Solar power – even when the power grid goes down “Battery Backup Generator”

Solar power – even when the power grid goes down

A SOLAR GENERATOR provides energy savings and energy security, with no maintenance, no moving parts and no fossil fuel.


I am often asked or hear the question: will my grid tied solar pv system stay up and running when the utility power grid goes down. If you have a battery backup system installed with your grid tied solar system the answer is a simple YES. If you do not have battery backup then NO, not even with a fossil fuel generator hooked up to it. A battery backup system tied in with your solar system is called a solar generator. You have options for a solar generator.

Solar Generator in a Nutshell

Without a solar generator, your solar energy (PV) system shuts down when the electric utility has a power outage.

To appreciate the extra value a solar generator offers, you have to understand how a typical grid-tied PV system works. A grid-tied solar PV system produces electricity that is sent into the home or business first if there’s a demand for electricity. If you are not demanding electricity, or if the demand is being satisfied, then any additional electricity from the PV array goes back out to the grid through a bidirectional meter that banks or counts the units of electricity you’re sending back to the grid. Those units sent back out to the grid are there for you to withdraw whenever needed. In this sense the grid becomes a big storage battery that is 100% efficient. However, this “net metering” only works for you when the utility grid is on. When the grid goes down because of storm damage or other reasons, there is no power on the grid for you to withdraw. Your PV system shuts down as well –to prevent back-feeding of electricity onto the grid that can potentially hurt or kill utility employees who are working on the power lines.

The typical grid-tied inverter inverts DC power from the solar modules to an AC power that is usable in your home by replicating the “sine wave” (a graph of voltage produced by a generator as a function of time) it sees from the grid. An off-grid inverter will actually create this sign wave, thus allowing it to operate independently from the grid. But as you can see, since the normal grid-tied inverters need to see a sine wave from the grid to operate, without the sine wave (ie grid), there is no power production from your array.

Solar Generator

A battery backup PV system (solar generator) that is also connected to the grid will cost more than your typical whole-house fossil fuel standby generator. But a solar generator provides you with a seamless transition from grid to batteries. You won’t even notice the transfer of power. With a solar generator, you could stay powered for years without the utility as long as the sun keeps coming out! It all depends on the electrical load or demand (amps), demand duration (amp hours), the number batteries you have and the size of the solar array. Of course we always want the utility to hold our units of electricity for us but across the country, people live every day with pure off-grid homes that have never and will never be connected to the grid!


The pros:

  • Uninterrupted power for the times the grid goes down and normal day to day utility bill reduction when tied to the grid
  • Critical loads are always energized like your well pump, furnace, refrigerator, and microwave, sump pump, general lighting, oil or gas water heater.
  • Safe, secure and flexible installation. Since your battery array operates silently and has no hazardous fuel, fumes or exhaust, it can be installed just about anywhere –in the basement, garage, utility room, etc.
  • No maintenance There are no moving parts in a solar generator –nothing to break down or malfunction. The sealed, deep-cycle batteries we install are designed for the typical charging cycles of a PV system, and should last 10-12 years.
  • Monitoring The batteries are constantly monitored and exercised for peak performance
  • Remote or urban installation settings on any scale assuming the resources needed are available

The cons:

  • Size Limited supplies of electricity based on battery capacity, available solar irradiation, and size of solar pv
  • Cost more than a traditional fossil fuel generator
  • Batteries that have a life expectancy of 10-12 years and will degrade to a point where they cannot be recharged or reconditioned


Limitations Of Solar Generators To Bear In Mind

To live within the means of your solar generator, it’s important to understand the limitations of your system. A solar generators’ battery bank is designed to provide power to a selected group of “critical loads” in your house:

Rules of Thumb

  • 1 watt of charge controller per 2 watts of solar
  • 100 amp hours of battery per 1 kw of PV
  • 1 watt of charge controller per 1 watt of back up load
  • 100amp/hours for every 1kW of solar

Solar energy can sometimes be scarce – such as during cloudy weather or at night. Therefore, it’s important to size your battery bank to facilitate the powering of these loads, so they can operate for a predetermined period of time without receiving a charge from the Solar Modules or supplementary generator. This rating is called “autonomy”. From there, the rest of the system must be designed to support this desired autonomy.

As with every electrical appliance, a battery has a specific set of electrical characteristics depending on its construction. Typically, a specific discharge rate is used to determine the amp/hour capacity of the battery. This describes how long a load (or loads) can be powered from the charge contained within the battery.

How Much Power Do You Need?

Once critical loads are selected, and their consumption of power and average run time defined it’s possible to determine how many amps (current) of electricity is needed to power those loads for the period of autonomy you’ve set as your goal.

If an appliance needs 1 amp of power to run, and is set to run for 10 hours per day, you know that you will need 10amp/hours of energy per day to run that appliance. If the desired autonomy is five days, your battery bank will need to hold a minimum of 50amp/hours of energy to power that specific appliance off the battery bank for five full days without a charge from an external source.

There are many steps when describing how a solar generator is sized, and they that may seem a little confusing at first blush. The good news is that if your system is designed and installed correctly, you can enjoy power off the grid for extended periods of time without experiencing the problems associated with a fossil fuel generator.