Can customer batteries provide backup power?

What are the issues for the grid and customers connected with the use of residential battery backup power systems to store solar energy? Photocredit: SolarCity
What are the issues for the grid and customers connected with the use of residential battery backup power systems to store solar energy? Photocredit: SolarCity
What are the issues for the grid and customers connected with the use of residential battery backup power systems to store solar energy? Photo credit: SolarCity

What are the issues for the grid and customers connected with the use of residential battery backup power systems to store solar energy? Photo credit: SolarCity

By Dr Geoff James
Previously published on www.energystorageforum.com and republished with permission.

Residential customers wanting to manage their power are likely to be the fastest-growing market for energy storage, and especially grid-connected batteries.

So being clear about why these customers have a big appetite for storage will be important for developing the right storage products, services and owner/user experiences. Backup power seems an obvious motivation.

After all, the largest battery installations in the world exist to supply emergency power to secure facilities and data centres.

Residential owners of energy storage will also expect their lights to stay on when the rest of their street is blacked out.

But mass-market storage systems will have different capabilities to large specialised installations, and the energy storage industry should set realistic expectations so that early adopters of residential storage are not disappointed.

Grid safety and energy storage

Safety is everyone’s business and is the main concern of distribution grid operators. Their grid connection standards vary significantly between countries and states.

But one universal requirement is that when there is a problem with the grid, or when part of the grid is de-energised for maintenance, customer energy sources including batteries must be disconnected from it.

This protects line workers from being electrocuted by wires they thought were inactive, and prevents fallen wires remaining live and endangering the public after storms and accidents.

The technology that implements this safety requirement is called anti-islanding protection because it prevents ‘islands’ of power remaining when power is no longer being delivered by the grid.

It must be implemented by all power conversion systems approved for mass-market sale and grid connection. This includes battery systems as well as solar PV generation.

A combination of voltage, frequency and impedance presented by the grid is typically used to detect when grid power is no longer present. How hard can this be?

Well, it becomes problematic when the penetration of local generation is so high that it can supply the total load in a local grid region, such as a single distribution feeder. This is a real prospect in some present-day grids.

Limits of battery capacity

In most situations, at some additional expense, a battery system can incorporate switching so that the residence can be safely re-energised after the grid power is interrupted.

This means backup power is provided, only not continuously, and perhaps to a limited number of circuits.

Which brings us to another difficulty with backup power: supplying the normal household load at times of peak usage can drain a battery quickly.

Even a large residential-scale battery of, say, 10kWh capacity, would last only about two to three hours during the evening peak period of many all-electric houses.

This assumes the battery is fully charged at the time of grid interruption, and that it can be fully discharged, which is not desirable for most technologies.

The power conversion system also may not have the capacity to supply peak load.

The conservative approach to backup power is to energise a selection of critical circuits so that the battery capacity can support a longer period without grid power.

Many customers would be delighted if their lights, Internet, entertainment and main kitchen appliances continued to operate during a blackout. This would require modest additional wiring at the switchboard.

At present, there is no universal approach to battery connection for residential customers, and standards and accreditation bodies are engaged in the task of documenting recommended wiring systems and their benefits.

Recognising multiple benefits of storage

Getting back to why residential customers have a big appetite for energy storage, it’s important to realise that backup power is only one of several key motivating factors.

One more is that batteries value solar energy at the retail price of grid-delivered energy. Customers still need the grid because solar PV panels are a variable energy source.

Their output is predictably zero at night, and during the day solar panels produce less than their rated output when the sun is low, obscured by cloud, eclipsed or shadowed, or if its rays are not normally incident on the panels.

When PV panels are producing less than household demand, a customer has to buy energy at retail prices from the grid.

When they are producing more, the surplus is delivered into the grid and usually earns much less than the retail price.

Even if it presently earns more due to generous government incentive schemes, such as feed-in tariffs, many customers are rolling off these in the next few years.

With batteries to store the surplus, customers can use most or all of the energy they produce, effectively valuing solar energy at the retail price that would otherwise apply to their energy consumption.

This creates a strong incentive for solar customers to buy energy storage.

  • Dr Geoff James previously worked for 22 years with the Commonwealth Scientific and Industrial Research Organisation, which will be present at Energy Storage World Forum Conference and Exhibition in Sydney.

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