How to Size Your Off-Grid Solar System?

Off-grid systems are increasingly becoming popular in Australia. The continuous rise of energy bills has pushed more Aussies every year to ease their dependence on the grid and among the many ways to do this, solar power & battery storage systems are dominant. 

The combination of solar panels and batteries in an entire off-grid system has benefitted thousands of households in the country. Aside from offering an eco-friendly way to power up home essentials, these power packages significantly help lower or completely eliminate your monthly energy bills. 

How do you know how big of an off-grid system you need, though? The keyword here is power consumption. You need to figure out your daily power consumption before going through other factors such as budgeting and selecting your power plan. If you're thinking of signing up for an off-grid system, ask yourself these questions first:

• How much power output do you require?
• Are you going completely off the grid or do you want to leave some loads on the grid?
• How much budget do you have in mind?
• What are the average peak sun hours in your current location?
• How many days are you willing to stay off the grid in case of a power outage?

Once you've answered these, it's time to start calculating the size of the off-grid system best suited for you. 

Sizing Your Off-Grid System: A Step-By-Step Guide

Step 1: Make a list of the appliances that are to be put off-grid and include the running watts of each one

Here's an example of this: 

Load	Qty	Power Rating	Run hours	Running Watts (Power Rating x Qty)
Fridge	2	350W	12	700W
Juicer	1	300W	0.5	300W
Lights	Multiple	150W	6	150W
		TOTAL		1,150W

Step 2: Get the starting watts of all off-grid loads

Load	Qty	Power Rating	Run hours	Running Watts (Power Rating x Qty)	Starting Watts
Fridge	2	350W	12	700W	1,400W
Juicer	1	300W	0.5	300W	600W
Lights	Multiple	150W	6	150W	0
			TOTAL	1,150W	2,000W

* The running watts of an appliance are usually indicated on its sticker, but for the starting watts, it’s best to check with your manufacturer. The example above uses an estimate of twice the running watts of the appliance provided.

Step 3: Compute the total load in watts

To get this, add the highest starting watts and total running watts.

1,400W + 1,150W = 2,550W (2.55kW)

Step 4: Determine the KVA rating you need for the inverter

KVA rating of inverter = total load in watts / nominal power factor (0.8)

2.55 / 0.8 = 3.1875kVA

A standard size of 3.1875kVA or 3.5kVA generator will be enough to power all your loads.

Step 5: Size your battery according to your preferred days of autonomy.

Days of Autonomy: Number of days with no power generation from the solar panels due to weather conditions.

Let's assume you can go off-grid for a maximum of 2 days. You need to calculate your total energy usage in one day (watt-hours) first to size your battery properly. (Watt-hours = Running watts x run hours).

Load	Qty	Power Rating	Run hours	Running Watts (Power Rating x Qty)	Starting Watts	Watt-hours
Fridge	2	350W	12	700W	1,400W	8,400
Juicer	1	300W	0.5	300W	600W	150
Lights	Multiple	150W	6	150W	0	900
			TOTAL	1,150W	2,000W	9,450Wh

Total energy consumed in 1 day = 9,450Wh / battery voltage (12V battery)

Daily Ah consumption = 787.5Ah

Battery Ah needed = Daily Ah consumption x Days of Autonomy x Load Expansion Factor / DoD

Assuming that your load expansion factor is 20% (1.20) and you want a lithium battery with 80% DoD, the calculation will be:

787.5Ah x 2 x 1.20 = 1,890 / 0.8 

Battery Ah needed = 2,362.5Ah

If you’re using a standard 500Ah battery, this would mean you have to supply around 5 of them for your off-grid system. 

Step 6: Find out the size and number of solar panels you will need to power your load during hours of sunlight and charge your battery at the same time.

To accurately determine the solar panel watts you need to power loads and charge your battery, use this formula:

Wh load of batteries x solar panel power loss factor correction / Maximum hours of sunlight. 

Let's say you have the following metrics:

• Power loss factor correction: 1.3
• Maximum sunlight hours: 5
• Preferred solar panel rating: 300W

2,362.5 x 12 = 28,350 + 9,450Wh (Total watt hours) = 37,800Wh

Solar panel watts needed: 37,800Wh x 1.3 / 5 = 9,828Wp

No. of solar panels needed: 9,828 / 300W (preferred solar panel rating) = 32.76

For your off-grid system, approximately 33 solar panels are required to successfully run loads and recharge batteries during your entire off-grid time. 

Having an idea of the correct off-grid system size for your home is extremely helpful in making sure you make the most out of the plan you’ll get. It can also save you from providers offering inferior power systems, which can result in more expenses instead of the other way around. 

Curious about the available off-grid packages you can get? Reach out to us and get a FREE quote. We’re more than happy to assist you from sizing to installation & and even after sales, so take this chance to join the growing number of Aussies who are currently reaping the benefits of using solar energy!