How To Size An off-grid PV System? (Expert advice)

Knowing how to size an off-grid PV system is different to calculating your power needs for a grid-tied system.

Living off-grid means you will need to focus on your overall kWh usage as apposed to focusing on your annual electricity bill. 

Although off-grid solar systems are more expensive than any grid-tied system you'll find, for some it is worth the money. 

Being off-grid means you will be 100% reliant on your own energy production and storage.

This article aims to teach you exactly how you can size your off-grid solar system. 

Let's get started. 

Sizing an off-grid solar system

In the following section of the article we are going to outline the steps you need to take in figuring out what size battery bank will suit your off-grid solar system.

Step 1 - Figure out what your energy needs are

First things first, you are going to have to figure out exactly how much electricity each one of your homes appliances uses. 

Doing this can be somewhat of a tedious task, but it is absolutely critical in correctly sizing your off-grid PV system. 

You are going to need to go through each one of your homes appliances and check their power consumption. 

You can usually find a label on the back which states their wattage draw. 

For example like on this label of a washing machine: 

Note that it draws 470 watts. 

You are going to have to make a notes of this, for each one of your appliances, so I recommend getting out a pen and paper or alternatively open up a word document on your computer and get tabulating. 

For example, we are going to make an example list of all the things that run in our small wooden cabin:

Load (watt) Duration of use (hours) Power (watt hour)
300 watt fridge 24 7200
2000 watt oven 1 2000
10 watt light bulbs x8 7  560
Total power 9,760Wh

 

Now, remember this is just an example, of course your home may have much more appliances that draw more load. That's fine, continue tabulating your list until you have covered each appliance. 

Step 2 - Add inverter load

 If you plan on using an inverter to convert DC to AC power then you'll have to account for inverter efficiency losses. 

Inverters always consume a small fraction of generated power when running. 

Therefore you are going to have to add the consumptions rate of your inverter to your daily total. 

Each inverter has it's own consumption rate so you are going to need to look at your own one to see what it draws. 

For example, let's assume you inverter uses 50 watts and runs for 8 hours a day, you are going to need to add that to your power load: 

Inverter load = 50 watts

Watts x 8 hours = 400Wh

Total load including your inverter = 9,760Wh + 400Wh = 10,160Wh

Also, you are going to have to account for inefficiencies in your overall system. 

Efficiencies usually range between 5% - 15% and depend on the type of inverter you use and how much load you draw. 

This is quite an important step to consider when sizing your off-grid solar battery, so make sure not to skip it. 

Step 3 - Calculate your battery size

Batteries store collected solar energy for later use. 

the size of your battery will depend on the amount of backup power you will require to run your appliances. 

You may find that in the winter months it may make sense to add a backup solar generator to your off-grid system as there may not be enough sunlight to power your homes main solar system. 

When choosing your off-grid solar systems battery size, you need to consider system inefficiencies and temperature coefficients. 

the rate of inefficiencies depends on your overall solar setup (as always).

In order to compensate for these inefficiencies you will need to oversize your battery bank.  

In most scenarios a batteries output voltage will be rated either 12V, 24V, 48V or 120V. 

Your first step is to decide what battery voltage you are going to use in your PV system.

Here's a rule of thumb to help you make your decisions: 

  • 12V = perfect for smaller solar systems
  • 24V, 48V = ideal for medium to large solar systems
  • 120V = great for very large solar systems. 

Account for inverter inefficiencies and temperature coefficients

Take a look at your inverter spec sheet and see what the efficiency rate is, if it is 90% you will of course have to add 10% to your daily load sheet.

So assuming your daily consumption is: 10,160Wh.

10,160Wh x 1.1 inefficiency = 11,176Wh.

Next account for temperature fluctuations. 

Temperature fluctuations affect battery efficiency, you should expect batteries to become less efficient in winter. 

Both lead-acid and lithium-ion batteries operate and varying efficiencies in colder weather. Lithium-ion tends to operate at a better efficiency in winter.

Assuming you are using lead-acid batteries in your off-grid PV system you will need to multiply your battery capacity by 1.59%

11,176Wh * 1.59 = 17,769Wh (lead-acid)

If you use lithium-ion batteries in your build, multiply your required capacity by 1.2

11,176Wh * 1.2 = 13,411Wh (lithium-ion)

You also have to account for efficiency loss when charging and discharging the battery. 

  • Efficiency loss for lead-acid = 20%.
  • And for lithium-ion = 5%

17,769Wh x 1.2 = 21,322Wh (lead-acid)

13,411 x 1.05 = 14,081Wh (lithium-ion)

Days of autonomy

Lastly, you will need to figure out your days of autonomy. 

After you have correctly sized your battery, you will need to figure out how much power you will need to recharge it. 

Additionally, you will need to figure out how many days you will want your battery system to be able to supply you with energy incase there is a number of consecutive overcast days with no sun. 

Generally speaking, it should be between 2 - 5 days at least. 

Now, multiply your battery size by the number of days you want power should there be absolutely no sun. 

In this example, we will use 4 days, and assume we are using lithium-ion battery technology. 

14,081Wh x 4 = 56,324Wh. 

As I am sure you have already noticed, temperature changes, inefficiencies, and battery type, make a major difference on the overall size of your required capacity. 

Convert from watt hours to amp hours

Most batteries are not measured in watt hours but instead in amp hours. Therefore, it makes sense to convert your amount into amp hours or Ah.

To do this, simply divide the battery capacity by its voltage:

56,324Wh divided by 12 volts = 4,693Ah. 

Most solar systems will use 200Ah batteries, therefore: 4,693Ah = 23 solar batteries for your off-grid system. 

Final thoughts

Keep in mind that you will also need to account for your batteries depth of discharge rates, for lead-acid it is 50% and for lithium 90 - 100%. 

If you plan on building a lead-acid system you will essentially need to double your required battery capacity in order to have the correct capacity. 

Please feel free to ask us any questions in the comments section below should you have any.

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