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Battery based back-up systems

Storage of electricity is inherently difficult and expensive. Batteries are required when grid independence is desired for grid outages or off-grid situations. Such grid-independent systems can be designed and customized for many different situations and for a given budget.

Simple DC battery systems

The simplest battery based systems are DC (direct current) systems that operate on 12 or 24 Volt DC. Appliances that work in cars, RV’s and boats can be used with these systems. This can involve lighting, refrigeration, pumps, entertainment, mobile phone equipment and computers. These systems can be used in off-grid cabins or they can be integrated in new homes as an alternative system to augment regular AC wiring. Cost for this type of installation is $1,000 and up. This may include a 100 watt solar panel and a charge controller.

Add an inverter

If an inverter is added to a DC system it becomes an AC (alternating current) system which in principal can then power all the standard house appliances. Typically a back-up system for a home would only power critical appliances during a power outage as for example a water pump, furnace, pellet stove, refrigerator and a few lights and outlets. These so called critical "loads" are powered from the inverter by using the power stored in the batteries during a power outage.

Instant inverting

The batteries can be charged by the grid and are always kept ready and fully charged by a charge controller. During an outage the inverter instantly goes into inverting mode and powers the critical loads as needed and without any interruption even if no one is home. If the power comes back after a few hours the inverter/charger recharges the batteries and keeps them fully charged until the next outage. As a rule of thumb, a modest but adequate backup system costs about $5,000 with a battery pack for one day. Each additional day adds about $2,000 in batteries.

What if there is a long outage?

However if an outage lasts for days the batteries will have to be recharged from a different source. The most common source is solar panels. Wind turbine and water turbines are less common but many work in certain situations better than solar. Generators can also be used to recharge the batteries as a last fall back solution.



Although back-up systems like this are far more expensive than cheap generators their advantages are significant. They are:

  • quiet (no noise whatsoever)
  • odorless (no gasoline smell)
  • independent from a external fuel source if solar panels are part of the system
  • always on—power is always available 24/7 not just when a generator is running
  • automatic start and stop (system will start within milliseconds of an outage and go back into standby mode when the grid is back on and stable for several minutes)

An existing or new grid-tied PV system can be expanded to become such a power source to refill batteries and power your critical "load" during an outage. In this way your PV system will provide power to your own home during an extended power outage.


Stefan Maier, Design and Engineering

Domestic DC Systems

Orange, Massachusetts, USA