number of solar panels needed- appliance calculation

How many solar panels do I need to power my home

The number of solar panels needed to power a home is highly dependent on the average power the home would consume. By the end of this article you would be able to determine the required power rating of solar panel, the type of P.V panel and the number of the panels needed.

The calculation can be divided into 5 simple steps

1. Find out the total power requirement.
2. To account for the loss in efficiency due to inverter and battery (if battery is to be included).
3. Find out the solar irradiance(w/sq.m) and sunshine hours.
4. Calculate the number of panel and the roof area required
5. Reducing the dependence on solar, if roof area is less than required.

Step 1

Calculating the power needed

The power consumption can vary based on the number of people residing in the house, the number of electrical appliances run, location/amount of sun light the solar panel would be exposed to.

Be sure to read the article about How to calculate the cost of setting up solar panel?

Electrical appliances, to estimate the total power needed to solar power your home:

1. Fans

  • Ceiling = 65–175 W
  • Window = 55–250 W
  • Furnace = 750 W
  • Whole house = 240–750 W

2. Microwave oven = 750–1100 W

3. Personal computer

  • CPU – awake / asleep = 120 / 30  W or less
  • Monitor – awake / asleep = 150 / 30  W or less
  • Laptop = 50 W

4. Refrigerator (frost-free, 16 cubic feet) = 725 W

  • 14 cubic feet = 330 W
  • 12 cubic feet = 240 W

5. Televisions (color)

  • 19 inch = 65–110 W
  • 27 inch = 113 W
  • 36 inch = 133 W
  • 53″ – 61″ Projection = 170 W
  • Flat screen = 120 W

6. Vacuum cleaner = 1000–1440 W
7.Water heater (40 gallon) = 4500–5500 W
8. Clothes washer = 350–500 W
9. Clothes dryer = 1800–5000 W
10. Dishwasher = 1200–2400 W

11. Lamp

  • Tube light with electronic choke  = 35-45 W
  • CFL = 5-25 W
  • Incandescent bulb  = 40-60 W

The  given values are taken as a reference from  http://energy.gov

From the above data, pick out the appliances you use. Take the average hours of  usage of the chosen appliance in a day(taking into considerations the variability of usage during weekends) .

Avoid using the heating devices such as water heater, clothes dryer with solar panels, since P.V panels are horribly less efficient in powering these devices. You can separately purchase “solar heating panels” which are a lot better with these heating devices. 

Multiply the respective wattage of the appliances with their respective usage hours, which would give you with the Watt hr. Add up the total Watt hr in order to get the final power used. Now divide the total by 1000 to get the final Kilo Watt hr.

Example: If you use the below mentioned appliances for the time provided, taking into consideration the fact that laundry, vacuum cleaning and other activities are done in the weekends. hence finding the use per day by dividing the total use in a week by 7 days,  you would end up with the total  maximum power requirement of  5.95  KWh.

 

S.No.

Appliance

Nos.

Watts

Hours of usage/day

Total wattage (Wh)

1

Ceiling fans

2

65

5

650

2

Television 36″

1

133

4

532

3

Laptop

1

50

5

250

4

Microwave oven

1

750

1/2

375

5

Refrigerator (after allowing for the recirculation, maintenance of interior temperature)

1

330

4

1320

6

Vacuum cleaner

1

1000

2/7

285

7

Clothes washer

1

350

2/7

100

8

Clothes drier

1

1800

2/7

514.3

9

Water heater

1

4500

1/4

1125

10

Lights

4

40

5

800

Total

5951.3

* The 2/7 hour refers to 2 hour total usage in a week

*The lower wattage for appliance is used because of new Energy star rated appliances (As appliance like vacuum cleaner, washer and drier are not used every day, a single higher value of wattage can be included for all of them together and they can be planned to be used one per day for effective utilization of energy and for optimizing the panel requirements.)

Calculated total = 5.95 kWh The wattage calculated is for above average family usage, which consists of more than 4 family members.


 

Step 2

Accounting for the inefficiencies

It’s important to add the inefficiencies of inverter and batteries. Some energy is lost during the conversion of energy or during the storage of energy, hence we need to have extra wattage to accommodate it.

In case of a grid tie system, the total power would be 5% more than the calculated wattage to account for the inefficiency of the inverter. In case of a off-grid system, the total power would be 5%(for inverter)+5%(for battery).

If you’re new to the term “grid tie” I would suggest reading choosing-between-grid-and-off-grid article.

Hence in the above report if the solar system is grid tie system then total power=5.95+5.95*0.05= 6.2475 kWh.
If off grid then total power=5.95+5.95*0.05+5.95*0.05= 6.545 kWh.

This value is the total energy needed to power your house per day, which is to be stored by the battery in case of a off-grid setup. Hence if you have a off-grid setup, the battery you would have to purchase should have a rating of 6.545 KWh.

Purchasing a solar panel with a rating little higher than the value calculated is recommended to avoid inadequate energy supply.


 

Step 3

Find out the total solar irradiance

We got to find the total solar irradiance specific to your region. Now I want you to head over to http://solarelectricityhandbook.com/solar-irradiance.html to calculate the solar energy available in your region.

Enter your country, location, the direction your panel would be facing. You will get the value for each month, find the average of the total sum of irradiance for all 12 months, that would result in average solar irradiance. We are now set to move to the step 4.


 

Step 4

Calculate the number of panel and the roof area required

We may calculate the power required for the house and come up with an appealing value of units required (KWh), but if the roofing cannot accommodate the P.V panels then the calculation would be a waste of time.

Roofing area as a factor

Having the necessary roof space is important in determining the number of solar panels needed. You can go in for a less efficient solar panel such as polycrystalline panel if you are running low on money, provided you have a good amount of roof space for the P.V panels.

Polycrystalline panels are less efficient and need larger surface of exposure to the sun to produce adequate energy. But having a monocrystalline panel over the roof  once and for all is advisable, since it’s better to have efficient panels occupying a portion of the roof rather than having inefficient panels all over the roof. Anyway it’s the purpose which dominates.

For location based analysis the two parameters that can change are

1.    Average solar irradiance ( 400 -1000 w/sq.m)
2.    Duration of sunshine (2 -10 Hours)

Calculation

I have devised two methods to find out the roof space and the panels required. The first method is the simple method.

Method 1

We need three things to calculate using this method –

  • Area of single solar panel.
  • Efficiency of solar panels you are going to purchase.
  • Roof area which is available in your terrace.

Since the single panel area varies with each manufacturer, it’s better to get the panel area of the panels you are planning to purchase. If you haven’t decided on the manufacturer yet you can go with the standard size of 0.56 sq.m.

  • Area of the panel required = (Total power required /irradiance )*(100/panel efficiency)
  • No. of panels required = Area of the panel required / single panel area

In our example case, say solar irradiance is 5 kWh/sq.m/day and standard panel area = 0.56 sq.m, let’s have the panel efficiency as 20%., then

  • Area of the panel required = 5.95/5 *(100/20) =5.95 sq.m
  • No. of panels required = 5.95 / 0.56 = 10.625 =11 solar panels

Hence if your roof area has 5.95 sq.m, it can hold your 11 panels spanning 5.95 sq.m.

A much more comprehensive way to find out the area and the number of panels required is listed below. The following method needs you to have a technical background to understand. It’s perfectly fine if you could work out your panel needs using the method 1, you can directly move to step 5. Method 2 is just to illustrate the principle used in method 1.

Method 2 

In this method you are required to know the solar irradiance of your location and the sunshine hours in your area.

Total Watts needed = X w
Average Number of hours of sunshine/day  = Y (Panel generation factor)
Panel watts = Total watts/Panel generation factor = X/Y W
Panel efficiency=20%

  • For an average solar irradiance of 800 W/sq.m.
  • Total module required = Panel watts/Module watts (160W/sq.m)= (X/Y)/160 = Z solar panels

In the above calculation module watts is 160W/sq.m because out of 800W only 20% of it used for energy generation.

  • Area of the panel for 80w = 5 -10 sq.ft.(Area varies with manufacturer)
  • Roof space required = Total No.of module x Panel area = Z  x 6 sq.ft/10.7498 (sq.ft/sq.m) = R sq.m

The value 6 sq.ft is the assumed standard panel area, since the solar panel area varies with manufacturer. The 6 sq.ft is divided by 10.7498 (sq.ft/sq.m) to convert the value from square feet to square meter.

We are going to workout 3 different examples where the solar irradiance and duration of sunshine varies to illustrate the above calculation.

Number of solar panels and roof area required under ideal condition(solar irradiance=800 W/sq.m, 6 hours sunshine)

Total Watts needed = 5951 w
Average Number of hours of sunshine/day  = 6 (Panel generation factor)
Panel watts = Total watts/Panel generation factor = 991.8 W

1. For mono crystalline silicon cell (Efficiency = ~20%)

  • For an average solar irradiance of 800 W/sq.m.
  • Total module required = Panel watts/Module watts (160W/sq.m)= 6.198 = 7 solar panels
  • Area of the panel for 80w = 5 -10 sq.ft.(Area varies with manufacturer)
  • Roof space required = Total No.of module x Panel area =7  x 6 sq.ft/10.7498 (sq.ft/sq.m) = 3.90 sq.m

2. For poly crystalline silicon cell (Efficiency = ~17%)

  • For an average solar irradiance of 800 W/sq.m.
  •  Total module required = Panel watts/Module watts (136W/sq.m)=  7.29 =8  solar panels.
  • Area of the panel for 80w = 5 -10 sq.ft.(Area varies with manufacturer)
  • Roof space required = Total No.of module x Panel area =8 x 6 sq.ft/10.7498 (sq.ft/sq.m) = 4.465 sq.m

Every solar panel has its own efficiency of energy conversion. Each solar panel comes in standardized size. The size of the roof is important while focusing on the efficiency and the number of solar panels required.

Typical calculation for change in solar radiation (400W/sq.m instead of original 800 W/sq.m, 6 hours sunshine)

1. For mono crystalline silicon cell (Efficiency = ~20%)

  • For an average solar irradiance of 400 W/sq.m.
  • Hours of sunshine = 6 hours
  • Panel watts = Total watts/Panel generation factor = 991.8 W Total module required = Panel watts/Module watts (80W/sq.m)=  12.39 =13 solar panels
  • Area of the panel for 80w = 5 -10 sq.ft.(Area varies with manufacturer)
  • Roof space required = Total No.of module x Panel area =13 x 6 sq.ft/10.7498 (sq.ft/sq.m) = 7.256 sq.m

2. For poly crystalline silicon cell (Efficiency = ~17%) For an average solar irradiance of 400 W/sq.m.

  • Hours of sunshine = 6 hours
  • Total module required = Panel watts/Module watts (68W/sq.m)=  14.58 = 15 solar panels
  • Area of the panel for 80w = 5 -10 sq.ft.(Area varies with manufacturer) Roof space required = Total No.of module x Panel area =15 x 6 sq.ft/10.7498 (sq.ft/sq.m) = 8.37 sq.m

 Typical calculation for change in duration of sunlight ( solar irradiance=800 W/sq.m, 10 hour sunshine instead of original 6 hours)

1. For mono crystalline silicon cell (Efficiency = ~20%)

  • For an average solar irradiance of 800 W/sq.m.
  • Hours of sunshine = 10 hours Panel watts = Total watts/Panel generation factor = 595.1 W
  • Total module required = Panel watts/Module watts (160W/sq.m)= 3.72 = 4 solar panels
  • Area of the panel for 80w = 5 -10 sq.ft.(Area varies with manufacturer)
  • Roof space required = Total No.of module x Panel area =4 x 6 sq.ft/10.7498 (sq.ft/sq.m) = 2.23 sq.m

2. For poly crystalline silicon cell (Efficiency = ~17%)

  • For an average solar irradiance of 800 W/sq.m. Hours of sunshine = 10 hours Panel watts = Total watts/Panel generation factor = 595.1 W
  • Total module required = Panel watts/Module watts (136W/sq.m)= 4.375 = 5 solar panels
  • Area of the panel for 80w = 5 -10 sq.ft.(Area varies with manufacturer)
  • Roof space required = Total No.of module x Panel area =5 x 6 sq.ft/10.7498 (sq.ft/sq.m) = 2.79  sq.m.

 

Step 5

Reducing the dependence on solar, if roof area is less than required

Compare the roof area you have over your terrace with the panel area. If the roof area required is more than the area the panels are going to occupy, then our solar project is in tune with the available roofing area! You have finished the calculation!!

If not, we have to redo the calculation with the available area and find how much of the energy requirement can be met from solar.

In this case we have to go back to step 1, decide on which appliances will be solar powered and which others will be run using the grid electricity.  Filter out unnecessary appliance which don’t need solar power from the list. Follow the other steps appropriately.

Need any help in the calculation of the number of panels? Please leave your comment below. I’m always there on the helping side.