In domestic applications, the most relevant technology for stand-alone operation is a conventional battery system (as commonly used with PV systems), although flywheel storage may have a role as products become cheaper.  However, in general, it may be assumed that, for grid-parallel operation, the grid itself is the "battery" as regards mass electrical storage.

Some manufacturers are considering the inclusion of very small electrical storage components to provide sufficient power to start the engine in the case of grid failure (so-called "black start" capability) and possibly as a load balancing mechanism for use during grid failure so that a separate UPS (Uninterruptible Power Supply) circuit can be maintained at all times.  Such a circuit might for example power lighting, heating ancillaries, TV and refrigeration.  This particular feature, although somewhat unnecessary in UK and most of Western Europe, still has an appeal to many technophiles and those who want some "independence" from the central supply system.

Other potential technologies (not yet shown here) include pneumatic and (super) capacitors.  The former can be rechargeable and act as emergency back-up power, whilst the latter tend to be used for very short term storage with their characteristic high charging and discharge rates.

Lead acid and other rechargeable batteries have been the standard electrical storage medium for decades.  They have a relatively low initial cost, but maintenance costs are high and performance tends to fall off throughout their life.

From an environmental perspective, there are also issues relating to recycling and disposal of components.

A UPS comprises an electrical storage component together with the necessary controls, including a charger and an inverter to convert the stored DC into AC for use in the home.  Depending on the requirements of the user, the UPS may store only sufficient power to safely power down computers and other vulnerable equipment, for continuous powering of emergency appliances for an extended period, or the total loads of a selected number of appliances (lighting, TV etc.) or even the entire domestic load for off-grid systems.

In order to provide seamless back-up power suitable for computer protection, it is necessary to maintain simultaneous feeds from the battery (through the inverter) and from the mains/generator.  This leads to small, but continuous electrical losses within the system and reduces the environmental benefit for micro CHP applications.

A flywheel energy storage system draws electrical energy from a primary source,  and stores it in a high-density rotating flywheel. The flywheel system is actually a kinetic, or mechanical battery, spinning at very high speeds (>20,000 rpm) to store energy that is instantly available when needed.
Upon power loss, the motor driving the flywheel acts as a generator. As the flywheel continues to rotate, this generator supplies power to the customer load. 

Whilst the URENCO system was designed for large power applications, Beacon Energy have developed products which, although still too costly for typical domestic applications, are viable for bespoke applications such as remote communications stations.
Flywheel-based energy storage systems, unlike lead-acid batteries, are sustainable “green” technology solutions that do not use hazardous materials for production, nor create them during operation. Unlike batteries, flywheels operate reliably for many years with little or no maintenance. 

Despite higher initial costs than battery systems, flywheels offer an attractive and long-term cost-effective energy storage alternative.

Chemical storage is similar in concept to that of a fuel cell, in that chemicals are fed directly into the regenerator to produce electricity and processed back into their former state when electricity is supplied.  As with battery storage, however, the chemicals involved are hazardous and consideration needs to be given to recycling and eventual disposal of components.

The production of hydrogen from low carbon electricity and the subsequent reconversion into electricity by fuel cells is a form of longer term energy storage; hydrogen also has the valuable characteristic of being suitable for utilisation in other applications such as a replacement fuel for internal combustion engines in the automotive sector.

URENCO