Solar power - Types of systems

Quick clarification... Being on the grid means you are connected to the power lines, a power utility that's supplying power from hydroelectric, nuclear, coal generators, natural gas generators, ... Here in North America we refer to the complex web of power lines and power generating systems as "the grid". Being off the grid means you have no external source of power, you are not connected to the power lines, your only source of power is your own solar, wind, micro-hydro or small generator.

Off-grid

This involves getting power from solar power with an optional generator as backup. See the diagram below. Instead of a connection to the grid, there can be backup power from a generator for when there are too many cloudy days in a row. Optionally the generator can be started and stopped automatically based on the battery voltage. In the diagram below it is a feature of the inverter. If your inverter doesn't have this feature, separate automatic generator starter units are available. Automatically doing this is better than having to run out and start the generator manually when the batteries get too low for two reasons: convenience and you don't want your batteries to go below 50% (without an automatic starter, you'd have to constantly be checking the battery voltage.) As an aside, the diagram below also shows a separate display and control unit, often available depending on the manufacturer of the charge controller and inverter.

For far more detail see the dedicated section on off-grid solar power including how system design and maintenance information.

Off-grid system.
Off-grid solar power system diagram.

The following photos show a typical system. Most of the wires have been run behind the wall and enter the components from their backs to prevent someone from accidentally hurting themselves.

You'll also notice some things called lightning arresters in the photo on the left. These protect the various components from lightning strikes. If lightning hits a solar panel then the very powerful electricity from the lightning could travel along the wires going between the solar panels and the charge controller and damage the charge controller. For this reason an DC lightning arrester is connected to that wiring and is shown at the bottom of the photo. Similarly there are AC lightning arresters used in case lightning strikes the generator or the wires coming from the generator and in case lightning strikes the household wiring.

Off-grid system on wall.
Off-grid solar power system components on wall.
Off-grid system including batteries.
Off-grid solar power system components.
Motorhome with solar system.

For the full step-by-step gory details of an off-grid installation see this off-grid cottege system installed on a motorhome.

The following is a detailed video tour of a completely off-grid home with a large solar system and a backup generator.

For far more detail see the dedicated section on off-grid solar power including how system design and maintenance information.

Feed-in tariff (or solar premium)

This involves selling power to the grid and getting paid for that power. It is essentially being a mini-power utility. Due to the high cost of solar panels, the amount paid by the power company is usually above cost.

Feed-in tariff system.
Feed-in Tariff solar power system diagram.

One example of this is the microFIT/FIT Feed-in Tariff program In Ontario, Canada. Here the average consumer can pay around $0.20 per kWh for electricity (total bill amount/kWh used.) The Ontario Feed-in Tariff program pays up to $0.549 per kWh.

Net metering

This involves feeding power to the grid and using power from the grid, paying for only the difference. Notice from the diagram that there are no batteries and so no storage of power. If there is a power failure on the grid then the inverter stops supplying power, even though you still have solar panels.

Grid-tied system.
Grid-tied or net metering solar power system diagram.

Usually you do not receive any money for the power you put on the grid. Instead, when your bill arrives, if you used more electricity from the grid than you produced than you pay only for the excess. On the other hand, if you produced more electricity than you used from the grid then you pay nothing for the electricity from the grid and any excess that you produced is kept track of for some period of time. Note that there may still be charges on the electricty bill which will not be reduced.

Example: Ontario, Canada

In Ontario, the program is refered to as net metering. Any excess that you produced is kept track of and is used as a credit against future bills for up to one year. After that time, the excess is forgotten. If before the one year is up, you have a few cloudy months and end up using more from the grid than you produce, then the credits that you accumulated are applied to the excess to reduce or elminate it. So you still may pay nothing. If during that billing period you use up all of your accumulated amount, and then you produce an excess the next month, the one year accumulation period starts over.

In Ontario, there are charges on the bill that are never credited.

As the following photo shows, there are additional safety features needed. If you are feeding power to the grid then you are an electricity generator for the grid. If there is a power failure on the grid due to something happening in your area, say a lightning strike damages something, then the grid workers will come out to fix the problem. Normally you'd think that since the grid is down the workers would be safe to do what they need to do. However, that's not true since your system is putting electricity on the grid.

For that reason, a system that feeds electricity to the grid must have a disconnect switch that is outside (see photo below) so that the grid workers can disconnect your system from the grid. Another typical rule is that they must be able to lock the disconnect switch. That's so that no one else can come along and reconnect your system to the grid while they're still working.

Net metering system components.
Net metering solar power system components.

Note that normally a grid tied inverter would automatically realize that the grid has gone down and stop putting electricty on the grid. But there's always the possibilty of something being wrong with the inverter and it continuing to put electricity on the grid.

Solar backup (or grid interactive)

This involves getting power from the grid and using solar power and/or other renewable energy sources, such as wind and microhydro, as backup. See the diagram below. The inverter is fed power both from the grid (through the main breaker panel) and from batteries. An extra breaker panel, refered to as a subpanel, is installed. The subpanel feeds the loads you need even when the grid goes down (essential loads). While the grid is up, the subpanel is fed through inverter from the grid. When the grid goes down, the subpanel is fed through the inverter from the batteries. This switchover is handled automatically by the inverter. The batteries are kept charged from the solar panels (through the charge controller) and from the grid (through the inverter, as it contains a charger too.)

Solar backup system.
Solar backup system diagram.
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