Off-Grid Solar System Design for Home Application

How to Design off-grid Solar power system for Home Application

  1. steps
How to Design off-grid Solar power system for Home Application
  1. 1. Electric Load Estimation

2. Rating of Solar Panel

3. Battery Sizing

4. Inverter and Charge Controller sizing

5. Cable and circuit breaker sizes

6. Summary of the project

Off-Grid Solar System Design for Home Application

(Electrical engineer solar opinion and estimate of energy storage and usage limits)

1.   Electric Load Estimation

The following electrical appliances are considered as a load for the whole design.

Table 1: electrical load

No.AppliancequantityPower(W)Time (h)Energy consumption (Wh/day)
1TV2366432
2Refrigerator17564500
3Wall AC190065400
4Lighting209101800
5Hair dryer110000.25250
6Ceiling fans275101500
7Blender1400.145.6
8Coffee make18941894
9Dish washer1120011200
10Microwave oven114500.5740
11washer15120.85435.2
12Clothes dryer149000.852450
13home computer130082400
14Laptop computer260101200
15mobile charger55250
16Water heater1450029000
17Modem/wi-fi11024240
Total 15966 32496.8

The total energy consumption (Wh/day) is approximately 32,500Wh/day and the peak load is 15966w. We are not considered as all equipment are switched on at the same time. The most common diversion factor is 0.6, so the maximum load will be 9580W.

2. Rating of Solar Panel

Table 2: Electrical specifications of PV module *

ParameterVariableValue
Maximum PowerPmax335 W
Maximum Power VoltageVmpp42.2 V
Maximum Power CurrentImmp7.94 A
Open Circuit VoltageVoc51.1 V
Short Circuit CurrentIsc8.51  A
*Under Standard Test Conditions (Irradiance 1000W/m2, Module Temperature 25°C)

  • Total DC Amp-hours/Day= Total System Load / System Nominal Voltage

                                                            = (32,500Whrs/Day) / 48 Volts

 = 677 Amp-hrs/Day

  • Total PV Array Current = Total Daily Amp-hr requirement / Design Insolation*

=677 Amp-hrs / 5.0 peak solar hrs

 = 136 Amps

  • Number of Modules in Parallel=Total PV Array Current/ (Mod. Operating Current)x(DF)

=136/ (7.94 Amps/Module)*(0.95)

= 18.03 modules ≈ 18 modules

Therefore, 18 modules are connected in parallel.

  • Number of Modules in Series = System Nominal Voltage / Module Nominal Voltage

= 48 Volts / (42.2 Volts/module)

= 1.14 Module

Use 2 modules, therefore, 2 modules are connected in series.

Total number of PV module required = 18×2= 36 modules

The solar panel size should be selected in such a way that it will charge the battery fully during the one day time. We can assume 5 hours of effective sunlight (December @ Mexico) which will generate the rated power.

  • Total power of PV panel capacity needed = 32,500Whr * 1.25 /5hr = 8125W – minimum PV array watts. The 1.25 factor accounts for the battery charge/discharge efficiency losses over the life of the battery.
  • Number of modules = 8125W ÷ 335W module rating = 35 modules

Therefore. 38 modules would make the system more reliable and further reduce generator time.

3. Battery Sizing

Design Considerations: Desired Reserve Time is one day and Losses and the safety factor is 1.1. Based on these design considerations the Minimum Battery Capacity is calculated in the following way.

=677 Amp-hrs/Day X 1.1

= 745 Amp-hrs/Day

Minimum Battery Capacity = (745 Amp-hrs/Day X 1 Day) / 0.70

= 1064 Amp-hrs

Use TN1000 (1.2V 1000Ah) NICKEL IRON BATTERY

Nominal Voltage = 1.2 Volts

Rated Capacity = 1000 Amp-hrs

                                                =1064 Amp-hrs / (1000 Amp-hrs/Battery)

= 1.064 ≈1 battery

Therefore, the number of batteries in parallel is one battery      

=48 Volts / (1.2 Volts/Battery)

= 40 Batteries

Therefore, the number of batteries in the series is 40 batteries.     

The total number of batteries required for this project is 40 batteries.

4. Inverter and Charge Controller sizing

The Peak Load figure is used to size the minimum full power output of the inverter required. Choose an inverter with a power rating above the peak wattage that the loads can draw. The selected inverter is 5000w mppt Solar inverter 230vac 48vdc, this inverter has a built-in charge controller and AC input, so there is no need of calculating another charge controller, and its specification is;

Power:  5000w

Input voltage: 48V Dc

Output voltage:  230v AC

Type:  mppt solar inverter

Max. Input voltage: 500vDC

Current: 80A

The number of inverters required is calculated from the maximum load of the project, which is ;

= 9580/5000

= 1.9

The total number of inverters required for this project is two.

5. Cable and circuit breaker sizes

Based on IEEE standards and calculated values in above the circuit breaker/ fuse rating and size of cables in different places look like the following.

No.LocationCalculated currentStandard rating of circuit breakerStandard rating of the cableNumber of circuit breakerType of circuit breaker
1Solar panel to inverter terminal136A160A50 mm21DC
2Battery bank to inverter terminal200A225A120 mm21DC
3AC generator to Inverter42A63A10 mm21AC
4Inverter to load42A63A10 mm21AC
Table 3: cable and circuit breaker rating

6.                  Summary of the project

  • The total energy consumption (Wh/day) of the project is 32,500Wh/day and the peak load is 9580W.
  • Total number of PV module required 36 modules
  • Total number of batteries required for this project are 40 batteries.
  • Total number of inverters required for this project are two.

Figure 1: Diagram of the project

Figure 1: connection Diagram of the project

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