the 70% feed-in rule were needed. At the time, feed-in tariff was at 24.43 ct / kWh, so a break-even point after 14 years was projected.

This initial setup worked great from day one and has been producing consistent results since it was plugged into the grid.

Step 2: Measure and manage your power

I am data driven guy. About 5 years ago, I started to be interested in more accurate data about our energy consumption. How much electricity do we need? When and why? Could we be more efficient? And how come anyone's cell phone plan shows an up to the last call status online while your electricity meter is read once a year?

So my first attempt was the Wattson. A smart and good looking gizmo that clicks a sensor around your power phases and transfers real time data wirelessly to a stylish display in the living room. Great conversation piece and solid education tool for the kids. My first find was that our house had, unbeknownst to me, a "gutter heating system" which ran from November until March on a timer consuming a steady 1,000 watts. Can you believe it? 40% of our annual electricity bill (24 kW * 30 days * 5 months = 3,600 kWh) was heating the roof! Unbelievable - I immediately turned that thing off.

Next was the logical management extension of our solar array. SMA produces the home manager, which reads the power produced on the roof, but also the electricity pulled from or pushed into the grid on the main phases (it also monitors the inverters via Bluetooth for failures). This provides a complete picture of the electricity production and consumption and does a good job displaying it on the Sunny Portal. It goes one step further and allows to control consumers via Bluetooth sockets, so you can switch on certain consumers at peak solar production times only.
The only tricky bit with this product was the connectivity to the meters. Every energy company in Germany uses a different type of meter and standards are rare, so SMA cannot certify all of them as input sources. The only way to handle it is to install 2 additional suitable meters yourself (one for roof production and one bi-directional meter for grid input / output). These are relatively cheap and can be connected via S0 interface to the home manager. All worked smoothly from there on.

The home manager gave me good insights into our electricity usage patterns and in some cases lead to changes to increase self consumption.

Step 3: Add electric mobility

I will write about my experience with one year of electric mobility in a later post, but as part of our energy concept adding an electric vehicle was planned in from the start. The PV array produces about 25,000 kWh annually and our house consumption is under 5,000 kWh. The rest was intended for mobility and feed-in. So, thanks to the fantastic success of Workday, I was able to afford the EV of my dreams: a Tesla Model S (an S 85 P+ to be precise). Its 85 kWh battery give it a range of about 400 km and it really works as promised. We installed a KEBA KeContact P20 wallbox (Type-2 plug) on a separate 3-phase feed to not interfere with the existing house power. Given that the current main power is set at 35A as provided to the building, we limited the wallbox to 20A for the Tesla leaving 10 kW draw available for the house. The car itself is an earlier Model S and only has one charge module at 16A (newer models apparently have 2 chargers now for faster charge). But it loads the car over night just fine.

What does not work today as I intend is using the battery of the car to power the house at night. The concept of bi-directional charging and vehicle-to-grid electricity flow is not a reality just yet. Few EV manufacturers support / allow this, in consequence wallbox manufacturers don't support it and the utilities do not know how to handle this as a part of the existing feed-in tariff system. Approaches are emerging, but it is likely 2-3 years out before a real solution hits the market.

Step 4: Store your energy

Although I cannot use my EV battery as storage, I still wanted to increase grid independence, up my self-consumption and autarky and move closer towards grid defection as an option. I was looking for a 3-phase island solution that allows for future integration of EV storage, external AC sources and optimal management capabilities. Not too many vendors come even close to this specification, but the S-10 by E3DC does the trick. The unit replaces one of my SMA TriPower inverters and uses the second TriPower as externally generated power. We installed 13,8 kWh lithium ion storage (batteries very similar to what is used in the Tesla). The S-10 acts as the main power station for the house and sits between the 3 phases coming in and the house distribution panel. It satisfies the house power needs first, then charges the batteries and finally feeds in surplus electricity. In the night, the house runs entirely off the batteries until they are empty. Depending on the time of year, this enables different degrees if autarky but I am projecting 60-70% (including charging the EV!).

Step 5: Manage your consumers

Lastly, I added a 6 kW heating coil into the warm water tank to be controlled via Bluetooth socket. It is only switched on when at least that much is produced by the PV array and it heats up water to 60 degrees C. This way I can additionally convert electric energy into thermal energy and reduce gas consumption for warm water production. At this time, this is work-in-progress and I need to report back once it is running for a while.


Our house is a bit of a research lab for sustainable energy and "life as an energy  prosumer". Not all of what we have done is financially viable, at least not in the short term, but all of it makes sense to me to prove out what is possible. All of the planning and work has been completed by Solartechnik Kittel, which I can highly recommend as a savvy and visionary energy technology company. I am looking forward to upgrading our home as things become available for improved energy independence. In the meantime, all remaining electricity and gas is purchased from renewable sources via Naturstrom.

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