Photovoltaic systems (PV) – or small wind and micro-hydro systems for power generation – can be grid-connected or stand-alone systems.There is no difference between solar, wind and hydro Net Metering systems.
Connecting a PV solar system to the grid has obvious advantages: you can feed excess power into the grid when your are producing more electricity than that you are consuming (typically during the day, when you aren't at home); and you can draw electricity back from the grid.
You don’t have to have an expensive back-up battery system.
The grid-connection process involves a net metering agreement.
Net-metering agreements vary in their details, but they all state the right of the owner of the PV system to be credited for the electricity production fed into the grid, that is, the right to draw electricity back, when needed.
Electric utilities are compelled to offer these agreements to people with PV or other renewable systems, but most regulations do not compel them to buy excess generation.
You will also need a net meter, usually plugged into the standard meter.
Stand alone renewable systems are mainly used by homeowners, communities and farms located far away from utility lines. Net meters are bi-directional devices, designed to keep track of the power fed into the grid and drawn from it. The meter will spin backwards during the periods when your solar PV system generates more electricity than your consumption...
Off grid systems
Off-grid systems are expensive, due to their two main parts: the battery bank and the charge controller.
Batteries can be of two basic types: 1) AGM or gel cell sealed batteries or 2) FLA (Flooded Lead Acid) batteries. FLA batteries have a longer lifespan, but sealed batteries are maintenance-free.
Image: Energy.govDeep-cycle (lead-acid) batteries are the best option for renewable systems. Contrary to common car batteries, they can discharge and recharge 80% of their capacity many hundreds or even thousands of times; they have a very long life, contrary to shallow-cycle automotive batteries. Only deep-cycle batteries will ensure a lifetime around 6-10 years (depending on their exact type), which is anyway shorter than the lifespan of the other parts of the wind/solar/hydro system…
The charge controller is also a key component of off-grid systems. It controls the flow of power to and from the batteries, protecting them from overcharging: new charge controllers have a deep discharge protection in-built circuit.
Modern electronic controllers operate the system automatically.
Other elements of off-grid systems:
- a diesel engine-generator.
- an off-grid inverter, to convert the electricity of the system to alternate current (AC current), used in domestic and electrical appliances.
- AC and DC safety disconnects: to switch off the connection between the batteries and the inverter, to prevent electrical fires and other damages.
These elements increase significantly the cost of off-grid PV systems. Inform yourself. Your utility should provide you with a list of all the requirements for grid-connection and Net-Metering.