1. Determine the demands of power consumption (Wh)

The first step in designing a solar PV system is to find out the total power and energy consumption of all loads that need to be supplied by the solar PV system as follows:

1.1 Calculate total Watt-hours (Wh) per day for each appliance used.
Add the Watt-hours needed for all appliances together to get the total Watt-hours per day which must be delivered to the appliances.

1.2 Calculate total Watt-hours (Wh) per day needed from the PV modules.
Multiply the total appliances Watt-hours (Wh) per day x1.3 (the energy lost in the system) to get the total Watt-hours per day which must be provided by the panels.

2. Sizing the PV modules (Wp)
Different size of PV modules will produce different amount of power. To find out the sizing of PV module, the total watt peak (Wp) produced needs. The peak watt (Wp) produced depends on size of the PV module and system location. We have to consider “sunlight factor” which is different in each site location. For central europe the factor is around 3, and corresponds to about 3 hours of sunlight, per day, expected average over the whole year (including in the winter time).

To determine the sizing of PV modules, calculate as follows:

2.1 Calculate the total Watt-peak rating needed for PV modules
Divide the total Watt-hours per day needed from the PV modules (from item 1.2) by 3 to get the total Watt-peak rating needed for the PV panels needed to operate the appliances. The factor 3 corresponds to the day-hours sun radiation under central european conditions and all season working mode (also winter), if PV system working in spring-summer-autumn the factor will be around 4-5 and for systems working only in summer, around 6-7 hours sun per day (factor=6 to7).  

2.2 Calculate the number of PV panels for the system
Divide the answer obtained in item 2.1 by the rated output Watt-peak of the PV modules available to you. Increase any fractional part of result to the next highest full number and that will be the number of PV modules required.

Result of the calculation is the minimum number of PV panels. If more PV modules are installed, the system will perform better and battery life will be improved. If fewer PV modules are used, the system may not work at all during cloudy periods and battery life will be shortened.

3. Sizing pure sine wave PV inverter (230V/50Hz from 12/24/48V)
An inverter is used in the system where 230V-AC power output is needed. The input rating of the inverter should never be lower than the total watt of appliances. The inverter must have the same nominal voltage as your battery (12/24/48V).
For stand-alone systems, the inverter must be large enough to handle the total amount of watts you will be using at one time. The inverter size should be 25-30% bigger than total watts (W) of appliances. In case of appliance type is motor or compressor then inverter size should be minimum 3 times the capacity of those appliances and must be added to the inverter capacity to handle surge current during starting (first 3-6 seconds).

Example: if total watts (W) of working appliances is 150W, your PV inverter converting the electricity from batteries in 12/24/48V-DC into 230V/50Hz-AC, should be around 200W (150W + 30%). 

4. Sizing the battery (Ah)
The battery type recommended for using in solar PV system is deep cycle battery. Deep cycle battery is specifically designed for to be discharged to low energy level and rapid recharged or cycle charged and discharged day after day for years. The battery should be large enough to store sufficient energy to operate the appliances at night, cloudy days and if needed in winter. To find out the size of battery, calculate as follows:

4.1 Calculate total Watt-hours per day used by appliances.

4.2 Divide the total Watt-hours per day used by 0.85 for battery loss.

4.3 Divide the answer obtained in item 4.2 by 0.5 (refer inverter manufacturer) for depth of discharge.

4.4 Divide the answer obtained in item 4.3 by the nominal battery voltage.

4.5 Multiply the answer obtained in item 4.4 with days of autonomy (the number of days that you
need the system to operate when there is no power produced by PV panels) to get the required
Ampere-hour capacity of deep-cycle battery

Battery Capacity (Ah) = Total Watt-hours per day used by appliances x Days of autonomy (0.85 x 0.5 x nominal battery voltage)

5. Sizing the charge controller (A)
As mentioned above, the solar charge controller is typically rated against Amperage (A) and Voltage (V) capacities. Select the solar charge controller to match the voltage of PV system and batteries and then identify which type of solar charge controller is right for your application. Make sure that solar charge controller has enough capacity to handle the current from PV system.

For the PWM charge controller type, the sizing of controller depends on the total PV input current which is delivered to the controller and also depends on PV panel configuration (series or parallel configuration).
According to standard practice, the sizing of solar charge controller is to take the short circuit current (Isc) of the PV module, and multiply it by x 1.3

Calculations in practice

Example a house has the following electrical appliance usage:

1x 18W lamp with used 8 hours per day

1x 60W fan used for 8 hours per day

1x 75W TV that runs 8 hours per day

The system will be powered by 190Wp PV modules.

ad 1. Determine power consumption demands
Total appliance use =(18W x  5hours) + (60W x 4hours) + (75W x 7hours) =916Wh/day

Total PV panels energy needed = 916x1.25=1145Wh/day   [+25% reserve energy and losses]

 

ad 2. Sizing the PV generator (panels)
2.1 Total Wp of PV panel capacity needed =1145/3=382Wp   [factor 3 = average daily solar exposure in hours for central EU location]

Fundamental question: should the PV off-grid system operate over the whole year (summer and winter) or seasonally ?

If seasonally then the amount of solar sun hours should be related to the desired season (in Central Europe in winter about 2-3h up to 6-8h in the summer).

For all-year PV off-grid systems, the average daily solar exposure expressed in hours, should be related to the lowest solar radiation in winter (eg. 2-3 hours). And the daily solar sun exposure (h) will be different for Estonia, Slowenia or Spain, additionally the solar exposure for mountain hut at 2000 above see level will be much higher than in the city.
 

2.2 Number of PV panels needed =382/190= 2 module

Actual requirement = 2 modules a 190Wp

So this system should be powered by at least 2 modules of 190Wp PV module.

ad 3. Sizing the inverter (DC-230V-AC site)
Total Watt of all appliances = 18 + 60 + 75 = 153W

For safety, the inverter should be considered 25-30% bigger size.

The inverter size should be about (153 x 1.3) 200W or greater.

ad 4. Sizing the battery
Total appliances use = (18W x 5 hours) + (60W x 4 hours) + (75W x 7 hours)

Nominal battery voltage: 12V

Days of autonomy: 3 days

Battery capacity = [(18W x 5 hours) + (60W x 4 hours) + (75W x 7 hours)] x 3/(0.85 x 0.5 x 12)   [losses, ratio discharge of battery, voltage]

Total Ampere-hours required: 538 Ah

So the battery should be rated 12V and 538Ah for 3-day autonomy

So the battery should be rated 12V and 179Ah for 1-day autonomy

The main point here is the allowed discharge level of battery (here 50%) if more, the needed capacity (Ah) significant less.

ad 5. Sizing the charge controller
According PV energy demands from above, 2x 190Wp (eg. GermanSolar GSP6 Premium Line) and technical specifications:

Pm = 190Wp

Vmp = 24.77V

Imp = 7.68A

Voc = 31.15V

Isc = 8.28A

A (current)
The rated current for solar charge controller = (2 x 8,28 A) x 1.25 = 20A   [25% safety buffer]

The solar charge controller should be rated at 20A or greater and would be a little undersized, but OK.

V (voltage)
The PV voltage (Voc) of 2x190Wp panels, connected parallel (!) will be 31.15, multiplied x1.2 = 37Voc.

The max allowed voltage within an 12V PWM charge controller is 41Voc, and will be not exceeded (37V), thus is OK.

According example above (2x190Wp) an 20A PWM charge controller for 12V system should be chosen. And will work optimally at slightly cloudy weather, typical for central European location.

 

 

 

 

 

Ref:http://pvshop.eu/offgrid