When the light goes out: Solar systems and blackouts

The question of whether or not your solar panels can power a house when the grid goes down boils down to the type of grid connection you have. Let's do a quick recap: there are three types of PV systems.

Grid-tie. Your home is connected to the grid, but has solar panels at the same time. Solar energy powers everything in the house during the day and all the surplus goes into the grid, which allows you to gain credits from utility companies – it's called net metering. There are no batteries in this system. This is the most widespread type of installation in the US.

Off-grid. You are not connected to the grid and rely solely on solar energy to power your home. Solar array charges your batteries that you use at night or when panels don't produce enough energy for everything.

Hybrid system. You are connected to the grid, but use solar batteries at the same time. Energy storage comes into play when an electricity blackout occurs. Not all the inverters fit this type of system.

Now, let's look at these types closely and see what's going to happen with those systems when the light goes out.

Grid-tie systems are 95% efficient and a net-metering program can bring you around $2,500 a year in some states. But will solar panels save you from a power outage in a grid-tie system? No, they won't. There are two reasons for this:

1. Grid safety. When you produce more energy than you need, the inverter sends the surplus to the grid. When the grid goes down and electricians start fixing it, there must be no current in the power lines. This is why all inverters connected to the grid are designed to turn off automatically, so that workers can bring the power back safely.

2. Safety of your appliances. During the day the power output of your array changes depending on the amount of sunlight the modules absorb. Normally, all the excess goes into the grid. If this uneven flow of energy from solar panels goes directly into the electrical system of the house, your appliances might break because the flow of electricity isn't consistent.

A while ago new grid-tie SMA inverters appeared on the market with a feature of Secure Power Supply (SPS). These inverters can power the house with solar energy even during power outages. You don't need energy storage to use them.Thus, the general rule is that grid-tie solar systems don't function during a power outage, unless an inverter offers SPS. So the question stays: how to use solar panels during power outages? Is there a way?

To keep powering the house with solar panels during a blackout, you need energy storage. This is where we move to off-grid and hybrid systems.

Off-grid systems aren't affected by power disruptions in the grid at all, because they aren't connected to the grid to begin with. These systems are normally used in remote locations where it's hard to connect to the grid or the power outages are a constant problem. You are guaranteed uninterrupted power supply as long as your battery bank is big enough for your house and you keep the batteries charged.

Hybrid systems are somewhat of a compromise between the two previous types. You earn credits by sending solar energy to the grid, but purchase batteries that will save you in an emergency situation. It makes sense to go for supplementary energy storage if uninterrupted power supply is critical for you, your home or business. For example, if you work with medical equipment, sudden loss of power can be dangerous.

A battery bank that is big enough for keeping your whole house powered is expensive. Some experts propose taking into account only critical appliances when sizing your energy storage. Another substitute for solar batteries would be a backup generator for blackout events. A simple gas generator might be even cheaper than a battery bank. However, it requires some kind of fuel, it's loud and smelly. So, it can't be a good full-on alternative to staying solar connected.