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Toward grid independence, energy storage for efficient homes

This insightful article on the interesting results of Terry Teoh installing a battery in his very low electricity consuming home was originally written and published by ReNew magazine on the 27th of September 2017. We have proudly reproduced the article for viewing below.

What happens when a home with very low electricity use adds a battery? Terry Teoh describes his home’s interesting results.

Our house is an Edwardian three-bedroom brick home renovated in 2010 along sustainable design lines. With two occupants, our house achieves a very low average electricity consumption of 2.4kWh/day, though note that gas is (currently) used for space heating, cooking and boosting of solar hot water. We installed a 5kW solar PV system in December 2016. With the array oriented east and west, the seasonal difference in energy production is accentuated compared to a north-facing array: our system produces, on average, 26kWh/day in summer and 7kWh/ day in winter.

In April 2017, we added a 4kWh Sonnen eco8 battery to our system to provide solar load shifting – storing solar energy produced during the day for use at night.

In the first two months of operation (to June 2017), our house has moved from 30% to 70% grid independence—i.e. 70% of our energy is now generated by our solar system.

Interestingly, that 70% is lower than we expected, given a substantially oversized solar array and battery. It turns out that our standby energy usage is too low to be served by our inverter! However, it’s still a good result and the battery has lifted solar self-consumption from 5% to 50% and paved the way for us to disconnect from the gas network and move to an all-electric, renewably powered household.

Motivations

Our motivations for installing a battery system included a desire to maximise solar self-consumption and grid independence. The latter is not out of antipathy for energy companies or the grid. We want to stay connected to the grid. The grid is good; it will just be used in a different way in the future to support a decentralised energy system where consumers will have more control over how they make, use, store and share energy.

Our Battery Choice

Our two person (plus two pooches and two chickens) household’s daily electricity consumption of 2.4kWh/day is used primarily to run the fridge, lights, microwave oven and washing machine; we don’t own a TV. A solar water heater is supplemented with an instantaneous gas booster, and gas is also used for space heating of the lounge room and for cooking. The sitting room is also heated by a slow combustion stove, used five months of the year. We both work full-time, so energy usage is skewed towards the evenings which accounts for about 60% (1.4kWh) of our daily electricity use.

I work for ZEN Energy, so my inclination was to purchase our storage system from them. At the time, ZEN offered systems from Redback, Sonnen, and Freedom (Freedom being ZEN’s own product with full backup). The German-made Sonnen had more global deployments and seemed a more mature product with a sophisticated control system. Sonnen warrants the battery for 10,000 cycles at 100% depth of discharge or 10 years, whichever comes first. Sonnen is a system integrator; inside the battery case is an Ingeteam inverter and Sony batteries. Both these companies are well established with a strong reputation for build quality. Another factor was that Sonnen uses lithium iron phosphate (LiFePO4) battery chemistry, which I believe is a lower fire risk than other lithium-ion chemistries.

Australian-based customer support

For a good economic outcome, the battery should be sized to work as hard as possible within its warranted life. Our desktop assessment suggested we only needed one Sonnen module (2 kWh) to cover 1.5 kWh of load shifting. We chose to add an extra 2 kWh module to bring the installed capacity to 4 kWh. This future-proofs our installation.

The extra battery module requires a Sonnen cabinet expansion unit, which sits neatly underneath the wall-mounted base unit. Expansion of storage capacity then becomes a simple matter of slotting in more modules. Our plan is to upgrade to a full backup system (ability to run during a grid blackout) in conjunction with gas disconnection in the coming 12 months. Sonnen allows mixing of battery module vintages.

Thus, our system can be expanded incrementally during its warranted life, rather than having to write off the original modules and purchase a new set, as is generally the case with lead-acid batteries and some lithium-ion systems.

System setup

Our system is set up for solar load shifting, as are most residential solar batteries. That is, it uses solar to serve loads, with any surplus going into the battery, then exports when the battery is full. In the evenings, it uses the batteries to serve loads, then imports from the grid when the battery is empty. The Sonnen real-time optimiser applies these priorities and according to Sonnen also has the ability to forecast consumption patterns and adapt its control strategies based on self-learning.

The ‘load’ and ‘solar’ three-day forecasts seem to be quite good, but we have not had the system long enough to observe any self-learning. We are only using the Sonnen for solar load shifting and I can confirm that the controller handles this simple task very well.

Where I expect the battery to come into its own is if in future we introduce additional complexity (e.g. tariff arbitrage, load management, grid services). When we purchased the battery system, I was expecting to achieve close to 100% grid independence, reasoning that the solar array is oversized relative to load (20kWh per day solar production versus 2.4kWh per day of consumption) and the battery is oversized as well (4kWh), with a design depth of discharge of 40%.

Figure 1: A typical Saturday in winter. Solar generation (yellow) kicks in at 7.30 am and ends at 5 pm, peaking at 1.8 kW at midday. Consumption (blue) follows the household’s typical pattern. After rising at 7.30 am, the toaster is switched on at 8.16 am, drawing 1.2 kW. At that time, PV production was 460 W, and the Sonnen discharges (red) at a rate of 740 W to cover the toaster load. The underlying blue ‘square wave’ is the fridge compressor kicking in and out on a 1½ hourly cycle. The battery charges (green) and is full by 10.30 am (100% state of charge (SOC), purple line). The house is vacated mid-morning for shopping and coffee. At 11.15 am the washing machine gets turned on, drawing 2.3 kW which is met by solar and balance from the battery. At dusk, the household commences the evening routine. The Sonnen discharges to meet load (red), primarily lighting.

However, the Sonnen controls are configured to turn on the battery inverter when a minimum load of approximately 60W is present. Our household has a standby consumption of 20W. So this background load, plus a small draw to run the Sonnen controller, constitutes a residual load that cannot be eliminated by the battery.

Our pre-battery draw from the grid is already at a very low level (2.4kWh gross, 1.5kWh post-solar/pre-battery). In a more typical Australian household consuming say 20 kWh per day, this effect would in all probability not even have been noticed. It is likely that the reported grid independence will ‘improve’ as we add more electric appliances and go off gas simply from increasing our standby load. Covering the increased expected daily load of 8 kWh for an all-electric house may mean we need to add another module to take the energy storage total to 6 kWh.

Figure 2: Weekly profile for one winter week from 25 June to 1 July. The household pattern is quite consistent across the week, with the battery drawing down to 60% state of charge (SOC). This level of battery usage is consistent with initial estimates. Tuesday 27 June (3rd solar peak from the left) was a low insolation day. Solar self-consumption over the seven days was 45.6%, comparable to the 52.7% for the first two months after the battery was installed in April. Our east/west array produces significantly less energy during winter, so I would expect the annual figure to be closer to 30% due to substantial solar over-production in summer. During the solar boom, when systems were sized to sit underneath load, self-consumption was a good proxy measure of economic effectiveness. Post solar boom, with module prices having fallen significantly, an export-dominant solar system still pays back in about eight years. So we’re not that focused on self-consumption and ideologically take delight in spilling our surplus green energy into the grid. Grid independence over the week above was 66.2% and for the two months since the battery was installed, 73.3%. This result is mildly disappointing as I was envisaging achieving 95% to 100% grid independence. However, we take comfort that this reflects the residual grid draw needed to run our standby load which is so low as to be below the Sonnen battery inverter cut-in, hence cannot be served by the battery.

The Sonnen can charge from the grid. We do not use this feature because both solar and battery are oversized, with the battery only discharging down to 40% to 50%. So there is no reason for the controller to import from the grid to charge the batteries. The battery can also export to the grid, but we are not doing this at present. If demand response or other grid services become offered more widely and are economically attractive, we would certainly look at these.

Our 12-month pathway is to disconnect gas, retain our solar hot water system and retrofit it with a bolt-on heat pump, then tie the heat pump into the Sonnen as a controllable load.

Installation

The system is installed in a small storage room which abuts our main dwelling and complies with current Australian standards. The main requirement is that the battery is not located inside the house. Since the Sonnen is not weather-rated, the storage room was a logical place for it.

The storage room is insulated and ventilation tight. It has a small, openable sash window and will require a door vent installed before summer as a precautionary measure to avoid high ambient temperatures.

Adding grid backup

Our current system is the base Sonnen offering which does not include grid blackout backup. Backup has a simple meaning for the ordinary consumer, but in reality, has a bit of complexity to it. We are in the process of evaluating what level of backup we wish to have and will roll these objectives into the gas disconnection plan in 6 to 12 months.

One option is back up with Sonnen Protect. The Protect unit has a power rating of 2kW. Typically the simplest and lowest cost option is to wire a new circuit with two power outlets. Alternatively, reassigning circuits at the switchboard involves labour costs, but enables a more tailored approach (e.g. we could pick up the beer fridge, lights, and the wi-fi router). The Sonnen will drain down from the available state of charge at the time the grid failed. Charging from solar is not possible.

Sonnen does not yet offer a full backup option, which would be our preference. It generally works like this: upon grid fail, the battery inverter (or auxiliary device) would go into island mode and the solar inverter would stay on. Loads would be served from solar and batteries and, with appropriate rationing, we could sit out an extended blackout of several days.

Other ways of using the battery

I expect the ‘use cases’ for the battery to evolve during its 10-year warranted life, under the impetus of regulatory reform and energy market transformation, as well as changing household consumption patterns. Right now, we’re looking to the short-term future, adding electrical loads as we go off gas for cooking, space heating, and hot water.

What_is_the_payback_of_a_battery_storage

With the ability to add more storage, we have a pathway to lift our grid independence. The controller has the sophistication to forecast and implement optimal battery use and I’m sure we will start to take more advantage of that over the years ahead.

Energy consumption changes
Figure 3: How the energy consumption has changed at each stage of system installation. Note how the small evening load still results in grid consumption as the load is too low to trigger the battery’s inverter.

Disclosure: Terry Teoh is an employee of ZEN Energy, where he purchased his Sonnen battery storage system.

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