Grid-connected systems allow you to use electricity you generate yourself, alongside electricity from the grid. When these systems don’t meet all your electricity needs, or you aren't using all the electricity you’re generating, the grid provides a back-up which acts as a battery. These systems are also called grid-tied, or grid-interactive, and can use one or a combination of any micro-generation technologies.
A grid-connected system will be most cost-effective if you’re using all or most of the electricity you generate yourself, because this offsets electricity at the full retail price, which is much higher than the price your retailer will pay you for buying your electricity. For a solar PV system, that means using electricity in your house during the day when the sun is shining and your solar PV system is generating the electricity.
Stand Alone Power System
A Stand Alone Power System (SAPS) generally use a combination of renewable generation sources (such as solar PV, wind turbine or micro-hydro), a battery bank, smart controller/inverter and a back-up.
Batteries allow you to store electricity from micro-generation so you can use it later. Banks of 12 V to 48 V lead-acid batteries are most commonly used. You need to replace them every 6 to 12 years - depending on quality, size and how much they’re used. You’ll also need a controller and an inverter, as well as a back-up generator.
Batteries of different makes and ages shouldn’t be mixed together, which means it’s important to get the size of the battery bank right to begin with. Deep cycle batteries are designed to be charged and discharged regularly. They’re different to normal car and truck batteries, which deliver a short burst of energy to start an engine.
Most households use direct current (DC) voltages between 12 V and 48 V. Individual batteries are usually only 2 V to 12 V, so you’ll need to arrange them to make up the voltage and current you need.
Make sure you buy the right sized batteries and battery bank for your situation.
- If your battery bank is too small - it may be discharged to a low level too often, which shortens its life.
- If the bank is too large - it will cost more and it may not fully charge regularly, which can also shorten its life.
Batteries need regular maintenance. This means topping up your batteries with distilled water (if they aren't sealed) and cleaning away corrosion, dust and dirt. Batteries contain dangerous substances like as acid and lead, so you need to dispose of them correctly - most are suitable for recycling.
Inverters convert electricity from direct current (DC) electricity to alternating current (AC), which most household appliances use. Stand alone power systems and grid-connected systems use different types of inverter. In both cases, advanced inverters control how much electricity you generate and how much electricity you use at any time.
Because stand alone power systems aren’t connected to the electricity network, you usually need a back-up generator (diesel, petrol or biodiesel) - for example when it's too cloudy for your solar PVs or there isn’t enough wind to drive your wind turbine.