How Many Solar Panels Do You Need: Easy Calculator
Ready to start capturing energy from the sun? You'll have to calculate how many solar panels you need. I'm a solar expert that has built many different systems and I'm ready to spill the industry secrets - in a way that you can easily understand.
In this short guide, we'll cover how to measure energy use and match that to the power you can expect from solar panels. We'll also include other important details to handle real-world conditions and get you through cloudy days.
By the end, you'll be able to confidently figure out how many solar panels you need.
This is part of a series of articles about how to choose solar panels.
Power Use: Watts and Watt-Hours
While watts and watt-hours sound similar, they represent two very different electrical figures. And they're central aspects to any electrical or solar-powered system, so they're worth understanding.
Watts: Power in the Moment
Think of watts (W) like the speed of energy flowing through your system. It measures how much power something uses (or generates) at a specific moment.
Watts are like the speedometer of a car. They'll tell you how fast you're going at a particular moment. If the watts are really high, there's a lot of energy moving.
An LED lightbulb uses a small amount of electricity, about 10W. An air conditioner uses a lot of electrical, about 2,000W. The LED bulb is like a car crawling through a parking lot while an air conditioner is driving at highway speeds.
For solar panels, a small amount of wattage is ~500W or less, while a large home solar panel array can have 2,000W or more worth of panels.
Watt-Hours: Total Energy Over Time
Watt-hours (Wh) measure the total energy used (or generated) over a time period.
They're more like the odometer of car showing you how far you've traveled. A large Wh figure represents a long way traveled, meaning a lot of energy has been used (or generated).
It’s a combination of power (watts) and time (hours).
Formula: Watts × Hours = Watt-Hours (Wh).
When you compare the LED light to the air conditioner in terms of watt-hours, you'll see that the fast-moving A/C unit will cover more ground (use more Wh) much faster. For example, running a 2,000W air conditioner for one hour is 2,000Wh. You can run a 10W LED lightbulb for 100 hours with that 2,000Wh of energy.
For solar panels, if a 100W solar panel gets 5 hours of sunlight, you can expect it to generate 500Wh in perfect conditions. If you have 500W of solar panels and get the same 5 hours of sunlight, you can get about 2,5000Wh of energy.
Why Both Matter
To size your solar system correctly, you need to balance both measurements:
- Watts (W): Tell you the maximum power your solar panels can produce at any given moment.
- Watt-Hours (Wh): Tell you how much energy your system will produce over time.
Pro Insight: When shopping for solar panels and batteries, focus on watt-hours for total energy storage needs and watts for matching peak power requirements. By balancing these two aspects, you can create a system to meet your needs.
Solar Panel Wattage: How Much Power Can You Expect?
Solar panels are rated in watts, telling you how much power the panel can generate under ideal conditions. But you don't want to take it at face value.
What Solar Panel Wattage Really Means
The solar panel wattage rating comes from perfect conditions (known as Standard Test Conditions or STC). This is measured in direct sunlight with no obstructions, at a set temperature (usually 77°F/25°C).
Let's assume that a 100W panel is out in perfect conditions. In one hour, it would generate 100Wh of power under direct, unobstructed sunlight. In four hours, that would be 400Wh.
Formula: Solar Watts × Sunlight Hours = Total Watt-Hours (Wh)
In the real world, solar panels don't actually generate that much.
Solar Panel Efficiency
You shouldn't expect solar panels to operate at their full rated capacity. This surprises many solar newbies, but it's a reality of the industry.
Instead, you should expect 70% to 80% efficiency of any solar panel. So a 100W panel will only generate 70Wh or 80Wh in one hour, even during the brightest sunlight hours.
This real-world difference from Standard Testing Conditions is due to:
- Angle & Orientation: Perfect orientation all day is challenging.
- Temperature: Panels get less efficient when they heat up.
- Shade & Dirt: Partial shading can dramatically reduce output.
- Wiring Losses: Every cable and connection causes energy loss.
How Much Energy Can a Solar Panel Generate Per Day?
The other part of real-world inefficiencies is the number of sunlight hours you can expect. Although the sun may be in the sky for much longer, your panels will only reach their maximum output for the short time when the the sun is shining at its brightest.
This is known as peak sunlight hours.
You can typically expect 4-6 hours per day of peak sunlight in many locations. Fewer hours for latitudes farther from the equator and areas with more cloudy weather. More hours for sunnier locations closer to the equator.
Let's bring these points together. A 100W solar panel will generate about 80Wh during the peak sunlight hours. If you are in an area with 6 hours of peak sunlight, that's about 480Wh of energy generated per day. 100W x 80% x 6 hours = 480Wh.
Formula: Solar Wattage x Efficiency x Peak Sunlight Hours = Total Wh Generated
This formula will help you get a rough idea of how much energy your panels will generate each day. If your panels are at an awkward angle or experience shade during the day at all, you can expect even less energy. Cloudy or stormy days will also generate significantly less power, perhaps only 10% to 20% of the anticipated amount.
Device Power Usage: Calculating Energy Needs for Any Setup
To size your solar panel system correctly, you need to calculate how much energy your devices use. By understanding your power needs, you can match it to the number of solar panels you get.
How to Calculate Energy Usage for Any Device
- Find the Device’s Power Rating: Look for the wattage (W) on the device’s label, charger, or manual. If only volts (V) and amps (A) are listed, use this formula: Watts = Volts × Amps
- Estimate Daily Use: Determine how many hours per day you expect to run the device.
- Calculate Total Watt-Hours (Wh): Use the formula Watts × Hours Used per Day = Watt-Hours (Wh)
- Add All Devices Together: Sum the watt-hours for every device you plan to run.
- Include a Buffer: Add 20% extra to efficiency losses and unexpected usage.
Examples of Daily Power Use
Adding up all of your devices can be tedious. Instead, you can use these rough guidelines to identify your power needs and buy the right amount of solar panels for them.
Light Setup with Minimal Power Needs
For a simple power needs to use phones, lights, and small gadgets:
| Device | Power (W) | Hours Used/Day | Total Wh/Day |
|---|---|---|---|
| Phone Charger | 10W | 2 Hours | 20Wh |
| LED Light (x2) | 10W each | 4 Hours | 80Wh |
| USB Fan | 15W | 4 Hours | 60Wh |
| Tablet/Kindle | 15W | 2 Hours | 30Wh |
| Total | 190Wh/day | ||
| With 20% Extra | 230Wh/day |
Moderate Setup with More Devices
For RVers or those with a few more essential devices like fridges, lights, and laptops:
| Device | Power (W) | Hours Used/Day | Total Wh/Day |
|---|---|---|---|
| Mini Fridge | 50W | 12 Hours (cycling) | 600Wh |
| Laptop (x2) | 100W each | 4 Hours | 800Wh |
| LED Light (x4) | 10W each | 4 Hours | 160Wh |
| Phone Chargers (x2) | 10W each | 2 Hours | 40Wh |
| Small Fan | 30W | 6 Hours | 180Wh |
| Total | 1,780Wh/day | ||
| With 20% Extra | 2,140Wh/day |
Home Backup Setup with Fridge and Entertainment
This is for a more energy-intensive home system to provide power during outages:
| Device | Power (W) | Hours Used/Day | Total Wh/Day |
|---|---|---|---|
| Refrigerator | 150W | 10 Hours (cycling) | 1,500Wh |
| LED TV | 120W | 4 Hours | 480Wh |
| Wi-Fi Router | 15W | 24 Hours | 360Wh |
| Laptop (x2) | 100W each | 4 Hours | 800Wh |
| Lights (x6) | 10W each | 5 Hours | 300Wh |
| Phone Chargers (x3) | 10W each | 2 Hours | 60Wh |
| Total | 3,500Wh/day | ||
| With 20% Extra | 4,200Wh/day |
Heavy-Duty Home Backup Setup with AC
For homes needing air conditioning or other high-powered devices:
| Device | Power (W) | Hours Used/Day | Total Wh/Day |
|---|---|---|---|
| Air Conditioner | 1,000W | 8 Hours | 8,000Wh |
| Refrigerator | 150W | 12 Hours (cycling) | 1,800Wh |
| Microwave | 1,200W | 0.5 Hours | 600Wh |
| Electric Kettle | 1,500W | 0.5 Hours | 750Wh |
| TV + Streaming Box | 150W total | 4 Hours | 600Wh |
| Laptop (x2) | 100W each | 4 Hours | 800Wh |
| Total | 12,550Wh/day | ||
| With 20% Extra | 15,060Wh/day |
If you can't meet the entire day of energy needs, you can use these figures to see how many hours your solar backup system will last. For example, if you need 10,000Wh per day but only have 5,000Wh system, you can plan on having power for 12 hours (0.5 days).
Calculating How Many Solar Panels Match Your Energy Needs
Now that you understand how devices consume energy and how solar panels produce it, you can match the two to figure out how many panels your setup needs. Balance your total energy consumption with how much solar power you can realistically generate each day.
You also want to leave room for inefficiencies and less-than-perfect conditions. And if you need to get through stormy or cloudy weather with this solar-generated energy, plan on days without any sunlight by adding more watt-hour of battery storage.
Step-by-Step Guide to Sizing Solar Panels
- List Your Devices: Write down every device you plan to power, including its wattage and expected usage time.
- Calculate Total Daily Watt-Hours: Multiply each device’s wattage by the number of hours you plan to use it daily. Add up all the watt-hours to get your total daily energy need. Example: If your devices use 2,000Wh daily, that’s your target energy production.
- Factor in Peak Sunlight Hours: Divide your total daily watt-hours by the peak sunlight hours you expect in your location (typically 4-6 hours). Example: 2,000Wh ÷ 5 hours = 400W of solar panel capacity needed.
- Add a Safety Buffer: Multiply by 1.2 (or 20%) to account for weather, shading, and normal efficiency losses. Example: 400W × 1.2 = 480W of solar panel capacity recommended.
- Consider Cloudy Days: You may be able to meet your energy needs each day, but it's best to have more capacity if possible. That way you'll get through cloudy or stormy days without a hitch. The size of your battery is important here. It can be ideal to have at least 3x your daily energy needs in battery storage.
Real-World Example Calculation
Let's run through one final example to see how many solar panels you need.
If your devices use a total of 1,000Wh in one day and you get 5 hours of peak sunlight, then you need at least 200W of solar. Multiply that by 1.2 to add a 20% buffer, leaving you with 240W. This is how many solar panels you need to cover your daily energy needs.
If you want to generate excess power to get through stormy days, you need extra panels and the battery capacity to store it. That could mean as much as 3 times the daily power needs.
If your daily energy use is 1,000Wh, a battery with 3,000Wh capacity will last you three days. To fill 3,000Wh in one day from solar panels, you need ~720W of solar panels. 3,000Wh / 5 hours x 1.2 buffer = 720W.
At this point, it's time to identify your power usage and then calculate how many solar panels you need. You have the tools!
What's Next?
Now that you know how much solar power you need, it’s time to choose the right type of solar panels for your system. Should you go with rigid, portable, or flexible panels? What efficiency rating should you look for? Are there other factors to consider when shopping for solar panels?
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How to Choose Solar Panels for a Power Station: Brief Guide
- Step 1: How Many Solar Panels Do You Need: Easy Calculator (This Article)
- Step 2: Types of Solar Panels for Portable Power Station
- Step 3: Understanding Solar Panel Voltage and Current Output
Or skip over to our series on How to Connect Solar Panels.