What Does an Inverter Do?
Inverters convert DC (typically low voltage) DC into AC (at 230V 50Hz) as required for conventional appliances.There are generally two types of inverter available: off grid and grid connect.
Grid Connect Inverters
Grid connect inverters are supply driven - they provide all the power supplied from a DC source to the grid or mains. Grid connect inverters are usually optimised for one specific type of generator, e.g. PV and generally operate at a higher DC voltage than off grid inverters.
Grid connect inverters should NOT be connected to batteries and off grid inverters should NOT be connected directly to PV, wind turbines or the grid.
Off Grid Inverters
Off grid, or battery supplied, inverters are demand driven - they provide any power or current up to the rating of the inverter and assuming that there is enough energy in the battery.
Smaller systems with few appliances may have only DC power, but recent advances in inverter design, efficiency, and reliability have increased the potential of wind/solar systems considerably.
With the use of modern high efficiency AC lighting the majority of, if not all, loads can be operated on AC especially in larger installations.
We tend to use both AC & DC where each is most effective and economical - many DC appliances use less power than their AC equivalents (especially refrigeration, lighting & electronics) - but DC appliances tend to be harder to find & more expensive.
Where Does any Excess Energy Go?
This depends on whether the system is off grid or whether it is grid connected.
Storage batteries are the heart of all off grid wind/PV or inverter electrical systems. By storing excess energy when the wind or sun is strong, they offer a reliable source of electricity which can be used when solar or wind power is not available. Their function is therefore to balance the outgoing electrical requirements with the incoming energy supply.
Batteries are also able to provide short term power output many times higher than the charging source output.
For grid connected inverters energy not being used by on-site loads is fed back into the grid.
Who Needs a Generator?
In typical domestic situations, for most of the day, loads are very small - perhaps a few lights and other appliances. For a small proportion of the time, however, large loads such as washing machines, electric kettles, etc. must be powered.
Sizing an off-grid renewable energy system to meet this peak demand is, in most cases, prohibitively expensive (at least initially).The optimum way to incorporate a wind and solar energy is for these to supply the low loads required for most of the day, and allow a generator to start up automatically to meet the small proportion of loads for which a large capacity is required.
In such systems, batteries allow power to be available 24 hrs/day but means that the generator need only run for short periods to charge the battery.
Modern electronic inverters are very efficient over a wide range of outputs, but some power is required simply to keep the inverter running (the standing losses) and they are less efficient when running small loads. Consequently, sizing the inverter for its required purpose is extremely important.
- If it is undersized, then there will not be enough power - demanding more than their limit will shut them off.
- If it is oversized, it will be much less efficient (due to the standing losses) and more costly to buy and run.
A load seeking circuit is normally included to ensure that battery power is conserved for useful purposes by automatically switching the inverter on and off as loads are applied or discontinued.
In off-grid inverter sizing the most important factor is peak power consumption: the peak power demand should not exceed the rated peak output of the inverter. This is difficult when it is possible for many devices to consume power at the same time, and is further complicated by any electric motors in the system. Some types of electric motors require three times as much power to start them as is required to run them. If two or more motors are started at the same time the surge power demand is much higher than the average demand. Consequently, the inverter should be sized to be able to at least start the largest motor in the system and measures taken to ensure that all motors do not start at the same time. Proper energy management can reduce peak demand, and so the inverter can be sized closer to the average power demand, thereby increasing the system's efficiency and reducing hardware costs.
Inverters should be located in a dry, non-condensing, clean, ventilated, environment. Vented lead acid batteries can produce corrosive vapours and when on charge produce an explosive mixture of hydrogen & oxygen. So good ventilation is required for the battery, particularly at a high level to allow any hydrogen to disperse. Preferably, the battery should be in it's own cubicle, vented to the outside. If this is not practicable, don't mount the inverter directly above the battery or directly adjacent to it. In order to minimise the voltage drop in the connecting cables to the battery, these should be kept as short as possible and of sufficient size.