Energy storage is the next big step for distributed renewables like rooftop solar — and many in the industry believe that battery technology prices will follow the same trajectory as that of solar PV. Australia, the country with the highest percentage of solar households in the world, has been described as “ground zero for energy storage” thanks to a unique, perfect storm of factors that have arisen here. This combination of factors also has some Australian homeowners grumbling about going off the grid with solar-plus-storage. But how realistic a goal is this?
The short answer is “not very.” Despite the low price of solar PV and falling battery prices across the globe, the average Australian household would still need to invest a sizeable chunk of change to be able to free themselves from the grid without significantly altering their electricity consumption habits. This doesn’t necessarily mean, however, that it would not be worthwhile to install some amount of energy storage to offset household electricity usage or for backup power.
Let’s look at the example of a household thinking about going off the grid in Brisbane. We’ll use the following assumptions:
- Electricity demand is about 10kWh per day on average annually, 13kWh/day in the peak of summer (December) and 10kWh/day in the dead of winter (July);
- The household has already done everything possible to reduce their demand without compromising their comfort — they are not willing to cut their usage any further;
- The home has a spacious, north-facing roof tilted at latitude;
- The home’s household usage roughtly follows a “double hump” pattern (as per the graph below), but without any variation between seasons;
An example of the “double hump” electricity consumption curve — common to homes with school-age children. Consumption is high in the mornings and evenings.
- Whatever battery bank the home installs has a maximum (recommended) depth of discharge of 80 percent — meaning that only 80 percent of the battery’s nominal capacity is actually used.
What does this home need in order to be truly energy self-sufficient while using only solar and batteries? Let’s first have a look at what it would take to be self-sufficient for a single day.
The graph below (created using a simple storage size estimator tool Solar Choice is in the process of developing), models how much of this home’s energy usage would be covered by a 3kW solar system coupled with 10kWh of storage. Energy drawn from the grid is represented by the grey shaded area. The graph shows that the home is about 80 percent self sufficient on the average day with this system. The blue bars represent solar energy used to charge batteries, while the red bars represent “overflow” solar exported to the grid. The purple shaded area is battery usage, the purple line is battery state of charge, and the shaded orange area is solar self-consumption.
3kW of solar and 10kWh of energy storage (80 percent max DoD) on the roof of a north-facing Brisbane household with 10kWh of energy demand daily. The system isn’t quite large enough to meet 100 percent of the home’s demand.
How about if we up the PV system size to 5kW and double the storage to 20kWh? As the results below show, in this case the system produces and stores enough electricity to meet about 160 percent of the home’s daily electricity demand (on the average day) — with a bit of solar energy to spare. It also meets daily demand on the average day in December (about 140 percent) as well as July (about 150 percent).
5kW of solar and 20kWh of energy storage using the same parameters as above. In this case, the system meets the home’s electricity demand for the average day of the year — with a bit of solar to spare.
To feel truly confident about abandoning the grid altogether, however, this home will want to prepare for the worst case scenario — long stretches of bad weather where there is little sunshine. This would mean having a much larger battery bank and a much larger solar system to charge it. Let’s assume a worst case scenario of four consecutive dark, rainy days in the middle of winter. How big would the system need to be to get the home through such a period (i.e. four days of energy autonomy)?
It would need to be fairly large when compared to the average solar system for a grid-connected home (5kW): About 13kW of solar and 65kWh of storage would be necessary to be able get the home through four dark winter days. What would this system cost? Assuming about $15,000 for the solar (at about $1.15/W) and a very optimistic $65,000 (at $1,000/kWh) for storage, the system will end up costing close to $80,000. Keep in mind that although a solar PV system has an expected lifespan of 25 years, most batteries last only 5-10. Unless the home is a newly-built one in a remote area and grid connection is going to cost a small fortune, who would seriously consider it?
13kW of solar and 65kWh of storage will get this home through a winter – but the costs would be significant.
Most people in Australia go solar to save money on their electricity bills. Those who go solar to spite their electricity retailer and/or for enviornmental reasons may have the emotional or ideological motivation to splash out a bit to achieve a high degree of self-sufficiency — or complete self-sufficiency — but not many would have this kind of cash at their disposal in the first place.
If they’re really determined, there are ways get around this, of course: The household might be able to reduce the overall capital cost of the system by aiming for only two days of autonomy, with a diesel generator to fill in the demand gaps. Or they could compromise on their comfort a bit and find even more ways to reduce their electricity consumption, which would eliminate the need for such a large system.
But for the average homeowner in a densely-populated urban area, why bother? The grid is reliable, provides relatively inexpensive electricity and is there for public use. This is why the majority of homes who get solar and storage will stay grid-connected — at least for now.
