Capturing solar energy in your home is the first step to building energy independence, but the sun only shines for half the day .  How do you store its energy for use at night?

There are several popular techniques for storing solar energy.  They are divided into two groups, depending on whether you are using photovoltaic cells (PV) or solar heating.

Storing Electricity

PV cells turn sunlight directly into electric current.  There are two main ways of storing electrical power for later use:

  • Batteries
  • Grid Tie-in

Battery storage has been around since the beginning of PV solar power.  The concept is simple – when the PV system is delivering more power than your system draws, the excess charge is stored in a bank of batteries.  Then, when output decreases or you begin to use more energy than your cells can provide, the flow reverses and the batteries make up the deficit. 

The two most common types of batteries are lead-gel and lithium-ion.  Lead-gel batteries are cheaper and more common, although they can only be discharged to 50 percent of capacity and so offer less working storage.  The lithium-ion batteries used in home battery systems can be discharged to 70 percent, a distinct advantage over lead-gel. 

In either situation, though, when the batteries are depleted, you're out of power.

A grid tie-in is another option for storing PV solar power.  During the day when your cells are running at full capacity, excess energy is sold to the power grid and other consumers.  At night when production falls, you draw energy from the grid.  In the end you pay only for the balance, although there can be a significant difference between the price of selling electricity and the price of buying it.

A third option is to combine these two approaches so that your home can draw energy from the grid when your batteries are depleted.  This helps negate the drawbacks of both methods.

Storing Heat

The good news is that storing heat energy is much simpler than storing electrical energy.  The bad news is that unless your home has been designed for it, it is difficult to install a heat storage system.

The easiest way to store solar heat energy is to increase the thermal mass of your home.  Thermal mass is any substance that absorbs solar energy when exposed to sunlight, only to release it later when it is needed.  Brick, stone, and concrete are some of the more traditional materials used to increase thermal mass, and homes designed for passive heating feature large windows to let the sun strike these surfaces during the day and “charge” them.

These systems require practically zero maintenance, but their weight makes installation difficult.  Not all homes can be adapted to use passive heating.

Another way of storing heat involves a solar water heater.  This device rests on the roof like PV cells and produces hot water for your home instead of an electric or fossil-fuel water heater.  Hot water is usually one of the largest energy expenditures in a household, and a solar water heater can reduce or eliminate that expense.