Despite the Tesla hype, going off grid is not that simple

Dr Gordon Weiss is a Principal Consultant with Energetics, a carbon and energy management business that works with large energy users to manage carbon emissions and energy costs. He’s also designed and built his own off-grid system for his home in the beautiful Blue Mountains, two hours west of Sydney. He shares the techs and specs of his system with us.
The announcements from Tesla about the PowerWall energy storage has stimulated an exciting debate about how low cost batteries will advantage householders and whether or not it will create widespread defection from the electricity grid.
Take it from someone who’s knows … alas, it’s not that simple!
It’s the challenge I faced when building a weekender home in the Megalong Valley, Blue Mountains, NSW; the property, sub-divided farmland with no grid connection, water, phone or sewerage connection, but we wanted to construct a modern house where all utilities were provided on the site. In other words, we wanted to go-grid and the house – covering 300 sqm – had to supply power for between 4 and 7 people when occupied.
Of course, a connection to the local grid could have been installed and the local network service provider was happy to provide the approximately 2 km of HV power line and a local 25 kVa substation at a cost of around $300,000…that’s a lot of solar panels and batteries!
Designing an off-grid solar power supply
Our off-grid solar PV supply needed to account for three factors
Firstly, we had to consider the typical daily electricity consumption, as well as the typical consumption that occurs overnight. This influences the size of both the solar panels and the battery capacity.
The second factor was the peak power demand of the house, as we knew the inverters and batteries must be sufficiently large to meet this instantaneous peak demand.
And finally, the expected weather patterns and, in particular, cloudiness meant we had to trade-off between battery size and the likely use of a backup generator to top up the batteries.
You can see from the this graph the average daily solar exposure for Little Hartley, the closest weather station.

As well as highlighting periods where cloud cover reduced the solar exposure for several days on end, it shows the much lower solar exposure in winter compared to summer. Of course, placing the solar panels at a horizontal angle to the latitude will maximise the power generated over the course of a year.
The system design
Our solar PV system consists of 3kW of panels on a roof on a slope of 11 degrees plus 4kW of ground mounted panels at a slope of 45 degrees. Both are orientated due north and the purpose of the ground mounted panels is to catch maximum winter sun.

Modelling the demand of the house suggested that the average demand during the year was in the order of 15 kWh, which is less than the average Australian house – this is due to our design that included extensive LED lighting, no air conditioning and no electrically powered space heating. Despite LEDs, the additional hours of darkness in winter meant that the average power demand at that time of year was in the order of 20% greater than the summer demand, placing even more importance on the performance of the system in winter.
Inverters, batteries and a diesel generator
The other elements of our system are two Kaco Powador grid invertors, a Selectronic SP PRO inverter-charger and 32 kWh of battery storage. As the batteries are gel-acid batteries, there are constraints on the depth of discharge if battery life is to be maintained; the effective storage capacity is in the order of 20 kWh.

For extended cloudy days, backup is provided by a 5 kVa diesel generator. The size of the solar panel array and the capacity of the batteries was determined using HOMER, the free former US Department of Energy program for optimising off-grid systems. The design took into account the likely hourly power demand, the hourly solar exposure and the cost of the various part of the system including the cost of fuel.
How does it all perform?
It was interesting to have a look back over the four month period ending in May 2015.
Firstly, the maximum instantaneous power demand was 12 kW. This is most likely the result of the in-rush current of an electric motor, on top of other household appliances such electric kettles, and really highlight the importance of correctly sizing the system. An undersized system would have shut-down if the power demand was too high.
This next figure in the graph below shows the operation of the solar PV system from 4pm on one day through to 8pm the following day.

It also highlighted several important features of the system design:

• Once the batteries are fully charged (the state of charge equals 100%), the output of the solar panels is constrained so that it just meets the load.
• There is an evening peak that will partially discharge the batteries, an overnight load (including the refrigerators and the aerator of the waste treatment system) and a morning peak that often occurs before there is any output from the solar panels.
• Even with a significant daytime load (averaging around 2kW), the solar panels managed to fully recharge the batteries by the mid-afternoon.

The challenge of cloudy days
Data from the same time period on different days shows the challenges of off-grid when the sun is not shining.

The household load was greater overnight, leaving the batteries at just over 40% charged when the morning load rose. The system was approaching the point where the inverter-charger would shut down the load to protect the batteries.
At this point the diesel generator was required to raise the state of charge of the batteries (‘External supply’ on the graph). At that time, it had been just over a month before the generator was last tested and so it would have been necessary to test-run the generator.
This situation was further complicated by cloudiness on the day, which meant that the solar panels were not able to recharge the batteries before night time.
A clearer demonstration of the impact of cloudy days is seen in the last figure. The house was unoccupied at the time and despite the reduced power demand, the solar panels were not able to fully recharge the batteries. From this, it’s worth noting that:
Even on very cloudy days, the solar panels still have some output. The total generation on the two cloudy days was 4.4 kWh and 5.5 kWh, and instantaneous outputs as high as 1.5kW have been observed.
Recharging is fast on sunny days. The figure shows how the batteries were charged by the middle of the day (despite a cloudy period in the mid-morning).

(Note that the 2 kW spikes that occur every 12 hours are most likely a small hot water heater switching on to maintain the desired water temperature.)
Obviously, as the panels can still recharge the batteries even on cloudy days, the more panels installed, the better the system can deal with the morning power demand. This both reduces the probability that the generator will be required and also allows some trade-off between the size of the panels and the storage capacity.
What can we conclude?
Our experience going off-grid points to several conclusions:

• It’s vital to understand the daily and seasonal power demand of your house to properly size the system. The smaller the demand, the smaller the solar array and the batteries.
• Batteries with a capacity similar to the average daily demand of the house will be adequate except on very cloudy days.
• The solar panels need to be sufficiently large to both run the house during the day and also recharge the batteries.
• A larger array can do a better job at recharging the batteries early in the morning and meet the morning power demand.

Obviously, we’re pleased to have an alternative to spending $270,000 on a grid connection, but we’re also very pleased with the way the system has worked.  It’s been fascinating watching the system in action and what comes out on paper is that anyone wishing to go off-grid should install as many batteries as they can afford, so as to minimise use of a back-up generator.

The specs: Gordon’s system consists of 3kW of panels on a roof on a slope of 11 degrees plus 4kW of ground mounted panels at a slope of 45 degrees. There are also two Kaco Powador grid invertors, a Selectronic SP PRO inverter-charger and 32 kWh of battery storage (gel-acid filled) with a backup 5 kVa diesel generator.