How to Calculate Solar Battery and Inverter

Written by John Ho on November 23, 2020
How to Calculate Solar Battery and Inverter 2021

Living on an off-grid system is not a piece of cake as one would think. A lot of work goes in from selecting the right sized solar panels for your needs to maintaining those solar panels with care. In fact, it is far more complicated than just setting a solar panel and maintaining the machine.

There are some other factors that come into play, as well, like picking the right battery and inverter for your solar system. This is where most people who have no prior experiences with off-grid living lose sight of the decision they should make.

And believe me, if you mess up here and get the wrong battery and inverter for your solar panels. Your days as a happy man will be numbered. I am saying this because, for say, you get a battery that is too big for your solar panels to charge, then your battery will never get fully charged. In case you get a battery with less capacity, it will not be able to fulfil your needs.

So a perfect way out here is to get a battery that is a balance between both of these aspects. Figuratively speaking, it should neither be too small nor too huge. Same goes for inverters.

To find a system that is the best of both worlds, I have created this detailed guide to walk you through all of the things that you should keep in mind while buying a battery and inverter for your needs.

Below is the complete guide which will teach you how to calculate solar battery and inverter. The best thing about this guide is that it complies with a person who is unfamiliar with solar technical terms in mind, so you do not have to worry about consulting an expert to understand any terms used in this guide. You can truly read and implement it yourself.

Guide to figure out the right size battery for your solar system

Step 1 - calculating your power consumption

To figure out the right inverter for your off-grid system. First, we will have to calculate the total wattage consumption of your home.

The easiest way to find out the total wattage consumption of your home is to add the individual wattage values of all your electrical appliances together.

For instance,

If your microwave and toaster are 100 watts each and you only plan to run these total appliances on your off-grid system (no one would do that, I am just trying to explain in the simplest manner).

Then your total consumption would be,

100W+100W = 200W

Now also consider the time that each of these electrical appliances run in a day to calculate watt-hours

For instance,

If your 100W microwave runs for 6 hours in a day and 100W for 4 hours, then the total watt-hours consumption of your home would be,

100×6+100×4 = 1000 watt-hours

Tip: always multiply your total watt-hours by 1.7 to add all the power losses that occur in a real-world application.

So in our case,

1000×1.7 = 1700 watt-hours

Step 2 - finding the right inverter

Now that you have calculated the total power consumed of your home in a day, it is time to find the suitable inverter.

Consider that you want your batteries to juice up your appliances for three days in a single go,

1700×3×2 = 10200 watt-hours can also be written as 10.2KWh.

There is a technical reason behind multiplying the whole thing by 2. But since I have to keep this guide as simple as possible so I will not be diving deep into that. You should just keep this mind that you have to multiply by 2 in this step.

After this, convert KWh into Ah. This can easily be done by dividing the voltage value of the system by 10.2KWh.

Ah = KWh/V

For instance,

If the voltage of your system is 24V, Ah becomes,

10.2KWh/24 = 425Ah

So in our case, you will need a 425Ah battery to run the electrical appliances of your home smoothly.

Benefits of buying the right size battery

  • You will be able to store sufficient energy for your appliances.
  • There will never be a shortage of energy during overcast days.
  • Batteries will be charged in a conceivable amount of time.
  • Batteries will discharge less, thus giving them a long lifespan.

Guide to figure out the right size inverter for your solar system

Figuring out the precise inverter capacity for your off-grid system is a much more difficult task than finding an inverter.

Since most people know that the job of a battery is to store power, so I did not feel the need to explain the function of a battery. However, often, people ask me what role do inverters play in an off-grid system? This is why, before explaining the sizing of the battery, I have explained the function of the inverter.

If you already know what an inverter does, then you skip the next segment and directly move onto the sizing part.

What is the function of an inverter?

Simply put, an inverter's job is to convert DC (direct current) into AC (alternating current). To explain this further, the appliances in your home run on AC and the electric current generated by a solar panel is DC. So in order to make the current generated by solar panels usable, an inverter is required.

Now that I have explained the role of an inverter in an off-grid system. Let's resume back to the sizing.

Inverters come in various sizes, but it does not mean that you can abruptly install any size inverter with your solar panels and it will uninterruptedly get the job done for you. There are a lot of factors that come into play, and I will try to state those factors below so you can get the right size inverter for your needs.

The ultimate formula to sizing inverters

This is the easiest way to size an inverter. In this method, you will first have to find out the total DC rating of your solar panels.

You can quickly do so through an amp meter. In case you need to learn more about how to find the DC rating of your solar panels, here is a detailed guide on that subject.

After finding out the total DC rating of your solar panels, get an inverter which is 1-1.2 ratio of that rating. Hold on, I know you did not get the last sentence, try understanding from the example below.

Example

Suppose that the DC rating of your solar panels is 4KW, so one ratio would be 4KW.

As, 4KW/4KW = 1

So in case of a 4KW DC rating, you should buy a 4KW inverter.

But if you do not want to buy an inverter this size, then you can also go with inverters of 1.2 ratios and below.

So,

4KW/3.5KW = 1.14

This is just to explain that you are not bound to a 1 ratio inverter only. Somewhere between a ratio of 1-1.2, is good. However, I would not recommend you to higher than 1.2 as it can damage your system.

Bonus tip: inverters are sold in the unit of (watts) so if you want to convert KW (kilowatts) into watts. You can do so by multiplying KW by 1000

6KW×1000 = 6000 watts

The importance of charge controller

Charge controllers are crucial when it comes to deciding the longevity of a solar system as they are responsible for preventing the battery for overcharging.

So do keep charge controllers in mind too. You can learn in-depth about charge controllers in this guide.

Benefits of buying the right size inverter

  • You will be able to convert DC of your solar panels into AC conveniently.
  • There will be fewer power losses.
  • Home appliances will not get damaged.
  • An off-grid system will last longer.

FAQs

Summary

As the movement to make the whole world run on green energy progresses, more and more people are buying solar panels and moving off-grid.

But, since this new wave of people getting into off-grid systems is not that knowledgeable about the ins and outs of solar systems. They often find it difficult to choose the machines required to build a perfect off-grid system.

Seeing this, people who have past experience with solar systems like me have started to step up and share whatever wisdom they have in this domain so everyone can do a green living conveniently.

Article written by John Ho

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