How To Do Solar System Sizing For Your Home

how to size your solar system

Solar system sizing refers to the process of determining the size of the solar system that would meet your energy needs. There are many factors on how to figure out solar system size. Before we get into the factors, a solar system has different components like any other system.

These components contribute to the overall size of the system. The size of each will be proportional to the size of the entire system. Thus, increasing the system’s size usually means increasing the size of each element of the system.

The modularity of the photovoltaic (P.V.) system is one of its most significant advantages. You can replace components with others of higher capacity or increase their number, as the case may be when your energy needs increase without dismantling the system. Before getting into how to size batteries for a solar system, you need to know its components.

A solar array after undergoing a solar system sizing.
A solar array.

Parts of a Solar System

The following are the parts of a solar system.

  • Solar panels
  • Battery Bank
  • Charge controller
  • Inverter
  • A.C. & D.C. loads
  • Balance of system

Solar Panels

This refers to the photovoltaic panels that form a part of the system. These panels are the most prominent feature of a solar system, and you can have one or a few depending on their capacity and your need. When you have several of them together, they should be connected in series or parallel so that they can combine the power they generate to meet your needs. You should have these panels connected to have maximum exposure to the sun for the best results.

solar panels being placed by a worker on a roof
Solar modules being placed on a roof

Battery Bank

This component comprises batteries in which the power generated is stored. The battery bank includes single or multiple deep cycle batteries. Like the solar array, you connect these batteries either in series or parallel depending on factors such as the current capacity needed. Batteries in the bank become the center where the energy required is discharged for use.

Charge Controller

The primary purpose of the charge controller is to prevent the battery from overcharging. Batteries can be fully charged when the sun is still up. Overcharging isn’t suitable for your battery’s life span, so the charge controller contributes to the overall health of your solar power system.

Inverter

The current comes from the batteries as direct current, while most household appliances use alternating current. The inverter converts the current from the direct to alternating current to make it useful around the house.

solar inverters to convert D.C. power to A.C. power on display.
Solar inverters for converting D.C. to A.C. current

A.C. & D.C. Loads

These refer to the appliances that use power in the house. They can light bulbs fridges. The components in your home are an essential factor to consider when sizing your solar power system, as we will see later.

Balance of System

The balance of a system refers to several components that ensure the whole system is interconnected. The balance system also enables you to ensure the electric system you have made from the various parts is safe. Its components include circuit breakers, switches, cabling, meters, and other related components.

How to Figure out the Solar System Size for Your Needs

The question of how to figure out the solar system size for your needs is relevant from the moment you decide to install a solar power system. The following are steps to take.

  1. Determine how much power you need
  2. Optimize your consumption demands
  3. Determine the correct battery size for your power needs
  4. Find out sun exposure
  5. Determine the right size for the array

Determining how Much Power you Need

The easiest way to determine how much energy you need is by looking at your past utility bills. Your energy consumption is likely to remain within the same range as it has been in the past.

Some utility companies give the number of units (kWh) used for the billing period, typically 30 days. If, however, they have not given you the number of units you have consumed in the month, you can calculate them by subtracting your previous consumption from the current consumption.

It is essential to know your daily consumption because your solar system will generate power daily. Understanding daily consumption is vital because your batteries are unlikely to have the capacity to store energy for longer than a day or two. You will, therefore, need to divide the kWh you have consumed in a month by 30 to find your average daily consumption. Divide this figure further by 24 hours to determine your average hourly consumption.

The average power usage in an American home is 900 kWh per month which translates to 30kWh per day and 1.25kWh every hour. This is the amount of power you want your system to generate every hour, and you should, therefore, target to generate as much or more. Please remember that a solar power system doesn’t generate an equal amount of power consistently throughout the day.

Sizing by Appliances

One of the first steps to size a residential solar system is sizing the appliances. You can also determine the amount of power you need to generate by filling out a load sizing worksheet. Devices have a label indicating how much energy they use. List all the appliances you have in the house and check their power consumption. The wattage indicated in each device refers to the number of watts it uses per hour.

You should also indicate the number of hours you use each appliance. Multiply the wattage by the number of hours the device uses every day. Sum up the figures. You get to find out how much power you need every day. That is the amount of energy you need to generate daily.

It is always good to mark up the figure you get here by about 10% because the appliances don’t start and stop on automatic timers. You can underestimate or overestimate the number of hours you have the devices on. Then mark it up because it is always better to have more power than you need than to have less of it.

You can use these figures to see how much energy you need per week or month, depending on the plans you have for your solar power system.

Read More: How to Become Proficient in Solar Panels for an RV

Optimize Your Power Demands

Optimizing power demands means reducing the amount of power you will need to rely on the solar power system to generate directly. Optimization of power needs is a vital step in establishing any energy system because it saves you money. Reducing your energy needs also enables you to have margins in the power you generate. The following is how you can take to optimize power demands.

Identify appliances that consume large and variable loads. Examples of such appliances include; high wattage outdoor lights, water pumps, refrigerators, washing machines, etc. You can seek alternative power sources for this equipment.

For example, you can use propane for your water pumps instead of electricity. You can also buy appliances that use D.C. power and replace those that use A.C. Having DC-driven appliances reduces power loss which occurs as the inverter converts it from D.C. to A.C.

Sometimes, it is possible to convert A.C. appliance into D.C. Still, you often have to buy a new item if you want to make this change. D.C. appliances typically cost more, but they also function better and last longer. Ultimately, you will be saving money on power and the future cost of replacement.

If there are high-consuming appliances you can’t eliminate or change, use them only when the sun is hottest and the P.V. panels are performing optimally. If you consider a solar power system before you buy appliances, make sure your buying list for devices is well optimized. It would be better because you will not have to make changes later.

Consider the Amount of Sunlight in Your Area

The duration and intensity of sunlight on your location will determine the amount of energy to expect from your solar system. Areas with fewer hours of sunlight will require you to have more and probably, better performing solar panels than where there is more sunlight. There are reliable resources online that indicate the amount of sun at different times of the year. Renewable Resources Data Center is one such place. 

Multiply the kWh you need per hour for your home by 1000. This figure is the number of watts of energy you need to generate every hour. This conversion is necessary because your consumption is expressed in kWh while the performance of a solar power system is described in watts.

The watts you get show you how much energy your solar power system requires to generate every hour. Thus if you divide this figure by the number of peak sunlight hours every day, this figure will give you an indication of the amount of power you need to generate hourly to meet your demand. From this figure, you will determine how many panels of what capacity you need.

Sizing Solar Panel System

Solar panels are the most significant, most conspicuous part of a solar power system. Therefore, they generate all the power and, therefore, sizing them properly is a critical step towards adequate overall solar system sizing.

Technicians doing solar sizing
Workers fitting electric ducts

How to Comprehensively Size Your Solar Panels

The following are some thoughts to help you comprehensively size your solar panels.

Panel output efficiency

Solar panel size

Panel weight

Sizing the inverter

Panel Output Efficiency

Not all solar panels produce solar energy the same; some are more efficient. The efficiency of solar panels is a critical determinant of the ultimate size of the system. If the panels are less efficient, you will need to install many of them to satisfy your energy needs. P.V. panels come in capacities of between 250 watts and 370 watts. The wattage may be determined by the size of the panel or by its efficiency.

Some solar panels have grid lines on the face, and they are less efficient, and they get damaged easily, mainly due to peeling off of the lamination. On the other hand, the brands without grids generate more power, and they don’t delaminate.

There is a wide variety of solar panels in the market, making it difficult to give a breakdown of which panels are the most efficient. However, whenever you go to buy solar panels, have in mind the amount of energy you need and the peak performance of each panel. You can only learn of the panel’s efficiency by getting reviews from people who have used them in the past.

Talk to your technician and research the brands as much as you can. Online customer reviews are usually an excellent way to know. If you find a brand you are considering has a significant percentage of negative reviews, move on to the next one. Else you might invest in panels that will cause you loss.

After choosing the right solar panel brand, divide the hourly wattage needs for your home by the wattage of each panel. This will show you the number of panels you need for your home, which is an integral part of sizing your solar system.

Solar Panel Size

The size of solar panels is almost standard across brands and levels of efficiency. They typically come in 65 inches by 39 inches. The dimensions have remained the same even as their efficiency has increased over the years. The size of your roof, or the space you have to fix your panel. If you have a small roof, you could buy higher wattage panels to generate more power.

Panel Weight

Your solar panels will mainly be positioned on your roof, and your roof’s ability to carry the weight is a vital sizing consideration. The weight of each panel varies slightly, but it is always around 40 pounds, give or take a few pounds. Manufacturers indicate the weight of their panels, and you can calculate how much of it your roof can carry and decide accordingly.

Sizing the Inverter

We mentioned earlier that the effectiveness of your solar system would be optimum if your appliances use direct current. However, it is unreasonable to expect everything in your house to use D.C.; after all, you are probably seeking to install solar power after buying appliances.

Having everything being D.C. powered is also not prudent because, more often than not, you will need to supplement your solar power with power from the grid, which is A.C. You need an inverter, and it needs to be the right size.

Inverter Sizing Factors to Consider

The following are some inverter sizing factors to consider:

  • The inverter’s input rating should always be equal to or greater than the total energy needs as indicated in the total wattage of the appliances you calculated earlier. It is recommended that the inverter’s capacity by between 25% and 30% higher than the total wattage of your devices. Such a margin would give you vastly improved results.
  • The inverter’s nominal voltage should be the same as battery time.
  • If you have inverter or compressor appliances, the inverter capacity should be at least three times more than their wattage. Thus your calculation for inverter size should be the wattage of all the appliances plus triple the wattage of the compressor and motor appliances.

If your system is connected to the grid, the inverter input should be the same as the panel array rating. This allows for a safe, efficient transition to and from the grid.

Inside look of a solar system inverter
How a typical inverter looks like when opened up

How to Size Batteries for a Solar System

The battery is one of the critical components of the solar system, and it is recommended that you use deep cycle batteries. They have the advantage of charging quickly and releasing power slowly. This process can go on for years, and the batteries will still maintain their control.

The following are the steps to take when sizing your batteries.

  1. Calculate the total watt-hours you need for your household per day
  2. The battery loses some power; to factor this in, multiply your total wattage by 0.85
  3. Divide the figure you get in ‘b’ above by 0.6. This calculation helps you put the depth of discharge as part of your calculation.
  4. Divide the figure you get in ‘c’ above by the nominal battery voltage
  5. Multiply the figure you get in ‘e’ above by the days you may need this system to work when the panels are producing no power.

The formula for battery capacity would therefore be;

Battery Capacity (Ah) = Total Watt-hours per day used by appliances x Days of Autonomy

                                    (0.85×0.6 x nominal battery voltage)

Sizing Charge Controllers

Charging controllers should have the same voltage as the solar panels and batteries. You find the appropriate size by multiplying the short circuit current of the charge controller and multiplying it by 1.3.

Charge controller rating = Total solar panels short circuit current x 1.3

Cabling is another critical part of the system. You will get a cable length that is long enough to cover distances in the system. Other fixtures such as switches and sockets will be determined by the system’s design.

 Once you have made these calculations in the outlined way, you are ready to proceed to the next step and purchase the necessary components.

Read More:

What is Solar Power and how Does it Work?

Advantages and Disadvantages of Solar Energy

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