Last Updated: August 9, 2022
How Many Solar Panels Will I Need?
Find Your Answer in 8 Simple Steps!
How many solar panels do you need to provide the energy to support your home and family? It’s a big question because it directly affects the cost of your new solar energy system. Even though prices for solar panels have plummeted in recent years, you still don’t want to overpay by getting more panels than you need.
There are eight steps to calculating how many panels you’ll need and six factors that affect some of the decisions you’ll have to make.
We’ll walk you through both as you go through this article. You might want to get a pencil and paper to work on this as you go.
7 Factors That Determine the Size of Your Solar Energy System
As you make plans for installing a new solar array, here’s the list of the six key factors that affect how many solar panels your home, farm, or business will need.
- Your monthly and yearly power bill
- Your monthly and annual power usage
- Inconsistent power production
- Your daily power usage – kWh per day
- Average daily “direct” sunlight
- Square footage of your roof
- Solar panel efficiency – actual power production per panel
Once you’ve got this part down, you can also look at how much your solar power system will cost to install.
Factor 1: Your Monthly and Yearly Power Bill
Your power bill is the first thing to analyze. It forms the basis for your solar energy production goals.
For instance, if your monthly bill is $100 ($1200 for the year), then you probably don’t need a solar array that produces $2000 worth of energy.
Task 1: Task 1: Look at your bills – summer and winter – and see how much you’ve been paying.
Then, if you have them, look at your rates from five and ten years ago. One bill-payer we looked at is getting charged more than twice as much for energy in 2016 as he was in 2008, and he’s using about the same amount of energy.
Your solar panels will produce consistent power for over 20 years.
Factoring in this kind of inflation should be part of your consideration. Your cost savings from solar will increase as time passes, even if your usage doesn’t change.
Factor 2: Your Monthly and Yearly Power Usage
There’s the cost, and then there’s the usage. Your usage is listed in kilowatt-hours – or kWh. Look for it on your power bill.
For a reasonable estimate of your usage, add up the total kWh you have used in the last 12 months and find the average. So if your total over the year is 11,000 kWh, that would be 917 kWh per month. That gives you a starting point to estimate how much power you’ll need your solar panels to produce.
Usage is separate from the cost. Your usage may not change much for ten years, but the price per kWh will change. This matters because solar panels produce a certain amount of kWh, and that amount won’t change much. So even though costs rise over time with traditional power, your usage doesn’t have to. In fact, with improved energy efficiency, your usage could actually go down.
That’s why, in addition to your average monthly power usage, you need to think about how your life circumstances might change in ways that increase or decrease your energy usage.
- Will you have kids? They’ll require more energy (and not just from you!)
- Are you planning to add in a hot tub?
- Do you want a new home entertainment system?
- Buying a second fridge?
- Making significant computer hardware upgrades or additions?
- Purchasing an electric car?
These sorts of things will add significant kWh to your monthly usage, so if you anticipate a couple of big “power suckers” in your future, factor those into your future energy needs.
Task 2: Task 2: Think about how your future energy needs will change in 10-20 years.
If you’re averaging 1000 kWh a month now, it may go up to 1200. Or, if you’re planning to move to a new and larger home and want to put solar panels on it, that larger home will probably require more energy usage. Again, plan accordingly.
From these first two factors, you can now set a goal.
Task 3: Set a solar power production goal.
How much of your energy do you want to produce from solar? 80%? 50%? All of it? As much as you can afford?
Most solar installers recommend aiming for about 80% of your current power usage.
If you use 1500 kWh per month and want to produce 80% of that from solar panels, you’ll need a system that generates 1200 kWh per month.
Keep your goal number in mind as we look at the following four factors.
Factor 3: Inconsistent Power Production
July isn’t December. While we can disagree about which month is our “favorite,” we can’t disagree about which one is better for solar energy production in the northern hemisphere
Which months are best for solar varies by region, but it isn’t consistent year-round anywhere in the US. Understanding this matters for several reasons:
- What happens if your system produces excess energy in the summer months? Where will the excess go?
- Does your utility have a net-metering (solar buyback) program?
- Do you have a way to store the extra energy (solar battery storage)?
If you don’t want to buy battery storage, and if you can’t sell it back to the grid because your utility doesn’t allow it, producing more energy than you need does you no good whatsoever.
That’s why many people set a goal of less than 100%. If your goal is to produce 65% of your energy needs from solar, then it’s unlikely you’ll generate more than you need, even in the summer. And instead of having a zero power bill, you’ll just have a much smaller one. But you’ll avoid the risk of producing energy that doesn’t benefit you.
However, if you have access to a buyback or net-metering program (like the Tennessee Valley Authority offers its customers) or want to buy solar battery storage, setting a goal closer to 100% makes more sense.
But be aware – your solar panels will not produce consistent amounts of energy around the year.
Task 4: Re-consider your goal with excess energy in mind
Factor 4: Your Daily Power Usage – kWh Per Day
Yes, we’re back to your power bill again. But we’re also getting closer to the number you want to know – how many solar panels will you need.
Task 5: Divide your average monthly power usage by 30 days.
Use your expected average monthly power usage that you determined earlier to calculate your average daily usage. If you’re using 1000 kWh per month, that’s 33 kWh per day. You’ll want this number to help determine how many solar panels you’ll need.
Factor 5: Average Daily “Direct” Sunlight
This is a big one. The same-sized solar array in central Canada will produce far less energy than one in South Carolina. The reason is that they don’t get as much direct sunlight in Canada – and not just because of cloud cover. The farther north you go, your position on the earth changes, meaning the angle of the sunlight hitting your home is smaller than what hits the Southeastern United States.
What is “direct sunlight?” It’s sunlight that hits a particular spot without interference or obstruction. (Visit our solar dictionary for a complete list of non-technical solar definitions)
Most of the Southeastern US, including Georgia and South Carolina, gets about 5.5 hours of direct sunlight per day. Here, it’s actually highest in the spring and early summer. But the winter has the least amount. Still, 5.5 hours is a good average to use if you’re going solar in Savannah or elsewhere in the region.
And now, you’re ready!