The beauty of pairing battery storage with solar is that you essentially create your own miniature utility to power your home. This is especially useful during prolonged power outages because unless you have solar power battery storage, your solar system will be turned off by the local utility to prevent the backflow of electricity from injuring workers trying to repair the grid.

But if you have battery storage, your system can operate independently when the grid is down. This is called “islanding.”

How electricity does a solar system produce?

Solar panel systems are measured in kilowatts (kW) that represent the amount of energy the system can produce in an hour of peak sunlight. So a 5 kW solar system can produce 5 kWh of electricity per hour in ideal conditions. But since conditions aren’t always ideal, we typically assume a performance ratio of 75%.

The average number of peak sunlight hours per day varies from 3.5 to 5.5 in the US. Multiple the system capacity by sunlight hours and 0.75 to find the daily output of a solar system.

For example, here’s how you would find the daily output of a 5 kW solar system getting 4.5 peak sunlight hours per day equals:

5 kW solar system x 4.5 sunlight hours per day x 0.75 performance rating = 16.875 kWh per day

In many cases, that’s more than enough to power essential electrical systems and recharge a 10 kW battery to use overnight. But electricity needs vary from home to home, so let’s run through a some common appliances and how much energy they use.

Your electricity needs during a power outage

How long solar battery storage can run your home depends how much electricity you use. And how much electricity you use depends on which appliances and systems you’re running.

During a power outage, it’s recommended to budget electricity for the most necessary things first and then make a plan for the remainder capacity. For many homeowners, the list of priorities includes:

Refrigerator

Kitchen and cooking

Water heating

Lights

TV and device charging

Heating and cooling

Now, heating and cooling is last on the list not because it isn’t important, but because these operations take so much electricity they’ll likely have to be limited regardless of your battery capacity. So the wise move here is to budget your essentials first, and then get a sense of how much battery capacity is left for heating and cooling.

Note: If you have a gas furnace, you won’t need much electricity to keep the heat on.

Let’s run through an example scenario of powering essential systems during a 24-hour power outage to get an idea of how much solar and battery capacity you’ll need.

A refrigerator is among most important things to power during an outage so that you can avoid having your food and drink go to waste.

If you have a modern, energy star approved model, your refrigerator is using around 1-2 kWh of electricity a day. If your fridge lived through Y2K, you might be looking at closer to 5 kWh per day.