Understanding Solar System Calculations & Wattage Needs

Wondering how many solar panels it will take to power your home or business? You’re in luck! This post is dedicated to helping you properly size your solar panel system and also understand basic solar energy measurements and terminology such as kilowatt and kilowatt-hour.

Although MOXIE will walk you through all the important steps of sizing your system in detail, having a solid understanding of the basics will only make the process easier. Plus, we truly want you to fully understand the ins and outs of solar power!

Quick note: This post explains how to calculate the number of solar panels you need to offset your average electricity usage. While this is the most common way to size a solar panel system, it’s not your only option. Read our recent post on goals for installing solar panels to learn more about the benefits of creating a system that’s larger or smaller than your average usage.

Good data in, good data out

First, let’s debunk some common solar system sizing misconceptions. Using the total square footage of your building is NOT a great way to calculate your solar panel system size needs. It’s also not about how many people live in your home. This might give you a ballpark estimate of your energy needs, but when dealing with sizing a solar system, you want to be as precise as possible. Like any calculation, if you put bad data into the equation, you get a bad answer out of it.

Remember what you’re trying to do here. You’re trying to figure out how much energy your solar system should be capable of producing on any given day. Whether it’s generated from dirty fossil fuels or from clean renewable sunshine, energy is measured in kilowatts (kW) and you’re billed for it based on kilowatt-hours (kWh). Because of this, your previous electric bills are actually the best guide for how to properly calculate your solar panel system size requirements. More on this later.

What is a kilowatt-hour (kWh)?

First, a kilowatt is 1,000 watts. If you’ve ever changed a lightbulb you’re familiar with watts. Most household bulbs range from 25-watts to 100-watts. The more watts, the brighter the bulb and the more electricity used. Watts is simply a measure of power.

Kilowatt-hours (kWh) is a measure of energy used. If you turn on a 100-watt lightbulb for 10 hours, it would rack up 1kWh of energy. When it comes to your electric bill, kWh is a measurement of how much energy your home or business used in the previous month. If you look at a past bill, you’ll see that your final cost is a calculation based on how many kWh you racked up.

Solar panel systems use these same fundamental measurements of power supply and energy usage.

Solar panel sizing 101

Much like lightbulbs, solar panels come in different sizes. For the most part, individual solar panels range in size from 250-watt up to 400-watt. Once you know the total number of watts your solar system needs to be, determining how many panels you need is simple math. A 5kW system is going to require at least twenty 250-watt panels (250-watts X 20 = 5000-watts). MOXIE commonly uses 350-watt to 370-watt solar panels. This size panel is generally a good compromise between 1) available space for the panels and 2) cost per panel.

Pro tip: When performing solar system calculations you should also factor in seasonal weather patterns and equipment derating over at least 25-years.

Ok, but how do I calculate my total solar system size needs?

Alright, now it’s time for you to get those previous electric bills out. If you want to know how big your solar system needs to be, you’re really asking “how many kilowatt-hours of electricity do I generally use in a day?” You’ve probably never thought about this before, but luckily, you’re utility company documents this for you on every one of your bills.

Here’s how MOXIE will help you determine your optimal solar system size if you’re looking for offset your total energy usage:

  1. Look back at your energy bills over the past 12 months (each statement should tell you your monthly kWh usage).
  2. Take that annual total and divide by 12 to get your average monthly usage (some electric bills show each month’s average on the bill).
  3. Divide that number that 30.5 to get your average daily usage.
  4. Divide that number by the average daily sunlight hours for your region. This would be your proposed system size at 100% efficiency.
  5. To account for variables, use a derating factor for an 85% efficient system. This is your final proposed system size in kWh.

Pro tip: You can calculate your average hourly energy usage by dividing your average daily usage by 24. Then you can easily convert your KWh usage to kW needs.

Solar system calculations & final cost

Once you have your solar system calculations figured out, you can start to understand what your cost to go solar is going to be. This largely depends on the cost per watt for solar in your state, as well as the solar incentives you’re eligible for. Our financial experts will help you understand all your options and help you get the most savings possible!

Contact MOXIE today to get a free, no-obligation solar system sizing and turnkey cost estimate.

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