Updated: Mar 12, 2022
Investing in a solar system is a smart solution for homeowners. The latest solar panels and photovoltaic (PV) systems are easy to install, maintain, and operate, with long-term performance and energy savings.
To make the most of your grid-tie solar system, you’ll want to know how to correctly size the system to cover your energy use patterns without over-sizing your PV array.
Follow these steps to learn how to get a sizing estimate, calculate your solar needs, and select the right panels to get the most benefit out of your solar installation.
The process for sizing off-grid solar systems is different, due to the need to account for battery bank sizing. Click here for advice on how to size your off-grid solar system.
Getting Started with Solar System Sizing
Before you begin to size a solar system, you’ll want to figure out the main constraints on the project and use those restrictions as the starting point for the design. You can approach the project from one of three angles:
Budget constraints: Build a system within your target budget.
Space constraints: Build a system that is as space-efficient as possible.
Energy offset: Build a system that offsets a certain percentage of your energy usage.
Take into consideration other sizing factors and common stumbling blocks that may impact how to size a solar system:
Local levels of sun exposure
Orientation of the array (tilt angle)
Plans for future expansion
Product efficiency ratings
Natural degradation of performance over the life of the warranty
Once you’ve assessed your solar needs and established your approach to design, follow these steps to size a grid-tied solar system.
1. DETERMINE POWER CONSUMPTION DEMANDS
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 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 per day needed from the PV modules.
Multiply the total appliances Watt-hours per day times 1.3 (the energy lost in the system) to get the total Watt-hours per day which must be provided by the panels.
2. SIZE THE PV MODULES
Different size of PV modules will produce different amount of power. To find out the sizing of PV module, the total peak watt produced needs. The peak watt (Wp) produced depends on size of the PV module and climate of site location. We have to consider panel generation factor which is different in each site location. For Nigeria, the panel generation factor is 3.596. 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 3.596 to get the total Watt-peak rating needed for the PV panels needed to operate the appliances.
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. INVERTER SIZING
An inverter is used in the system where 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.
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 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.
For grid tie systems or grid connected systems, the input rating of the inverter should be same as PV array rating to allow for safe and efficient operation.
Another way to size power inverter is to use its power factor by using the formular below
Power of inverter = power requirement / power factor
4. BATTERY SIZING
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 and cloudy days. 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 0.6 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 0.6 x nominal battery voltage)
5. SOLAR CHARGE CONTROLLER SIZING
The solar charge controller is typically rated against Amperage and Voltage capacities. Select the solar charge controller to match the voltage of PV array 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 array.
For the series 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 array, and multiply it by 1.3
Solar charge controller rating = Total short circuit current of PV array x 1.3
Remark: For MPPT charge controller sizing will be different. (See Choosing the Right Solar Charge Controller/Regulator)
A house has the following electrical appliance usage:
One 18-Watt fluorescent lamp with electronic ballast used 4 hours per day.
One 60-Watt fan used for 2 hours per day.
One 75-Watt refrigerator that runs 24 hours per day with compressor run 12 hours and off 12 hours.
The system will be powered by 12 Vdc, 110 Wp PV module.
1. DETERMINE POWER CONSUMPTION DEMANDS
Total appliance use = (18 W x 4 hours) + (60 W x 2 hours) + (75 W x 24 x 0.5 hours)
= 1,092 Wh/day
Total PV panels energy needed = 1,092 x 1.3
= 1,419.6 Wh/day.
2. SIZE THE PV PANEL
2.1 Total Wp of PV panel capacity
needed = 1,419.6 / 3.4
= 413.9 Wp
2.2 Number of PV panels needed
= 413.9 / 110
= 3.76 modules
Actual requirement = 4 modules
So, this system should be powered by at least 4 modules of 110 Wp PV module.
3. INVERTER SIZING
Total Watt of all appliances = 18 + 60 + 75 = 153 W
For safety, the inverter should be considered 25-30% bigger size.
The inverter size should be about 190 W or greater.
4. BATTERY SIZING
Total appliances use = (18 W x 4 hours) + (60 W x 2 hours) + (75 W x 12 hours)
Nominal battery voltage = 12 V
Days of autonomy = 3 days
Battery capacity = [(18 W x 4 hours) + (60 W x 2 hours) + (75 W x 12 hours)] x 3/
(0.85 x 0.6 x 12)
Total Ampere-hours required 535.29 Ah
So, the battery should be rated 12 V 600 Ah for 3-day autonomy.
5. SOLAR CHARGE CONTROLLER SIZING
PV module specification
Pm = 110 Wp
Vm = 16.7 Vdc
Im = 6.6 A
Voc = 20.7 A
Isc = 7.5 A
Solar charge controller rating = (4 strings x 7.5 A) x 1.3 = 39 A
So, the solar charge controller should be rated 40 A at 12 V or greater.
To choose the right inverter for user requirements, they must know some basic calculations. Using this simple calculation the user can easily buy inverters according to their need.
Inverter must be bought according to the power requirement.
For example, If the user wants 2 tube lights, 2 fans, 2 CFL lamp and 1 television to run at the time of utility outage; then the power consumed by these appliances is supposed to be
1 tub light – 60 watts, 1 CFL -25 watts, 1 fan- 70 watts, 1 television- 120 watts.
Then the power requirement =2* 60 +2*20+2*25+1*120
So the total power requirement is 330 watts.
Then the power of the inverter is the ratio of power requirement in watts to the power factor (efficiency).
Power of inverter = power requirement / power factor
Most of the inverters have efficiency 60% or 70 %
Assume power factor =0.7
Then power of inverter = 330 / 0.7 = 471 VA, the inverter of specific rating can be choose.
How To Choose the Battery for Solar Inverter Wisely?
The life and performance of solar inverter depends upon the battery. The battery capacity means the maximum hours that the battery can run all the equipment. The battery capacity is termed as Amps-hour (Ah). The battery capacity can be defined as the product of power requirements in watts and backup hours in hours divided by battery voltage in volts.
Battery capacity= power requirement * backup hours / battery voltage.
Using this formula, the user can easily choose the battery for their requirement.
If battery backup is considered to be 3 hours and battery voltage is 12V then the battery capacity is calculated using above expression.
Battery capacity = (330*3)/12 = 82Ah.
Battery with particular capacity can be used.
Additional Considerations When Sizing Solar Power System
Sizing a solar inverter is an important part of any solar installation, big or small. Since your solar energy system is going to be producing and sending DC electricity to your inverter, you're going to need to have an inverter size that can handle the load and convert it to AC power. This requires knowing how to size an inverter properly.
If you're going to understand how to size an inverter you must first understand how inverters are rated.
How Inverters Are Rated
The first way inverters are rated is in Watts (or Continuous Watts).
1. Continuous watts is the total amount of watts the inverter can support indefinitely. A 2000 watt inverter can power up to 2000 watts continuously. A bigger inverter size could handle more.
For your inverter to be right for your system, it's watts rating must be approximately equal to your solar system's watts rating. This is the correct way to size an inverter.
Therefore, if your solar system is rated at 2000 watts, you'll need a solar inverter with about 2000 watts, maybe a little bit more. But not too much more or the efficiency will drop.
If you want to run multiple appliances at the same time and want to make sure your inverter can handle the load, just add up all the Continuous Watt ratings of all the appliances that may be running simultaneously.
Depending on the total continuous watts you get, you can determine if your inverter can handle it. This is also an important part of inverter sizing (how to size an inverter).
So if the total continuous watts of all the appliances that may run at the same time is 3000, it's too much, you'll have to run less appliances at the same time.
The second way solar inverters are rated is in Surge Watts.
Surge watts is the amount of power the inverter can support for a very short time, usually momentary. A 2000 watt inverter rated at 4000 surge watts can handle up to 4000 watts momentarily while starting things like motors - which usually require more power than normal to get started.
For your inverter to be right for your system, it's surge watts rating must be approximately equal to (or greater than) the potential surge watts of each appliance.
You can find this out by looking at the sticker on the back of all of the appliances you will be using with your solar system and checking the potential surge watts of each appliance. By doing this you can determine the minimum surge wattage you'll need your inverter to be rated for. Usually, you'll need about 1.5 to 2 times as much surge watts as continuous watts for a good measure of surge protection (more, if powering heavy duty equipment).
Therefore, if the highest surge watt rating on any of the appliances you plan to use with your solar system is 4000, you'll need a solar inverter with a little over 4000 surge watts.
Input Voltage - Should I get a 12v 24v 48v or more inverter.
The next rating you have to look at when sizing an inverter is the input voltage.
For correct solar system sizing... your solar panels, inverter and battery bank all need to use the same voltage.
So the input voltage of your inverter will depend on the inverter's power or watt rating. For inverters with a relatively small amount of power like 100 watts, the voltage will be 12V, 24V and 48V. For higher powered inverters, the input voltage will likely be more.
Length of Wire & Solar Inverter Performance
One of the factors that can affect your inverter's performance is the distance between your solar panel array and your battery bank. The longer the wire used here, the lower your inverter's voltage should be to perform optimally, because with long wires voltage drops and current increases.
The higher the voltage and the lower the current, the shorter length wires you can use. With longer wires, you would need to use thicker wires.
Inverter Stacking (Using Multiple Inverters)
Sometimes people connect more than one inverter together to "stack" up more power. This would typically be done if you have many smaller inverters and want to join them together to form a bigger one.
If your inverter demands increase in time (because you added more solar panels) you can either buy a bigger solar inverter or wire multiple inverters together.
When you install and wire two inverters together, it's called inverter stacking and it can provide either more power or higher voltage.
If two compatible inverters are wired together in series, you can double the output voltage. This inverter stacking technique would be used if you only had two smaller inverters and had to provide 120/240 (200/400) volts AC.
However, if you were to wire them in parallel, you would double your power (watts). This solar inverter stacking technique would be used if you had two smaller inverters but also had a solar system that was rated at much higher watts (power) than what a single inverter could handle. If you wired two 2000 watt inverters together in parallel, they would be able to handle 4000 watts (4KW) of power.
Inverter Performance With Less Sunlight
By correctly matching your solar panel's, your battery bank's and your inverter's rated capacities, you can improve the performance of grid-connected solar systems.
However, when the sun is not at it's brightest and the system isn't producing at close to full capacity the inverter will be operating at partial load and it's efficiency will drop.
Energy loss also occurs when an inverter is too small to operate in conditions of overload. Another important thing to consider in PV inverter sizing.
Solar Inverter Price
An average quality Modified Sine Wave solar inverter can cost anywhere from N100,000 - N500,000. These low to medium quality range inverters can operate with small to medium sized systems and relatively speaking provide good performance, reliability and consistency.
Obviously, unlike more expensive inverters (True Sine Wave inverters) there is typically a moderate amount of energy or performance loss, but not if your appliances aren't too high tech and your solar application isn't too demanding.
If you want to get a good quality inverter for a pretty big system, it would probably cost you about N200,000 to N350,000 for a 2000 to 3000 watt Modified Sine Wave solar power inverter.
If you want to be able to run basically anything plus have all the automatic features, you would likely have to pay about an extra N200,000-N500.000 for a True Sine Wave solar power inverter.
These higher quality Sine Wave inverters are computer compatible and computer controlled which will add automation and true convenience to monitoring and protecting your solar power system. Never forget to factor in convenience and practicality when doing your solar inverter sizing.
The solar inverter components must be protected from overload, lightning surges, etc. It must contain sensing circuits to sense the problems and automatically shut down. If the supply voltage is less, it must shut off. Otherwise the battery will over discharge and even damaged.
Points to be remembered when choosing an inverter:-
Sine wave inverter is better, because it reduces heating and will increase the longevity of inverter. The square wave inverter produces a humming sound. This humming sound can be reduced by using sine wave inverter. The sine wave inverter supply is better than the supply from grid.
Choose the inverter with capacity according to the load.
The idle power consumption of inverter must be low.
It must consume low electricity from grid.
The battery for inverter must be branded with warranty
Need Help with Solar System Sizing?
Of course, sometimes it’s easier to talk to an expert who knows how to size a solar system and can walk you through the design process. Once you’re ready, we do encourage you to schedule a free design consultation with us so that we can double-check your sizing, find compatible products, and ensure the system works within your constraints (budget, build space, and energy offset).
The fastest way to get a thorough evaluation of your solar needs is to WhatsApp or call us at 08093009019 and connect with one of our designers. We’d love to help you design the perfect grid-tied system to meet your solar requirements.