How to Size an Off Grid Solar PV System for the Home

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 What is currently happening in our world

With the planet continuing to warm up and the increased pressure on humanity to ditch fossil fuels and embrace other forms of energy that don’t harm the planet, renewable energy has seen a steady rise in its popularity and usage.

Amongst the renewable energy sources of wind, solar and geothermal, Solar is increasingly becoming dominant. The main component of any solar energy installation are Solar PV panels. Although still relatively inefficient, the rise of Solar PV panels has been astronomical.

Solar PV installation in the rooftop of houses are increasingly becoming popular as we race to reduce our carbon footprint and keep the planet safer.

Basic Things to know Before you Start Your Solar PV Sizing Project in the Home

To successfully implement an off grid solar PV installation in the roof top of your house, there are some basic things you need to be aware of before you embark on this journey:

1. What is your load requirement?

2. Do you want to power all your loads when off grid ? Or

3. Do you want to power some loads and leave some on grid?

4. What is your budget for this project?

5. Can your budget finance a project where all your loads requirements can be taken off grid and                powered by the Solar PV installation?

6. What is the average level of sunlight insolation in your location where the project is being                       planned?

7. How many days are you willing to stay off grid if you choose or if there is power outage. This is            called days of autonomy – days the installation can run on your battery bank without shutting                  down.

Basic components of a Solar PV Installation in the Home

A basic Solar PV installation for the Home consist of the following components:

1. Solar PV panels

2. Solar Charge Controller

3. Battery Bank

4. Inverter to power your Alternating Current (AC) loadings

5. Appropriate wiring

6. Appropriate protection against lighting, short circuits, and overloads.

See : A Guide to Understanding Solar PV Panels Power System Installations

Sizing Procedure for Solar PV Installation for Your Home

To correctly size your Solar PV Installation for Your Home, follow the following basic steps:

1. Determine the loads that are to be put off grid or during power outage

2. Calculate the running watts of all the loads

3. Calculate the starting watts (also known as surge watts) of all the loads

4. Determine the total load in watts by adding total running watts to the highest starting watts of the  load to be powered by the Solar PV installation

5. Determine the KVA rating of your Inverter by dividing by 0.8, the nominal power factor.

6. Determine the size of your battery bank to enable you stay off grid for the days of autonomy you    desire or when there is power outage for that long.

7. Determine the size of your Solar PV array that will power your load during hours of sunlight as              well as charge the battery bank at the same time.

8. Determine the size of your charge controller.

Sample Sizing Calculation of Solar PV Installation for the Home

Suppose I stay in a region with a maximum of 5 hours sunlight and want to install an off grid solar PV installation that can power my total loads shown below for two days continuously according to the allotted run hours without utility power. How many solar panels and batteries do I require? What size of Solar Charge controller is needed?

ELECTRICAL LOAD	QTY	POWER RATING	RUN HOURS
Deep Freezer	2	115W	12
Submersible pump (1Hp)	1	750W	0.5
TV	2	100W	12
Lighting Loads	Lot	200W	12
Juicer	1	400W	0.5

Steps 1 & 2  

As per step 1, all loads are to be put on the solar PV installation. The running watts have been given for each electrical loads, however total running watts for each electrical load is:

ELECTRICAL LOAD	QTY	RUNNING WATTS
Total Deep Freezer Load	2	2*115 = 230W
Total Submersible pump Load	1	1*750 = 750W
Total TVs Load	2	2*100 = 200W
Total Lighting Loads	Lot	= 200W
Juicer	1	1*400 = 400W
	Total	= 1,780W

Note that the running watts of an appliance can either be gotten from the name plate label of the device or you can simply multiply the rated current and voltage of the device on the name plate label if given by using the following relationships:

Power in VA (volt amps) = Rated current x Rated Voltage

Power in Watts = Power in VA x Power factor. 

To simply our calculations, we assume power factor = 1

Step 3: Calculate the Starting Watts of all the Electrical loads

ELECTRICAL LOAD	RUNNING WATTS	STARTING WATTS
Total Deep Freezer Loads	230W	2*230 = 460W
Total Submersible pump Load	750W	3*750 = 2,250W
Total TVs Load	200W	0
Total Lighting Loads	200W	0
Juicer	400W	2*400 =800W
Total	1,780W	3,510W

See how to size portable generators on how to calculate starting watts

Step 4: Calculate Total Loads in Watts Required.

Total Load in Watts = Highest Starting Watts plus Total running Watts = 2,250W + 1,780W = 4,030W = 4.03KW   (1KW = 1,000W)

Step 5: Determine KVA Rating of Inverter

KVA Rating of Inverter = Total Load in Watts/0.8  (Power Factor = 0.8)

                                          = 4.03KW/0.8 = 5.0375KVA

A standard size of 5KVA inverter will be able to sufficiently power the loads.

Step 6: Determine Battery Bank Size for Days of Autonomy

Here we want to be able to power all our loads for two days without power. First let us calculate the total energy usage of all the loads provided in Wh (Running Watts x Run hours of electrical load):

ELECTRICAL LOAD	QTY	RUNNING WATTS	RUN HOURS	ENERGY (Wh)
Total Deep Freezer Load	2	230W	12	230*12 = 2,760
Total Submersible pump Load	1	750W	0.5	750*0.5 = 375
Total TVs Load	2	200W	12	200*12 = 2,400
Total Lighting Loads	Lot	200W	12	200*12 = 2,400
Juicer	1	400W	0.5	400*0.5 = 200
	Total	1,780W		8,135Wh

Total Energy consumed for 1 day based on the allotted run hours by all electrical loads is = 8,135Wh

Using a 12Volts battery system, total battery Ah required daily = Total Energy (Wh) for 1 day /12 = 8,135/12 = 677.92Ah

Battery Ah needed =   (Daily Ah consumption * Days of Autonomy *load expansion      

                                         factor)/DOD (%)

Daily Ah consumption = 677.92Ah

Days of Autonomy       = 2 days

Load Expansion factor = 1.20 (We are assuming a load expansion factor of 20%)

DOD (Depth of Discharge) = 0.5 (We are using Lead acid batteries with DOD = 50%, for AGM batteries, it is 80%)

Battery Ah needed = (677.92 x 2 x 1.20)/0.5 = 3,254.016Ah

Using a standard 500Ah battery, we arrive at total number of batteries required = Total Ah/500 = 3,254.016/500 = 6.5 batteries.

Therefore, approximately 7Nos, 500Ah batteries connected in series are required to power the electrical loads for the allotted time for two days off grid. In this case an 84V, 5KVA DC-AC inverter will be required.

Step 7: Determine Number of Solar PV Panels Required to Power the Loads

SIZING OF NUMBER OF SOLAR PANELS REQUIRED
Size of Solar Panels required to supply load and charge batteries Wp =	(Wh of Power Required by Load * 1.3)/Maximum hours of Sunlight
Wh of load of batteries =	3,254.016 x 12 = 39,048.192Wh
Wh load of connected loads =	8,135Wh
Power loss factor correction for Solar Panels	1.3
Maximum hours of Sunlight in your location	5
Wp of Solar panel required =	(39,048.192+8,135) *1.3/5 = 12,267.63Wp
Chosen Solar panel rating	400W
No. of Solar Panels Required	12,267.63/400 = 30.67

For this project, approximately 31Nos**, 400W Solar panels will be required to successfully power the loads and charge the batteries during off grid time in the day with sun light hours and also be able to power the loads during nighttime.

Step 8: Determine Size of Charge Controller

There will be Seven (7) Nos, 500Ah batteries in series giving us a total = 12V X 7 = 84V DC

Assuming we are using the LG LG400N2W-A5 (400W) Solar Panel, the maximum power voltage is 40.8V and its current is 9.81 Amps.

To sufficiently charge the batteries, voltage applied to the batteries must be greater than 84V DC. Three (3) Solar panels in series will provide = 40.8 x 3 = 122.4V to charge the batteries.

Therefore, number of parallel strings of 3Nos solar panels in series will be = 31/3 = 10.33 = 11 approx

Total Current produced by 11Nos parallel strings of 3Nos solar panels in series = 11 x 9.81 = 107.91 Amps. 

Therefore, a charge controller of at least 120 – 150 Amps will suffice.

**Note that we calculated approximately 31Nos solar panels required. But for actual implementation, we shall require 33Nos solar panels.