“How many solar panels do I need?”…. This the the most common question asked by far from our solar design engineers. The answer is complex and most customers approach it from the wrong angle when they start to look into solar systems for their homes.
We are often asked to quote a system to power a 2-3 bedroom home or to support a family electricity usage, energy consumption of 4 people. In these situations, it is impossible to provide an accurate estimate until we know more about the household’s exact energy needs.
In reality, the best place to start is to evaluate your current energy use based on past electric bills. Past usage data is the best baseline to figure out how many panels you will need.
Your lifestyle, local climate and panel efficiency all play a role in figuring out the size of your solar system. Here is the process we use to make an accurate estimate.
How Many Solar Panels Do You Need To Power Your Home or Business?
Your energy usage in kWh (kilowatt-hours) will determine the size of your solar panel system. Solar panels have a wide range of wattages (310W-550W is common in 2023), and other factors like local sun exposure, mount orientation and the presence of a solar battery bank also play a key part. Read this article and use our solar cost calculator for help finding an accurate estimate for your electricity needs.
The number of solar panels you need depends on various factors such as your energy consumption, location, roof size, panel efficiency, and desired level of energy independence.
How many solar panels I need for grid tied systems?
Here’s a general approach to estimate the number of solar panels:
- Calculate your average daily energy consumption in kilowatt-hours (kWh) by reviewing your electricity bills from the past year. Summarize the daily usage.
- Determine the solar panel system size you need in kilowatts (kW) by dividing your average daily energy consumption by the average daily sunlight hours in your area. As an example, if your average daily energy consumption is 30 kWh and your area receives an average of 5 hours of sunlight per day, you would need a 6 kW solar panel system (30 kWh / 5 hours = 6 kW).
- Estimate the number of solar panels needed by dividing the total system size in kilowatts (kW) by the rated power output of a single solar panel. Solar panels typically range from 250 to 400 watts per panel, although higher wattages are also available. For example, if you’re using 300-watt panels, you would need 20 solar panels for a 6 kW system (6,000 watts / 300 watts per panel = 20 panels).
It’s important to note that these calculations are rough estimates and actual results may vary depending on specific factors such as panel efficiency, shading, and orientation. It’s recommended to consult with a qualified solar professional or use an online solar calculator that takes into account more detailed parameters to get a more accurate estimate for your specific situation. Additionally, local regulations, incentives, and installation requirements may affect the size of your solar panel system.
What is a kilowatt hour of electricity?
A kilowatt-hour (kWh) of solar refers to the amount of electricity that is generated by a solar panel system with a total capacity of one kilowatt (kW) over the course of one hour of continuous operation under ideal conditions of maximum sunlight exposure of your solar modules.
Solar panels generate electricity through the photovoltaic (PV) effect, where sunlight is absorbed by the solar cells in the panels, causing electrons to be released and creating an electric current. The power output of a solar panel is typically rated in watts (W) or kilowatts (kW), which represents the maximum power it can produce under standard test conditions (STC), which include a set of specific parameters such as temperature, sunlight intensity, and angle of incidence.
A kilowatt-hour (kWh) is a unit of energy that represents the amount of electricity used or produced over a period of one hour at a constant rate of one kilowatt. It is commonly used as a measure of energy consumption or production for residential, commercial, and industrial purposes. Therefore, when referring to a “kilowatt-hour of solar,” it generally means the amount of electricity generated by a solar panel system with a capacity of one kilowatt (kW) over the course of one hour of operation, assuming ideal conditions. The actual energy production of a solar panel system can vary depending on factors such as sunlight intensity, shading, panel efficiency, and system maintenance.
Lets take some examples
If an appliance rated for 1 kilowatt (1000W) runs for an hour, then one kWh of energy has been used.
The energy company measures total energy usage in kilowatt hours. Your total usage in kilowatt hours determines how much you are billed each month.
First Example: A fridge rated at 250 watts runs for 4 hours per day. 250W x 4 = 1000W, or 1 kW. This fridge uses 1 kWh of energy over the course of a day.
Second Example: An oven is rated at 2000 watts (2 kW). Cooking in this oven for half an hour would consume 1 kWh of power (2kw x 0.5 hours = 1kWh).
Find how many kWh of energy you use per year. That will give you a good jumping off point for estimating your energy needs – but you’re not there yet.
Divide that number by 365 to get your daily energy usage in kWh.
Once you have your daily energy usage, use this formula to estimate your total system size:
Daily Usage (kWh) ÷ Sun-Hours ÷ 0.9 inefficiency factor = Minimum Solar Array Output
Sun-hours refers to how much sun you get each day where you live. You can find that info on our Global Solar Irradiation Map (Sun-hours map). More on this a bit later.
The inefficiency factor simply accounts for circumstances that would make your system run below its optimal output, like shade, extreme temperatures, voltage drop and equipment inefficiencies.
Take your daily usage and divide it by these two numbers to get an estimate of the overall output of your system.
For example:
According to the European Union Statistics EUROSTAT an average household used 17.798 kWh of electricity while according to the U.S. Energy Information Administration, the average American household used 10,632 kWh of electricity annually. That’s about 29.13 kWh per day if we take the USA example as a base.
Let’s say you live in Texas, which gets 5.5 sun-hours per day.
29.13 kWh per day ÷ 5.5 sun-hours ÷ 0.9 = 5.884 kW capacity system.
That would give you an approximate system size of 5.88 kW, or 5884W (remember, 1 kilowatt = 1000 watts).
From there, the last step is to divide by the energy rating of each individual panel. Solar panels are graded by how much power they use. The panels you would use in a residential setting typically range from 330 to 550 watts per panel.
Let’s say we want to use Canadian Solar 335W monocrystalline panels. Take your system size and divide by the panel wattage to figure out how many solar panels you need in your system:
5884W ÷ 335W = 17.56 solar panels
Round up the final number, since you can’t buy partial panels. In this scenario, we would need 18 panels rated at 335 watts apiece to cover our energy needs.
We can’t stress this enough: this calculation is a very rough estimate. It should only be used to ballpark system size and make early pricing estimates.
But don’t take this estimate as gospel – there are too many factors that can change the size of your system in practice.
Additional factors considering an off grid solar systems
Calculating power consumption needs for an off-grid system is a tad more complicated. People who live off-grid need to focus on daily power usage rather than monthly or annual consumption.
You’re not staring down a power bill each month – you’re independent and responsible for covering your own day-to-day power needs. The system needs to be able to produce enough (and store enough) to keep things running smoothly.
Without power bills as a starting point, it’s best to start by listing out your major appliances and estimating how much you use them on a daily basis. Input this list into our Off Grid Solar Calculator.
If you’re not sure how much power an appliance uses, follow the appliance electrical consumption table as a guide. You can also check for the EnergyGuide sticker, or use a meter to measure energy consumption if possible.
This form will give an estimated daily usage.
Pay close attention to December and January when you estimate your energy needs. Those months tend to have the highest power usage and the lowest output by your system.
When you live off-grid, you will need a battery system that’s large enough to store enough power for the day and then use solar power to recharge them in a timely manner. It’s common to lean on a backup generator during the winter, when there won’t be enough sunlight to fully power your solar system.
Once you know how much power you use in kWh per day, a solar design technician can determine the minimum battery size needed with a formula that accounts for things like inefficiencies and temperature coefficients.
Here are the basic formulas we use to size off-grid systems:
Minimum Battery capacity (for lead acid batteries):
Daily usage (kWh) x 2 for a 50% discharge depth x 1.2 inefficiency factor = Minimum Battery Capacity
Minimum Solar Array Size
Daily usage (kWh) ÷ Sun-hours ÷ .9 inefficiency factor = Minimum Solar Array Output*
*Ensure solar array meets battery charge requirements, typically around 10 charging amps per 100ah battery capacity.
You may need a larger array or battery bank based on your location, ambient temperature, your usage patterns and other factors. Take a look at our Solar Battery Sizing Calculator to help figure out how many batteries you need to power your system.
Solar module quality vs. quantity: panel efficiency is not everything
Another aspect that affects the size of your system is the efficiency of the solar panels themselves. Most residential panels range from 265W to 450W. If you go with a 265 watt option, you’ll need several more panels to build your array. When it comes to solar, efficiency isn’t always the most important factor to consider when you build your system. It really depends on what your specific goals are.
For example, if you look at your solar panels primarily as an investment and a quick ROI is your biggest goal, you might be better off with a lower output, lower cost panel.
“If a panel is 50% more efficient, but costs 100% more, you are better off paying for a larger system of less efficient solar panels.”
–Adam Jackman, technical solar engineer at Segen
Adam Jackmanf, technical solar engineer of our partner company expert with Renusol, he had a few thoughts on this matter. Adam said one of the most common mistakes he sees people make is that they buy into ultra-efficient, high-quality, and technically advanced system, and it might not be worth it – especially if ROI is your biggest concern.
“While a lot of the solar pv systems like that have really good equipment, it is important to look at the cost per watt,” he explained. Adam urges consumers to consider this: “How much are you paying for the total wattage/production of the system?”
“It doesn’t really matter how efficient a panel is if it costs you a lot more. If a panel is 50% more efficient, but costs 100% more, you’re better off paying for (a larger system) of less efficient panels.”
But there are some circumstances where having a smaller yet more efficient solar system makes sense. For example, if your roof space is really limited, you might need a more efficient system to cover your energy needs within that given area.
Sun-hours based on your location
“Sun-hours” refers to how much solar energy hits a given area over a certain amount of time.
Your local climate determines how many peak sun-hours you get each day. This number can change drastically based on where you live. If you live in the United States, you can check out our Solar Irradiation Map to get an estimate of how many sun-hours you receive in your area.
It’s important to keep in mind that the term “sun-hours” doesn’t just refer to the hours of daylight that your area receives. The peak hours occur when the sun is at its highest in the sky, which will change based on the season and how close you are to the equator. In the winter, the average sun-hours in your location could decrease by 25% to 50%.
So how do sun-hours affect the number of panels on your solar system? If you live in an area with less sun-hours, you’ll need more panels to capture what you need to cover 100% of your energy usage. However, if you live in an area that gets 5-6 sun-hours per day, you might be able to get by with a smaller system.
Considering other variables when designing a solar system
We can not stress this enough: this breakdown only serves as a very rough estimate and a starting point for planning your system.
It’s great to get you closer to a ballpark figure on the cost of panels. It will help as a benchmark when you measure your roof to see if you can fit a system up there. But when you get deeper into planning your system, unexpected hurdles always come up and the system size tends to change. Here are some question to be considered:
- What if you decide to go with 265W panels instead of 450W panels because the cost-per-watt is lower?
- What if shade covers your system, or your roof doesn’t directly face the sun?
- What if harsh weather causes your equipment to perform below its rated efficiency?
- What if you start using more energy than you did in the past? Yes, you’ll need more panels.
Although this breakdown can give you an estimate of how many solar panels you’ll need in your array, at the end of the day it’s just an estimate. There are several other variables that can determine the size of your solar system, but this process is still important as it gives you an idea of what to look for before you seek out quotes on solar systems.
If you’re interested in going solar, the best thing you can do is speak with a solar design engineer that can help you find the ideal system for your unique situation. Bring your estimate as a starting point. We’ll go over any potential problems and tweak the design to suit your location and lifestyle.
