Solar Panel Formula
Approximate Number of Solar Panels* = (Hourly Energy Needs X Peak Sunlight Hrs) / Watts per Panel
*This formula is intended to roughly predict the number of solar panels needed to cover approximately 100% of your predicted energy consumption. If you want to over-produce (sell back to the power grid or store in a battery for lower light times) or under-produce (use a mix of solar energy and power from the grid), adjust this number accordingly.
The Variables
- Hourly Energy Needs : Average hourly household energy (past) usage plus estimated adjustments for any expected changes (see section below)
- Peak Sunlight Hours : Look up the average number of peak/direct sunlight hours for your geographic area as a baseline. Then, increase and/or decrease this by the percentage of blockage (by shade, buildings, mountains, etc) of the area where your solar panels will be mounted.
- Watts per Panel : This is determined by the type and size of solar panel you install. See rough wattage numbers for each solar panel type under “Typical Wattage”.
Now, let's examine this formula step-by-step to determine how many solar panels your home needs.
Estimate Your Energy Needs
Calculate Your Past Household Power Usage
Knowing how much power your household consumes on a daily basis can help you determine how much electricity you want to generate with your solar panels.
Take a look at the past few years of power bills. The invoice will tell you how many kilowatt hours (kWh) you consumed each month. Account for seasonal changes by combining your power consumption for a whole year. Then divide that by 365 days in a year to determine the average power your home consumes per day, and divide that by 24 to get your average hourly consumption. This should give you an accurate estimate of your current hourly energy consumption. However, depending on your situation, there may be additional factors to consider when calculating your future power needs (detailed below).
Other Power Usage Considerations & Changes
Beyond your home's average power usage in the past, you should also try to predict your power usage in the future. This may include more efficient appliances or the addition of new equipment added to your home. If any of these changes are planned for the future or happened recently, they will not yet be fully reflected in your past power usage and will need to be added to get a more accurate estimate.
- Changes in the number of residents and their power-usage routines
- New power-consuming purchases, like an electric vehicle that will charge at home.
- Power usage by new major appliances (fridge, freezer, heater, air conditioner, etc)
- Whether your furnace, cooking, or water heater are electric or natural gas
- Possible changes in lifestyle or routine
- Amount of time in your home office
- Installation of any features that will draw significant power, such as a heated pool, hot tub, or sauna
Each of these changes to your household can also change the amount of power your home will need in the near and distant future. Since usage varies by appliance, brand, frequency, etc., if it’s important for all your power consumption to come from solar, you’ll want to calculate using the higher end of an estimated range.
Typical Wattage Produced by Solar Panels
Most solar panels can produce between 250 and 400 watts or 1.5 kWh (kilowatt-hours) per day. However, different types of panels have different solar production values.
Thin Film Panels: under 200 W
- Description: Made from cadmium telluride, amorphous silicon, or copper indium gallium.
- Pros: More affordable than other panels
- Cons: The least efficient power generators and not as durable.
Polycrystalline Panels: 250 - 300W
- Description: Silicon fragments melted and poured into a square mold
- Pros: An affordable option with medium power generation and minimum silicon wastage when manufactured.
- Cons: Lower heat tolerance than microcrystalline panels.
Microcrystalline Panels: 320 - 380 W
- Description: Single-crystal panels made from pure silicon crystal cut into several wafers.
- Pros: Space-efficient and long-lasting.
- Cons: High price tag and some amount of silicon wastage in manufacturing
It’s important to keep in mind that these numbers represent an average or typical production per solar panel. However, actual production depends on a number of other factors, such as those outlined in this article.
Solar Panel Placement
Solar panels work best where they can catch the maximum amount of direct sunlight each day. In North America, this occurs at a 15 to 45-degree angle facing south. North-south angled roofs are the best for solar with panels mounted on the south-facing side. Custom mounting racks can help you achieve the best angle to catch some sun.
Mountains, tall trees, and nearby tall buildings can occlude the sun by creating shadows. Your region will determine how much sun you get at different times of year, considering latitude, geography, and cloud cover. The size of your roof can also allow you to plan for a larger number of panels or a smaller number of high-efficiency panels to produce the right amount of electricity.
The Amount of Solar Energy to Produce
Once you have calculated how much energy you are likely to need, you still must decide how much energy you want to generate. A zero-energy home could ideally produce (and store) enough power to go off the grid.
Going off-grid also requires batteries and a constant over-production of power to account for nighttime hours and cloudy days. If this is your goal, you'll want to drastically increase your solar production plans and installation budget.
Some homes simply offset their energy consumption with solar, reducing power bills while increasing sustainable energy use. You may be looking to produce 80% of your home's power needs with a solar array that could be built later to increase that percentage. Other homeowners choose to generate more than 100% of their power needs, ‘saving’ the extra for a cloudy day, or the winter season, or selling it back to the power company at a profit. Even if you don't plan to purchase batteries and go “off-grid”, there are
advantages to producing more solar power than you need and tapping into that energy later.
Save Time with a Free Solar Calculator
How many solar panels does your home need? It depends on how much power you want to produce. While there is a
formula for calculating your estimated solar power needs on your own, this can quickly become time-consuming and overwhelming. The great news? You don’t have to! The best place to get started is to make use of a free solar estimator. By filling out
a single form on asksolar.com, you can receive free estimates from multiple solar installers to help you understand your options and ensure you're getting a competitive rate.
Try our free solar calculator to begin your journey to residential solar! Or
contact us to learn more!