A Guide To Calculating Solar Panel Battery & Inverter Size

When it comes to making the switch to solar power it is important to understand what the best equipment will be for building your solar power system.

A Guide To Calculating Solar Panel Battery & Inverter Size

As well as knowing what equipment is best, it’s also important to know what the right steps to take are for building the solar power system too.

Some people tend to think that there aren’t many calculations involved when it comes to finding this equipment for their solar power system.

They believe that the only calculation needed is how much power there will be flowing through the entire system so that it totals to the amount they specifically need.

However, there are indeed other calculations that are necessary for building the most efficient solar power system.

Calculations for making your power inverter, charge controller, and batteries compatible are also just as important. There is a risk that your solar power system won’t work very efficiently or even at all if these elements are not compatible with each other.

Today, we will be looking at how you can work out these calculations for yourself so that you can find the best solar equipment for your specifications.

We will be looking at how you can calculate the amount of solar panels that you will require as well as what types you will need. We will also be looking at ways in which you can work out solar battery amp-hours so that you can pick the best power inverter.

Being comfortable with the equipment that you require will aid you in assembling the most efficient solar power system for your own personal requirements.

Solar Panel Battery & Inverter Sizing For A Solar Power System

As you may suspect, to be completely off-grid you will require a good amount of solar panels for your solar power system. However, you will require other elements too such as a battery bank in order to store energy and also an inverter or two.

This ensures that your system is not connected to the utility grid and is your own individual source of power.

You can in fact have your own home battery storage, even if you’re using the utility grid that your provider is connected to. These batteries are able to automatically switch over and turn themselves on if your typical electricity supply stops working.

In order to correctly size your solar power system, you will need to follow these steps:

  • Size your solar panels to fit with their energy consumption – Typically, to calculate the size of the solar panel you need, you will need to divide your daily kWh by the irradiance within your location so you know the solar kW rating.
  • Size your inverter to fit with your solar power system power rating – Once you have calculated your solar panel rating that will be the same as your inverter size.
  • Size your battery bank to fit with the amount of hours that you require it to run for – To calculate the battery size needed, you will need to multiply the number of autonomy days you need by the daily load amount. Then, you will need to divide this by the system volts so that you then have the right amount of amp-hours.

How To Calculate Solar Panel & Battery Size

How To Calculate Solar Panel & Battery Size

It is easy to assume that the solar panel size and battery size would be the same, however, this is not the case.

Your solar panel array size is actually dependent on your home’s energy consumption and the size of your battery is dependent on the autonomy that you require.

So, before calculating the sizes that you need, you will need to determine how long you actually want your battery to be able to supply you with power if there was a grid failure or a cloud cover.

The amount of time that you specify is the autonomy days that you require. From this, you will then be able to figure out what size battery would be best for you.

Calculating The Solar Power System Size

In order to work out what size of solar panel that you will need, you will need to know what the load is that is required. If you’re looking to power your home then you will be able to use the daily, monthly, or annual kWh to understand your load size.

The best way to find out the size of the power load is to look at your utility bill for the last year. From here you will be able to see a breakdown of your daily, monthly, and annual electricity consumption. This is your power load.

Once you have this information, you can translate it into generated solar panel energy and system rated watts so that you can figure out exactly how many panels you will need.

What Is The Average Amount Of Energy That A Home Consumes Each Day?

The average American household consumes around 900kWh per month or 30kWh per day according to the US government.

You can use this average energy consumption to help you understand the size of the solar panels that you will need as we will be able to work out the amount of solar power that will need to be generated.

What Effect Does Irradiance Have On Solar Panels?

Solar panel power output can be affected by many different factors, however, the sun’s energy (irradiance) is the biggest factor overall.

This irradiance can be measured in kWh per square meter per year or day, this is abbreviated to kWh/m2/day. This number will differ drastically depending on where you are geographically located.

These irradiance measurements are also referred to as peak sun hours.

Solar installers are the people who tend to rely on this measurement most of the time so that they are able to calculate how much energy each solar panel that is installed is able to generate.

A lot of the time you are able to find out what the irradiance is within your geographical area by looking it up in online databases such as GlobalSolarAtlas. All you need to do is find the city where you are located and take note of the peak sun hours in this area.

In this example we have taken 5 different cities from around the world and compared their irradiance so you can visually see how these numbers can differ even across various US states.

City San Fran., CaChicago,IlLondon, UKHouston, TxGlasgow,UK
Peak sun hours (irradiance)5.6994.0432.3744.2531.860

Solar Panel Sizing Guide

The irradiance found in Chicago is about the average found across the US, so when giving an example for calculating solar panel sizes I will use the average irradiance (4) as a guide.

  • Daily energy needed = 30kWh
  • Solar wattage needed = 30kWh/4 irradiance = 7.7kW (solar power)

If we say that the solar panels that we are considering are 300 watts, then our next calculation would be:

  • No. of solar panels = 7700 watts/300 watts = 25

So, in this example, we would require 25 300w solar panels in order to cover the energy consumption that we require.

However, it is also important to consider the losses that can impact our solar power system. The calculations we just did together assume that we are gaining all of the power that is being converted. This simply isn’t the case.

Solar power system losses are usually around 25% on average. So, we will need to add another 25% onto the amount of panels that we need in order to cover these losses.

  • No. of solar panels needed = (25 x 2.5) + 25 = 38

In order to cover our solar power system losses, we will need 38 300w panels in total.

Solar Panel Losses

When doing the calculation above, we factored in a loss of 25%. This may be an exaggeration, however, it is better to use an over-estimation rather than underestimate.

You will find that solar modules aren’t too costly and so it is better to have more capacity than not enough in the long run.

There are ten main solar power system loss factors. These are:

  • Shading losses (7%)
  • Dust and dirt (2%)
  • Thermal losses (4.6%)
  • Array mismatch (0.7%)
  • Reflection (2.5%)
  • DC cable losses (1%)
  • Spectral losses (1%)
  • Inverter losses (3%)
  • Irradiation (1.5%)
  • AC cable losses (0.5%)

So, where there is 100% of solar power input you will find that you are only achieving around 76.2% of solar energy output.

Calculating Inverter Size

Calculating Inverter Size

Usually, it is best to use the power rating of your solar array as the size of the inverter that you will need. This is best because inverters are more efficient when they are fully loaded.

A decent quality inverter is often at a full load efficiency of 97%. If your inverter is lightly loaded you may find that your energy suffers from more losses.

On top of this, your inverters are best when not oversized. All in all, you want to aim for the same power output as your solar array.

What Size Battery Is Needed?

In order to work out the best storage capacity for your battery you need to know what the length of time is that you require your battery to be able to supply the home.

As well as this, you will need to know what your energy consumption is using the steps we covered earlier.

So, if we assume that your energy consumption is that of the average American household (3kWh per day) then we can work out the size of the battery that is needed.

Let’s say you want your battery to be able to cover you for a full 24hrs of use, then the kWh of your battery would need to be roughly equal to that of your daily energy consumption.

If you need your battery to cover you for 48hrs, then you would simply need to double this battery capacity. Typically, an off-grid storage system accounts for 48hrs of autonomy.

This is to make sure you are covered if there is a lot of cloud cover that affects your power losses.

  • Battery size for 48hrs = daily energy consumption (30kWh) x 2 = 60kWh
  • Battery Ah = 60kWh/battery voltage (48) = 60000/48 = 1250Ah

It is important to note that usually batteries that are lead-acid and deep-cycle usually have their own recommended “Depth of Discharge”. This is calculated around 50% typically. So, you will need to double the amount of Ah that you calculated earlier.

  • Battery Ah = 1250Ah x 2 = 2500Ah

How Many Batteries Are Needed For A Solar Panel System?

If we use the result of our calculations above and assume that we are using 200Ah batteries that are deep-cycle, then:

  • No. of batteries needed for 48hrs autonomy = 1250Ah/200Ah = 6.25 batteries.

We would round the number up rather than down so that we can cover all the power we need. So, in this case we would require 7 batteries each at 200Ah. However, it is also important that we take the depth of discharge into consideration from earlier:

  • No. of batteries = 6.25 x 2 = 12.5

Again, we would round this number up meaning that 13 200Ah batteries would be required.

Is A 5KW Solar Panel System Enough To Power A House?

In order to answer this question you will need to know what your energy consumption is per day. For this example we will use the average American household energy consumption (30kWh per day).

We will also factor in the losses of 1.44. Now, we can calculate the amount of energy that a 5kW system would be able to generate.

  • 30kWh x 1/44 = 43.2kWh

Using the average irradiance, we know that 1 solar watt is capable of generating 4 watts per hour per day. With this, we can calculate the amount of solar energy that this household requires:

  • 43200/4 = 10.8kW

Now we have done this calculation we can see that a 5kW solar power system wouldn’t actually be enough to power the average American household. You would need over double the amount of power in order to be able to power your home.


It is quite normal to feel like these calculations are a little overwhelming at first.

However, if you have the right information in regards to your daily energy consumption then you will have no probable calculating the right solar panel size, inverter size and battery sizes needed for your home.

Soon, you will be able to power your home entirely off-grid, potentially saving yourself a lot of money in the long run. Of course, it is an up front investment, but this investment is more than worth it in the long run.

We hope that this information has been helpful in order to aid you in calculating the required battery, solar panel, and inverter sizes for your home.

Joe Danner