Electric Vehicles

Off-grid refers to a PV system that is not connected to the electric grid. If you do not have access to the electric grid, this is the type of system that you would install.

The system consists of:

• PV panels to convert sunlight into direct current electricity
• Batteries to store the energy produced
• A charge controller to regulate the current from the PV panels to maximize charging but protect the batteries
• Safety equipment to protect the panels, charge controller, batteries and you
• (Possibly) an DC to AC inverter if you need 120V and/or 240V AC power

PV Panels

These are flat panels of typically silicon-based cells. Typical panels of today will generate 400-500 watts per two square meter panel. They are totally passive (no moving parts) and tend to have a guaranteed life of from 20 to 30 years. 

Batteries

This is where you store energy produced by the panels so you can use it when needed. Deep cycle lead-acid batteries have been the norm for decades but lithium ion batteries have finally become available at a realistic price. LiIon batteries have many advantages including longer cycle life and significantly less weight. Of the LiIon choices, Lithium Iron Phosphate (LiFePO4) batteries are the best choice as they have higher cycle lives than other lithium batteries, can be fully charged/discharged without degradation and cost less.

Charge Controller

This is a device that regulates the flow of current from the panels to the batteries. The least expensive charge controllers operate in a pulse width modulation (PWM) mode to limit the charge to protect the batteries.

An maximum power point tracking (MPPT) controller adjusts the PV load to maximize the charge rate of the batteries while still preventing overcharge. There are two advantages here:

1. You no longer have to match the PV panel voltage to the battery voltage. Besides a general advantage (for example, a 24 volt panel charging a 12 volt battery would be very inefficient with a PWM controller) you can connect more PV panels in series offering higher voltages at lower currents decreasing the cost of wiring.
2. Even with a decent match between panel and battery voltage, you will probably gain around 30% more energy from your panels.

Safety Equipment

Fuses or circuit breakers between the panels and charge controller and between the charge controller and battery should be installed to protect the equipment. You also should put a circuit breaker between the battery and the load.

A second important piece of safety equipment is lightning arrestor. This will protect the charge controller if a panel gets struck by lightning. Note that the metal frame of the panels needs to be connected to a good electrical ground.

Inverter

With the above equipment you have a "DC" system. What that means is that you only have the output of the batteries to run your house. Smaller systems tend to use 12 volt batteries and lots of 12 volt appliances and lights can be purchased so that may be sufficient. If, however, you want to run typical household appliances such as a refrigerator you will want to add an inverter to the system. The inverter converts the DC voltage from the battery to 120 volt and/or 240 volt AC current.

There are lots of considerations for sizing such a system -- number of PV panels, power handling capabilities of the charge controller, battery voltage and ampere hour capacity and inverter size. If there is sufficient interest I will add a pace on this information.