8760 Solar’s Guide to Solar Batteries: How to Understand What Size Battery You Need

Having a solar system installed is an amazing and clean way to significantly reduce your energy bills.

In most cases, solar systems are connected to the grid. During the day, when the sun is shining, the system harvests the energy that your farm either uses or sends to the grid as a credit to be used at a later date.

During times of the day when sunlight isn’t as plentiful, the system draws power from the grid to keep everything running. Essentially, the grid acts as a giant battery that you send and draw power to and from.

But what happens during power outages when the grid is not available? And what about remote areas of your farm that are not attached to the grid but also require power, like cabins and irrigation systems?

Battery storage is an ideal solution to these dilemmas. But what size battery does your farm actually need? Let’s take a look at how you can work that out.

Why Have Battery Storage?

In rural communities, power outages can be common. When we looked at the power outage stats for Colorado, almost 3,000 people were currently without electricity. 

Most residential homeowners can light a candle and make do until the lights go back on, but a farm is an entirely different beast. Loss of power means loss of revenue. If you can’t operate your farm because of a power outage, how much money will you lose?

This is why solar batteries are such a useful addition to your solar system. The battery stores the solar energy you produce and allows you to keep your farm running when the grid is down. 

They also give you the ability to power remote parts of your farm without having to resort to noisy and expensive generators. And batteries can provide a much-needed boost during peak demand without having to draw (and pay for) power from the grid.

Why Does Solar Battery Size Matter?

While a battery won’t power your farm long-term, it can often be enough to keep the essential things ticking over while the grid is unavailable or if the solar system doesn’t have enough light to harvest energy. The key is to select the correct battery size for your needs. 

If you choose one that is too small, it may only power your farm for an hour or two, or worse still, not at all. 

Additionally, solar batteries are expensive, so if you pick one that is too large for your requirements, you will have spent money unnecessarily. It’s also worth pointing out here that a large battery will be ineffective if your solar system is not powerful enough to charge it fully. Therefore, the size of your solar system limits the size of the battery that you can have.

You want to choose a solar battery that is in the “Goldilocks zone” for size. One that is neither too big nor too small but just right!

Solar Batteries Sizes: Things to Consider

Image credit: Tesla

A battery’s storage is measured in kWh. You can get huge commercial-grade batteries capable of storing 100 kWh+ right down to small batteries designed for residential use. 

Several batteries can also be stacked and used together to create a larger capacity.

Depth of Discharge

One very important thing to keep in mind is that solar batteries must be measured by their usable capacity rather than their total capacity.

The usable capacity is referred to as the “depth of discharge” (DoD), and most batteries’ DoD is around 80%. Draining the battery entirely can be dangerous as doing so negatively affects the chemical composition of the battery’s components. Draining the battery also has the added disadvantage of significantly reducing the length of the battery’s useful life.

Rate of Discharge

Another consideration is the rate of discharge of the battery. This is how much kWh power it can deliver at a given moment.

Imagine you have a gallon of water in a sealed bottle. You make a small hole in the lid of the bottle so the water trickles out. Eventually, you’ll get all your water, but it’ll take a long time to receive the whole gallon since only a little bit of water can exit through the hole at once. 

On the other hand, if you take the lid off the bottle, the water gushes out. You get all the water very quickly, but the bottle is soon emptied.

This is essentially how the rate of discharge works:

  • A battery with a high capacity but low rate of discharge may only be able to power a few essential areas of your farm, but will last a reasonably long time before it needs to be charged again.
  • A battery with a lower capacity but a high rate of discharge output may be sufficient to power everything on your farm, but only for a short time.

Therefore, it’s important to go for a battery that will deliver enough power for your needs over a long enough period of time.

The Temperature

The temperature where the batteries are stored also has an effect on the rate of discharge. Solar batteries discharge their power at a higher rate in hotter environments, and at a lower rate when it’s cold.

Colorado’s average annual temperature is 45.15 F, and its average winter temperature is 25.8 F. It’s not a particularly warm state, even though it is generally sunny. Therefore, you can expect the rate of discharge to be a bit lower than if it were situated somewhere warmer.

How Much Solar Battery Storage Do I Need? How to Calculate the Size

Solar batteries can be added to new solar systems or retrofitted to existing systems. During the installation of a new system, your solar developer will work closely with you to determine the right battery for your requirements. They can also help you fit a battery into your existing setup. 

However, there is a calculation you can do yourself that will give you a general idea of what size battery is required for your needs. Doing this will help you get a clearer overview of the expected costs.

Here’s the information you need to gather:

  • Your average daily power usage: How much kWh does your farm consume on a daily basis? Most power bills will show you what your average daily kWh is. If not:
    • Take a look at your monthly power bills and divide each amount by 30 (this gives you each month’s daily kWh rate);
    • Now look at the daily rate for each month and work out what the overall average is.
  • Work out what the DoD is for your chosen battery type.
  • The upward adjustment for the colder Colorado temperatures – your solar developer can advise on the exact percentage.
  • How many days worth of backup do you need? In other words, how long do you want your battery to last?

Next, you can work out the size battery that is required by doing the following equation. For this example, let’s assume that the DoD of the battery is 80%, the adjustment for the temperature is 40%, and we want the battery to last three days:

Daily consumption rate x 1.2 (80% DoD) = kWh amount x 1.4 (40% cold adjustment) x 3 (number of backup days required).

So, if your farm uses 10 kWh daily, the equation would look like this:

  • 10 x 1.2 = 12 
  • 12 x 1.4 = 16.8
  • 16.8 x 3 = 40.4 total kWh required

If you want a battery to power one specific part of your farm, such as an off-grid cabin or irrigation system, you will need to work out its individual daily kWh consumption rate.

Don’t forget that the size of your solar installation will also affect battery size and may place a limit on how large it can be. We invite you to check with your solar developer about the maximum battery limit for your solar array.

If you don’t want to spend the time manually working out the figures, try out this handy solar battery calculator for size. 


Solar batteries are a great – and highly useful – addition to your farm. They remove the need to draw power from the grid and give you access to power when there is none available elsewhere. 

However, take the time to understand which size battery you need. This will avoid any efficiency problems and ensure you get the power you require when you require it.

At 8760 Solar, we want to reassure you that you don’t need to spend lots of time trying to work out your ideal battery size. We can do all of that for you!

When you get in touch, we can talk to you about your vision for solar and run a thorough analysis of your farm. This will allow us to determine what kind of solar setup you need – including what size battery is best.

We’d love to hear from you. Text “READY” to 719 470-0254 or get in touch via email: sales@8760solar.com, and we’ll get you started on your solar journey.

Frequently Asked Questions

Are Solar Batteries Worth It For My Farm?

Solar batteries are worth it because they give you access to power during grid outages or during peak usage hours, allowing your farm to remain operational. Additionally, solar batteries can be used to power off-grid areas of your land, such as cabins or farm machinery. 

How Much Battery Storage Do I Need for Solar?

The size battery required for your solar system can be worked out by performing the following equation: Daily consumption rate x DoD (depth of discharge) = kWh amount x cold percentage adjustment) x number of backup days required.

However, if you are unsure, talk to your solar developer, who will be able to give you an accurate battery size for your installation.

How Long Will a 5kW Battery Last?

A 5kW battery can last up to ten hours if it is only being used to run a couple of appliances and a few lights. If you want to run larger appliances or equipment, then this time will be considerably less. Usually, a 5kW battery is not sufficient for a farm.

Is a 10kW Battery Enough To Run a House?

A 10kW is sufficient to run a house for around 8 – 10 hours. If you are planning to use a solar battery for an off-grid weekend cabin, you will need at least twice this amount to ensure the cabin can be powered for the whole weekend.

How Many Solar Panels Do I Need For a 10kW Battery?

A 10kW solar battery requires around 24 solar panels for it to charge fully.

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