Solar Battery: How To Size A Battery Bank In 3 Easy Steps

Introduction

Choosing a solar battery can be a daunting task, but we’ll show you how to work out what your storage needs are.

In this guide, you’ll learn to size up a solar battery for home and office use, and you’ll be able to choose the correct battery for you.

Contents

Fundamentals

How do they work?
The fundamentals of Solar Batteries have been mentioned before in our Solar Battery Guide, which you can find here. However, I’ll add them here for beginners.

You may want to skip this section if you are familiar with the basics.

What Is A Solar Battery?

Solar batteries are a part of a solar power kit that contains an inverter, solar panels, switches, mounting and cabling.

Solar batteries store the overproduced solar power during the day to provide you with usable solar power in the evenings and during grid failure.

The more solar power you store and use during downtimes will help you use less grid supply, saving you money on your electricity bill.

Lithium Iron Phosphate

Or LiFePO4 for short.
These batteries form part of the newest in Solar Battery technology. They have  low toxicity levels, well-defined performance and long term stability .

These are a few features that make LiFePO4 batteries the favourite in the solar industry.

From Solar Storage to motor vehicles, their popularity is ever-growing.

kWh - Kilowatt Hour

kWh is a measurement of energy.

It corresponds to using 1kW of power for an hour.

E.g. You can run a 10W light bulb for 10 hours with a 1000W system.
So a 2.4kWh battery means you can run 2400W (2.4kW) of power for an hour.

OR

You can run 1.2kW of power for 2 hours.

What Does DOD Mean?

DOD stands for Depth of Discharge.

Typically, the DOD of LiFePO4 solar batteries is between 80% and 100%. Traditional batteries (Lead-Acid) have a DOD of 50%

Therefore, you can use 80% of the battery daily and still prolong the battery's life.

What Does Cycle Mean?

Cycles refer to the  process of charging and discharging .

One cycle is one full discharge and one full recharge.

When you see “6000 cycles” it means that the battery can be fully charged and discharged 6000 times.

 DOD plays a factor in how many cycles you can get out of a single battery  if you increase the DOD to 90% your number of cycles decreases.

What is the Life Span?

The lifespan of Lithium-ion Solar Batteries is 6000+ cycles.

Compared to Lead-Acid batteries which stand at 300 - 1350 cycles.

To work out the life span, take the number of cycles divided by 365 (days in a year):

6000 / 365 = 16.44 years
NB: This is based on one cycle per day.

You can expect a LiFePO4 battery to last upwards of 10 years!

Appliance Ratings

How do they work?
Let’s start by listing some appliances you will most likely have in your home and regularly use.

The ratings of the appliances below are based on averages.

To get an accurate usage for your own home, check what ratings your appliances carry.

Run all day:
A few hours a day:
A few minutes a day:
  • Modem – 10W
  • Alarm – 6W
  • Fridge Freezer – 150W
  • TV – 200W
  • LED Lights – 25W each
  • Dishwasher – 1200W
  • Washing machine – 800W
  • Tumble dryer – 3000W
  • Kettle – 1200W
  • Toaster – 850W
  • Blender – ​​500W
  • Hairdryer – 1500W
  • Microwave – 1000W

Working Out your Solar Battery Bank Needs

How do they work?
Now to work out what size solar battery bank you’ll need.

You’ll need to work out what you want to run off your battery and for how long you need to run these appliances.

I’ll refer to load shedding, evenings, nights and mornings as ‘downtime(s)’ as this is the period you’ll not be producing solar power.

There are three main questions you need to answer:

  1. What appliances need to run off the battery bank?
  2. For how long do these appliances need to run off the battery bank?
  3. Will your battery be used during load shedding, evenings, nights and mornings?

Whatever your answer is, you’ll be able to work out your kWh needs by following the steps in this section.

STEP 1

Add the power ratings of all the appliances you’d like to run off your solar battery bank.

It is a good idea to check each of your appliances for their ratings to get an exact idea of your usage.

STEP 2

Next, we’ll need to work out the amount of time you typically use these appliances.

You’ll use your appliances for varying amounts of time throughout the down phases. Like a TV might be on for 4 hours while your microwave is only on for 3 minutes.

This section is entirely reliant on your usage.

STEP 3

How will you use your solar battery? Load shedding? Non-production time? Both?

Load Shedding
In some areas, they can expect 2.5 hours of load shedding, and in other areas, they can have up to 4 hours of load shedding. It’s essential to consider these times.

Evening Usage
The average evening usage is around 4 hours for appliances that we use when we’re awake from when we come home from work until we go to bed. We’ll work from 18:00 to 22:00.

Nighttime Usage
Say you’re getting in a good night sleep at the recommended 8 hours a night. Most of us still run a few essential appliances like fridges, alarms etc.

Morning Usage
We use a lot of power in the mornings because we use appliances like kettles, toasters, microwaves and hairdryers. All of these appliances pull large amounts of energy.

Usually, between 06:00 and 10:00, we don’t produce enough solar power to supplement our load. Therefore, it is beneficial to have a big enough battery to cover as much of our usage in the morning as possible.

As a rule of thumb, we suggest being mindful of what you are running off your battery bank, but mornings are different because who wants to start their day de-caffeinated with a severe case of bedhead?

Case Study

How do they work?
Now we get to the fun part: working out our usage!

Below, I’ll give you an example based on my home and the appliances we own with their ratings, how many times we use them, and how long we use them in 16 hours.

I have broken them down into mornings, evenings and nights.

Morning Usage 06:00 - 10:00

Appliance Watts Uses Duration of usage Total Watts
Espresso Machine 1450W 2 20 minutes 484W
Toaster 850W 1 3 minutes 43W
Microwave 1200W 1 1 minute 20W
Hairdryer 1000W 1 10 minutes 166W
5W LED x 10 50W 1 3 hours 150W
160W TV x 2 320W 1 2 hours 640W
Fridge freezer 150W 1 4 hours 600W
Alarm 6W 1 1 hour 6W
Modem 10W 1 4 hours 40W
Heat pump 1250W 1 1 hour 1250W
Total: 3399W

Evening Usage 18:00 - 22:00

Appliance Watts Uses Duration of usage Total Watts
Toaster 850W 1 3 minutes 43W
Microwave 1200W 2 3 minutes 120W
5W LED x 10 50W 1 4 hours 200W
160W TV x 2 320W 1 4 hours 1280W
Fridge freezer 150W 1 4 hours 600W
Modem 10W 1 4 hours 40W
Heat pump 1250W 1 1 hour 1250W
Total: 3533W

Night Usage 22:00 - 06:00

Appliance Watts Uses Duration of usage Total Watts
Fridge freezer 150W 1 4 hours 600W
Alarm 6W 1 1 hour 6W
Modem 10W 1 4 hours 40W
Phone Chargers x 4 @ 20W each 40W 1 2 hours 80W
Laptop Chargers x 2 @ 45W each 90W 1 3 hours 270W
Total: 1678W

Case Study Findings

Now that we have totals for each time frame, we can determine my family’s overall usage and battery needs to cover 100% of our load using our solar battery bank.

We’ll add all three totals together: 8610W or 8.61kW.

To choose a battery, I’ll need to consider the battery’s DOD (Depth of Discharge). If I choose a battery with 80% DOD, I’ll need to subtract 20% from the overall rating of the battery.

I could consider these options to cover my usage completely:

  • 4 x PylonTech US2000C Batteries giving me 9.12kWh @ 95% DOD
  • 5 x Dyness 2.4kWh Batteries giving me 9.6kWh @ 80% DOD
  • 3 x PylonTech US3000C Batteries giving me 9.975kWh @ 95% DOD
  • 2 x Alpha ESS 5.8kWh Batteries giving me 10.44kWh @ 90% DOD

You can view these products here:

Solar Battery Prices

How do they work?
We all know that battery prices are pretty high. But if you’re cycling a battery bank once a day and have 6000 cycles, that gives you over 16 years of battery life.

But what can you expect to pay?

Lithium-ion prices range from just under R12,000 for singular batteries and upwards of R53,000 for large battery banks with multiple batteries.

Solar product prices are also falling yearly. By the time you will need to replace your batteries, you’ll most likely find that solar battery prices will be far more affordable than they are today.

This fact may put you off buying batteries now, but consider how much money you will still save on your electricity bill by using solar power.

You could even start small by only covering your evening usage and expand your battery bank as you go. One of the joys of Lithium-ion batteries is that you can add new batteries to older batteries without causing any damage to the new ones.

Depending on your budget, you have a couple of options:

Have a big enough solar battery bank and array to ensure you use as much solar power in the morning as possible.

Or

Supplement your load demands with grid and solar battery with a Hybrid Inverter Solar Power Kit.

Either way, you will still be saving money every month on your electricity bill.

Cost Per Cycle

How do they work?
If you still need some convincing, I'll show you how much a solar battery will cost per cycle for the duration of its life expectancy.

The below calculations are based on a PylonTech US2000C 9.6kWh Battery Bank at R53,832.93 with 6000 cycles.

Let's start by working out your cost per cycle:

R53,832.93 / 6000 = R8.97 cost per cycle.

1 Cycle Per Day

R53,832.93 / 6000 days (16.44 years) = R 8.97 cost per day

2 Cycles Per Day

R53,832.93 / 3000 days (8.22 years) = R 17.94 cost per day

While it may be a significant amount to front initially, as you can see, the cost per cycle is highly affordable.

Solar Battery: Conclusion

We hope that you have enjoyed this article and better understand how to calculate what your battery needs are.

Remember, if you need any help with sizing a solar battery bank, you are welcome to contact us. We are available on our website chat, telephone and email.
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