How Many Watts Of Solar Does My House Need?
Determining the appropriate solar system size for your home is an important first step when going solar. Properly sizing your system will ensure it meets your home’s energy needs and provides maximum return on investment. Undersizing can lead to continued reliance on utility power, while oversizing wastes money on excess capacity. Calculating your solar wattage requirements takes some initial legwork, but ensures your system is just right for your home’s energy appetite and budget.
This guide will walk through the key factors in identifying how many solar watts your house needs. We’ll look at calculating current and future energy usage, equipment selection, site considerations, storage options, and financial incentives. With some basic information about your home, utility bills, and location, you’ll be able to determine the ideal solar system size to maximize savings and get the most out of your solar investment.
Calculate Energy Usage
The first step is to check your average monthly energy bills over the past year to determine your household’s current electricity usage. Look at your kWh usage on your utility bills. Add up the kWh used over the year and divide by 12 to calculate your average monthly usage.
It’s also recommended to do an energy audit to analyze exactly how much electricity your household uses. An audit will account for high energy appliances like air conditioners, electric vehicles, pools etc. An energy professional can conduct an on-site audit and provide a detailed breakdown of your home’s energy consumption. There are also do-it-yourself audit checklists you can follow.
Once you know your average monthly kWh usage, you can determine the solar array size needed to offset that demand.
Factor In Future Energy Needs
When calculating how many solar panels you need, it’s important to factor in future energy needs, not just current usage. Over the 25+ year lifetime of a solar system, your energy consumption will likely increase as you add new appliances and equipment.
Some key things to consider for the future:
Electric vehicles (EVs) – Charging an EV can add 8-12 kWh per day in energy usage. If you plan to own an EV in the next 5-10 years, account for this added electricity demand (according to Do-It-Yourself Home Energy Assessments – Energy.gov).
New appliances and devices – Consider upgrading appliances to more energy efficient models. However, even efficient appliances like refrigerators and AC units will increase your overall energy use (reference How to Calculate Future Energy Consumption – YouTube).
Climate change – Depending on your region, rising temperatures may result in greater cooling needs in the summer. Factor in a 10-20% increase in future AC usage.
Home additions – Finished basements, in-law suites, swimming pools, and other additions increase the square footage to cool and power. Account for any future home expansions.
Over the system’s life, it’s reasonable to estimate a 30-50% growth in your home’s energy demand. When sizing your solar system, add this anticipated increased usage to ensure your system will sufficiently meet your household’s needs for decades to come.
Determine Solar Panel Wattage
When determining the wattage needed for your solar panel system, you’ll need to consider the power output per square foot of your solar panels as well as the efficiency rating. Most solar panels today have a power output ranging from 15 to 21 watts per square foot, with high efficiency monocrystalline panels towards the upper end of that range at around 20 watts per square foot.
Panel efficiency is another key factor, as it measures how much of the absorbed sunlight is converted into usable electricity. Standard panels tend to have 15-20% efficiency. Premium monocrystalline panels can reach up to 22% efficiency, generating more wattage from a smaller surface area.
To calculate the wattage, you can multiply the panel size in square feet by the watts per square foot rating. For example, a 300 square foot array of standard panels at 18 watts per square foot would provide 300 x 18 = 5,400 watts or 5.4 kilowatts. Going with more efficient 22% panels at 20 watts per square foot would require only 5.4/20 = 270 square feet for the same output.
Keep in mind that panel output degrades slightly over time. Most panels lose around 0.5-0.8% of their power output per year. This factor along with your energy needs and available space will determine how much solar panel wattage your system requires.
Calculate Total System Size
To determine the total wattage needed for your solar system, you’ll need to add up the wattage requirements for all the devices and appliances you plan to power with solar. This includes essentials like lights, refrigerator, furnace, etc. as well as any luxury items you may want like a pool pump or AC unit. Go through your usage and create a list of all devices and required wattage.
It’s important to oversize your system slightly to account for inefficiencies in the system. Things like shading, high temperatures, dusty panels, aging equipment and angle of the sun throughout the day can all degrade performance. Experts generally recommend oversizing your system by about 25% to ensure it can meet your household’s needs year-round.
Additionally, oversizing allows your system to better handle future energy needs if you add new appliances or electric vehicles. It’s recommended you estimate your energy usage needs 5-10 years out when sizing your system. This way you won’t have to expand as soon. Oversizing upfront does cost more, but it will save you money in the long run.
Choose Equipment
Selecting the right equipment is critical for getting the most out of your solar system. The main components to evaluate are solar panels, inverters, racking and mounting hardware.
Solar panels convert sunlight into electricity. Monocrystalline and polycrystalline panels are the most common. Factors like efficiency, durability, size and cost should be considered when selecting panels [1]. Higher efficiency panels produce more electricity but often cost more per watt. Panels rated to withstand high wind and snow loads may be preferable in certain climates.
Inverters convert the DC electricity from solar panels into usable AC electricity for your home. central inverter systems are common for residential setups. When choosing an inverter, consider the power rating, warranties, monitoring capabilities and price [2]. Oversize the inverter by 10-25% to allow for future expansion.
Racking, mounts and hardware securely fasten the solar panels and electrical components. Rooftop systems use roof mounts while ground mounts use poles anchored in concrete. The mounting method must comply with local building codes and wind/snow requirements [3].
Consulting with solar professionals can help select the optimal solar components for your unique situation and goals.
Understand Site Factors
Before determining solar panel wattage for your home, you need to thoroughly evaluate your roof and site to understand if it can support a solar PV system. Some key factors to consider:
Roof size – Your roof needs to have adequate square footage to accommodate the number of solar panels needed. Panels vary in size, but on average you need about 100 square feet per kW generated. Make sure to account for spacing requirements between panels. Also factor in any roof obstructions like vents or chimneys that will reduce usable space. A roof inspection can determine exactly how much usable area you have. (Source)
Orientation – The optimal orientation for solar panels is due south. Panels facing southwest to southeast can still be effective. Try to avoid north-facing roofs if possible. Use a compass and solar assessment tools to determine your roof’s exact orientation. (Source)
Shading – Solar panels perform best in full, direct sunlight. Look for shading from nearby trees, buildings, chimneys, etc throughout the day. If more than 10% of the system is shaded, it can dramatically reduce output. Use a solar shading tool to identify shading impacts. (Source)
Spacing/positioning – Panels need to be spaced apart to allow air flow and maintenance access. Leave at least 3 feet between array edges and roof edges. Follow local fire codes for spacing around chimneys and vents. Consult your solar installer on optimal panel positioning.
Storage Capacity
When sizing your solar system, it’s important to consider your battery storage needs, especially if you want a backup power source or to go completely off-grid. There are different battery options to choose from including lead-acid, lithium-ion, and saltwater batteries. Lead-acid batteries are the most affordable option but require more maintenance. Lithium-ion batteries are lighter, longer-lasting, and more efficient but also more expensive upfront. Saltwater batteries are an emerging technology that may provide greater capacity in the future.
To determine the right battery size, calculate your average daily energy usage in kilowatt-hours (kWh) and multiply by the number of days you want to be able to go without sunlight as a backup. For off-grid systems, size your battery bank to at least 3-5 days of backup capacity. Make sure your solar charge controller and inverter are also sized appropriately to handle the full charging and discharging loads. It’s recommended to oversize your battery bank by 25% to account for efficiency losses over time.
There are solar battery bank sizing calculators available online that can help determine your specific storage needs based on factors like solar array size, load demand, days of autonomy, etc. Properly sizing your battery bank will ensure your solar system can provide reliable backup power or go off-grid when needed. Consult with a qualified solar installer for the most accurate storage system recommendations.
Financial Incentives
Saving money is often the top argument for installing solar panels on your home. Between federal tax credits, state rebates, net metering, and more, there are many financial incentives for home solar panels. Let’s look at the key incentives that can help offset the cost.
Federal Solar Tax Credit – The federal government currently offers a 26% tax credit for installing solar panels on your home. This tax credit applies to the cost of both equipment and installation. For example, if you install a $20,000 solar system, you’d receive a $5,200 tax credit. This incentive alone typically covers a significant chunk of the overall cost.
State/Local Rebates and Incentives – Depending on where you live, your state or utility company may offer additional rebates and incentives. For example, some states give cash rebates per installed watt of solar capacity. Other local programs provide low-interest loans or property tax exemptions. Check with your state energy office to see what solar incentives may be available.
Net Metering – Most utility companies offer a billing mechanism called net metering, which allows you to get credit for excess solar energy fed back to the grid. When your solar panels produce more power than your home is using, the extra electricity goes back into the grid and spins your meter backwards. This effectively reduces your electric bill, since you only pay for your net usage.
Conclusion
In summary, the key steps for properly calculating your solar system needs are determining your current and future energy usage, calculating the required solar panel wattage based on available sunlight and space, selecting the right equipment for your site, and understanding financial incentives. Taking the time to accurately size your solar system will ensure you get maximum return on investment and energy savings.
Installing a properly sized solar system provides many benefits. You’ll generate enough clean energy to offset your utility bills without overproducing unneeded surplus energy. Your system will seamlessly provide power even during grid outages. Right-sized solar panels will efficiently use all available roof space. And getting solar tax credits and incentives depends on system size calculations. Taking a methodical approach helps ensure your solar system investment is cost-effective.
