The answer is yes and no. No you cannot likely afford enough batteries to power your entire home if the grid is down for a week. But yes you can power the Critical Load items of your home or business from a photovoltaic battery backed-up system.

If we use a 10kW photovoltaic system as our base line example, it is realistic to run that power through an inverter/battery combo system that would be very capable in keeping one’s home/business operating for a few days without the grid. It requires a process of either physically segregating certain consumption items in your main circuit breaker panel to what is called a critical loads circuit panel or electronically controlling which circuits receive the battery/PV power during grid outages.

  • Items we recommend you power on the critical loads panel are:
    • Furnace blower motor and igniter
    • One frig freezer combo A few lights
    • A few outlets – router, modem, phone chargers, toaster oven Garage door electric opener
    • Medical health equipment

What to learn more… read on 

When deciding on how powerful a battery system and how large a solar system you need, there are three factors to keep in mind that function together but are separate limits on power availability when the grid is out.

  • Inverter size, and AC output as measured in kW
  • Batter size, measured in kWh
  • Critical load panel loads, measured both in kW and kWh

So back to our example, a 10kW DC solar system with a 7.6 kW AC inverter coupled with one 10 kWh batter is a very common set up. During an average day of consumption with good solar radiation this system would power the critical loads panel circuits and charge the battery. At night or with grid outages this battery has a max discharge rate of 5kW, which if used fully it would be depleted in just under 2 hours. Instead if you just had one 100 watt light bulk running the battery would last 98 hours (100w x 98 hours = 9800 watt/hours) (The batter will never discharge to zero).

If one increased the battery size to 2 – 10 kWh batteries you could have 10kW of instantaneous draw capability and double the life, or 20 kWh. It is important to keep in mind that with a battery system we are not attempting to power the entire structure, just the critical items that maintain some normal functionality. To power everything would be either an off grid system or a very large battery bank.

So as a consumer, what should you do before you decide to start shopping for a battery and photovoltaic system? Determine what items you really want powered during power outage and for how many days you think you want these items powered. In that way a solar integrator can start by sizing a battery to meet your goals and work backwards to size an inverter and photovoltaic system that all have the capability to perform as desired.

One final thought, remember you are designing a system for the worst case scenario so you cannot assume that the photovoltaic system will be producing power during the grid outages as a blizzard that took down the grid is also covering up your solar panels.