How many batteries do I need to power my house?

When it comes to whole house batteries, things are changing very fast! And for good reason. This technology has the ability to impact virtually every aspect of our lives. From cars, aircraft and to even lawn mowers, things are moving towards electric.

Fortunately for those who appreciate renewable energy, the new energy storage is scalable and as such, can be sized to fit your house.

So how big of a battery does a house need?

To calculate how batteries you will need to power your house, simply take the hourly electrical load of your home (in watts) and multiply it against the number of hours this load will run.

By doing this, you should be able to size a battery storage system that is able to power your home for the length of time that you require.

Sizing A Battery For My Home

Obviously, the larger the reservoir to store energy within, the more energy there will be to use. And getting this reservoir right is crucial for those extended periods where power input is interrupted.

To begin, the measurement for stored energy in whole house batteries is in watts – specifically kilowatts (1kW = 1000 watts). For example, the battery bank in our house is rated at 60kW or 60,000 watts.

Now that we’ve established the nomenclature, it’s time to discuss some numbers. And the most important thing about these numbers is a term called ‘usable energy’.

*It is absolutely imperative that you understand this concept, as numbers can be very misleading.

For example, in the case of the 60kW battery bank mentioned above, there is actually only 12kW usable for my setup. This is because the lead acid batteries that I am using have a longer shelf life if they are only discharged down to 80% charge.

In comparison, the Tesla Powerwall is rated as a 13.5kW battery – far below the 60kW rating of the battery bank that is in our home. However, the Powerwall has ‘usable’ energy storage of 13.5kW – meaning it actually offers more energy for use, despite having a lower overall storage number.

Always make sure that you are working with the usable energy number (kW) and not the total energy (kW), as the later is, for all practical purposes, irrelevant to your needs.

So how many kilowatts of usable energy storage do I need?

For the sake of example, let’s start with a hypothetical 12kW.

If I had an electric heater that was rated at 1500 watts (1.5kW), then that heater would be able to run for 8 hours as 1.5 divides into 12 eight times. (12kW storage / 1.5kW heater = 8 hours of run time)

But what if you were also running a electric hot water heater that was rated at 4000 watts (4kW)?

Now your load (household energy demand) is much higher as the 4kW is on top of the already needed 1.5kW. Consequently, your 12kW of stored energy will only last a little over 2 hours as 5.5 divides into 12 @ 2.18.

Now what if you run the microwave… dishwasher… clothes dryer… ect.? In this scenario it is very easy to see how 12kW might not go very far.

So how is our house able to do it?

Our 12kW of usable energy is primarily to get us through the night when the demand for energy is very low.

During the day, our 7.8kW solar array takes over. And as our house load rarely exceeds 2.5kW, this means that extra power is available for storage.

### Note ###

Whether your home is off-grid (where you supply all of your own energy) or you are grid-tied (connected to the electric utility), whole house batteries must be of adequate size, otherwise you risk finding yourself in the dark.

Batteries And Discharge Rate

Another crucial aspect of whole house batteries is a term called discharge rate.

Simply put, discharge rate is the maximum amount of stored energy that can be released for use at any given moment.

To understand this, think of a water facet. A low discharge rate is where the water is just trickling out. A high discharge rate is where the water would really be pouring out.

It’s very much like this for whole house batteries.

For example, in the hypothetical 12kW scenario, if maximum discharge rate was set to 1kW then that would mean that whole house load could never exceed 1kW.

In other words, the 1.5kW heater mentioned above would not be able to run as this demand is higher than the 1kW limit set by discharge rate.

Likewise, if discharge rate was set to 5kW, then you would easily be able to run the 1.5kW heater, as well as, another 3.5kW worth of load – say a microwave, water pump, ect.

Like ‘usable energy’, having an adequate ‘discharge rate’ is absolutely critical to powering your house.

Even if your battery has a high usable energy rating, if that battery is unable to deliver power in the volume that your house requires, then you will find yourself in the dark.

Batteries And Cycles

Where usable energy and discharge rate deal with the performance of your whole house battery at any given moment, cycles is more relative to the life of your battery. And given the cost of whole house batteries, this is important.

Simplified, a battery cycle is one charge and discharge.

Obviously, a potential buyer is going to want to see batteries rated at a very high cycle number. And each battery will have its own warranty specifications regarding this characteristic. But sizing your system correctly will have an effect on the longevity of your purchase.

For example, if you have a heavy energy demand, then a battery with 10kW of storage is going to be ‘emptied’ a lot less than if you only had 5kW of storage.

While both the 10kW and 5kW batteries might have an adequate discharge rate to keep your house running, the 5kW battery has half the energy stored and consequently, will empty out faster. In other words, it will cycle much faster and more often – shortening the life of your battery.

Conclusion

The home battery market is one that is quickly evolving. What was the norm for decades is very much looking like the stone wheel from long past.

Improvements are coming at a rapid pace, with the capability curve appearing to be a near vertical climb.

It can be exciting or overwhelming, depending on your perspective.

However, the core components of household power demand have not changed. Usable energy and discharge rate are fixed/relevant, no matter what storage technology is employed.

In short, it’s not a matter of understanding the specifics of some new groundbreaking technology, it’s a matter of understanding how much power your daily life requires.

Once you have defined your needs (usable energy stored and discharge rate), then choosing a battery technology comes down to what appeals to you.