There are three main disadvantages of all engine-driven generators:
- They emit toxic fumes and should be run only outdoors;
- Some portable generators produce low quality power, which may not be suitable for some electronics;
- They require frequent oil change and other maintenance.
That’s why battery-based backup systems sounds appealing to many homeowners. Unfortunately, the description of the systems provided by the manufacturers and their advertised capacity can be confusing to people with no engineering background. This guide will explain the features of such systems, discuss your main options and help you choose the right one for the home or business.
The two main configurations of battery-based generators are portable and permanently connected. The permanent ones are in turn divided into grid tie and standby. Let’s review these types and discuss their pros and cons.
Some companies sell portable backup systems which can be plugged into a wall outlet and provide electricity at their built-in AC outlets. When line voltage is present, their charger keeps the internal battery charged. When electrical outage happens, a built-in transfer relay switches the output outlets to the inverter, which runs off the battery. Such systems are sometimes advertised as home battery back up. In reality, they are nothing else than uninterruptible power supplies (UPS) similar to those that sell for computer applications. Even though such a device may be advertised as a home backup, it does not provide power to the house wiring. In addition to this, the capacity of corded systems is limited by the current you can draw from an AC outlet. Since a standard duplex wall outlet in North American homes NEMA 5-15R is rated at 20A combined current (and 15A maximum per each receptacle), you can’t draw out of it more than 120×20=2400 volt-amp total power. A recommended continuous load is actually 80%, which is 1920 VA.
- Lowest cost;
- Easy to use;
- No professional installation is required.
- Emergency power is provided only to the devices that are plugged into it;
- No easy access to the internal battery bank in case you want to increase its capacity;
- Rated power is limited to about 1920VA.
See my home-made battery backup system for gas-fired steam boiler.
FIXED STANDBY SYSTEMS
Such systems likewise contain a battery bank, a charger, and an inverter. However, unlike portable ones, they don’t have a line cord and AC outlets.
Instead, their inverter normally has input and output terminal blocks, which have to be hardwired to the home electrical system. The input has to be hooked up after the main service disconnect. The AC output may be connected to an auxiliary electrical panel that contains critical electrical branches (see the diagram to the right). Non-essential branches are supplied from the main distribution panel. When there is a grid failure, the inverter’s internal transfer relay will automatically disconnect the sub-panel from the utility line and connect it to the inverter. The inverter module usually includes the charger and the relay. Normally, the battery bank is external and may be sold separately. It can be scalable, so you can size it for a desired runtime. If the power rating of the inverter is sufficiently high, you can connect it to the main panel and use it as a whole-house battery backup. However, you would then need a 240VAC device with a center tap for neutral line. For your safety the wiring should be done by a licensed electrician. And always make sure the main service disconnect turned off before the installation.
- Provides emergency power to the home wiring;
- Automatic hands free operation;
- Allows connection of multiple batteries for longer runtime.
- Requires professional installation of a sub-panel and re-wiring of some circuits;
- Higher cost.
GRID TIE SETUP
Both portable and stationary systems discussed above provide power either from the utility or from the backup battery, but not from both. In contrary, grid tie systems are designed to work parallel with AC line. They use special sine-wave inverters that match amplitude, frequency and phase of their output voltage with the mains. In such systems, the electricity to your home can be provided from two or more sources simultaneously. Additional sources of energy in grid interactive systems are usually solar panels and/or wind generators. Under normal conditions, a small portion of energy is used to keep the batteries charged. If electric service fails, an internal contactor disconnects mains. The inverter then continues powering your residence from the battery. Note, that not all grid tie systems allow emergency battery back up- there are batteryless renewable energy systems that simply shut down during a blackout. The purpose of such batteryless systems is reducing your energy bills. If you are not using a renewable energy, obviously there is no point in buying a more expensive grid tie inverter.
PROS AND CONS OF BATTERY BACKUP
Here are the main pros and cons of home battery backup systems.
- Indoors operation;
- No pollutions;
- Unlike most generators, sinewave inverters produce clean well-regulated power;
- No maintenance is required.
- Not suitable for long outages because of low energy density of the batteries;
- Average life expectancy of deep cycle batteries is 3-5 years.