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Considering installing a residential solar power system to save money on energy bills and reduce dependence on the utility grid?
Calculating how much electricity you need to generate to meet your goals is an essential first step.
Our 4-step guide and free solar panel calculator are all you need to estimate how many solar panels you need at your location and what type of system is best to meet your goals: grid-tied, off-grid, or hybrid.
It requires some data and a few calculations on your part, but we’ve kept it as simple as possible.
Ready to get started?
Let’s dive in.
How Do I Calculate How Much Solar Power I Need?
Calculating how much solar power you need, what type of photovoltaic modules you should purchase, and how many solar panels you require to meet your electricity generation goals at your location is a multi-step process.
But that doesn’t mean it has to be painful.
Once you determine the unique answers to the questions below, you can plug the numbers directly into our free solar panel calculator at the bottom of the page.
Let’s take a look at the process step-by-step.
1. Determine Your Monthly Energy Consumption (kWh)
First, a crucial distinction.
Power in watts/kilowatts (W/kW) is how much electricity your appliances require to start and run.
Energy is how much power your appliances and home systems consume over time in watt-hours/kilowatt-hours (Wh/kWh).
For example, a 150W television running for 1 hour consumes 150Wh of electricity.
You’re likely familiar with kilowatt-hours because your energy bills charge you for electricity consumption in cents per kWh (¢/kWh).
The average ¢/kWh rate in the US is currently about 0.19/kWh, though it varies significantly from state to state.
Calculating your average monthly energy consumption helps you evaluate how much money you can save on electricity bills with solar panels and estimate your return on investment.
Fortunately, estimating monthly electricity consumption is easy as long as you’ve lived in the same home for at least a year.
Collect your previous 12 monthly electricity bills and total your consumption in kWh.
Divide the total in kilowatt-hours by 12.
The resulting number is your monthly average consumption in kWh.
For reference, the average US household consumes:
- 10,566kWh annually
- 880kWh monthly
- 29kWh daily
Your home may consume significantly more or less.
If you live in a location with significant seasonality — like frigid winters and sweltering summers — pay close attention to monthly variations in your bills.
For example, if you use air conditioning in the hot summer months but use gas to heat your home, your electricity consumption is likely substantially higher during June, July, and August than during December, January, and February.
If seasonality is a significant factor in how much electricity you consume, it’s essential to factor it in if you’re considering reducing electricity consumption by 75% or going entirely off-grid.
Ensure that your solar + storage system size is sufficient to supply enough energy during peak demand months, not just your monthly average.
If you’re considering a fully off-grid or hybrid solar system to meet all your household electricity needs, the process for sizing your solar panel array, solar inverter output, and solar battery storage is significantly more complex than the process outlined here.
Learn how to calculate off-grid solar power system size.
The method we’ll explore is better suited to grid-tied or hybrid systems that supplement utility power or provide whole-home backup during power outages.
If you’re looking for an extremely rough estimate, skip averaging your monthly bills and enter the kWh consumption from a recent energy bill — ideally one with the highest consumption from the year…
2. Average Peak Sun Hours per Day at Your Location
All solar energy systems that generate electricity from sunlight use the photovoltaic effect.
PV modules like solar panels utilize photovoltaic cells that capture photons from visible light to produce direct current (DC) electricity.
Depending on your balance of system type (grid-tied, off-grid, hybrid), DC is converted to AC (household) electricity for immediate use or transmission to the utility grid.
In off-grid and hybrid systems, DC power passes through a charge controller, is stored in a solar battery, and is converted to AC by an inverter for on-demand household use.
No matter what type of solar panels or system you choose, peak sunlight hours at your location largely determine how much electricity it can generate.
Fortunately, the National Renewable Energy Laboratory offers a free tool — the PVWatts Calculator — that can estimate peak sun hours at your address using historical solar irradiation data and other considerations such as local weather patterns.
Rated power in watts is a standard spec for PV modules like solar panels, and it’s tested using peak sunlight.
For example, a 400W rigid solar panel outputs a maximum of 400W power during ideal daytime conditions reproduced in a laboratory using Standard Test Conditions.
Standard Test Conditions for Solar Panels
| Condition Type | Standard Test Condition | Real-World Conditions |
| Solar Incident Angle | Always zero, irradiation beam always normal to the PV panel* | Variable, and depends on time, date, and site latitude. In the case of rooftop systems, roof orientation, and inclination govern system capacity. |
| Solar Irradiation | Always equal to 1000 Watts/m²* | Variable and depends on the time, date, and site latitude. Limited sunshine hours bound system capacity. |
| Ambient Temperature | Always 25°C* | Variable and depends on the time, date, weather condition, and site latitude. Higher ambient temperature degrades PV panel efficiency and reduces system output. |
| Air Mass Coefficient (AM) | Always equal to 1.5* | Variable and depends on the time, date, and site latitude. Higher AM with higher latitudes. |
| System Losses (e.g., Wiring, Inverter) | Always Zero* | Variable and depends on the design and location of PV panels, inverter, and grid meter. |
*Cannot be achieved in real-world operation (Source: ResearchGate)
PV modules don’t generate their full rated power during every hour of daylight.
As you can see in the table above, solar panel specs are determined using solar irradiation of 1000W per square meter of photovoltaic material — the rough equivalent of direct sunlight at the brightest time of day.
PV modules continue to generate electricity any time there’s visible light, but production is significantly lower in the early morning, early evening, and on cloudy days.
Historical peak sun hours at a specific location are the industry standard for determining how much electricity a solar panel array can be relied on to generate on an average day, month, or year.
Learn more about calculating solar panel output.
Once you’ve determined peak sun hours at your location, move on to the next step.
3. Electricity Bill Offset Goals
For many homeowners, going off-grid with solar power is a realistic goal.
However, it’s far from the only — or even the best — way to save money on energy bills.
Grid-connected and hybrid solar power systems offer the convenience of utility power while removing the burden of relying entirely on solar panels to meet all your household electricity needs.
One crucial consideration to keep in mind…
Grid-tied solar power systems don’t work during a blackout.
A hybrid solar + storage solution like EcoFlow DELTA Pro 3 can provide whole-home backup during an outage and save you substantial money on electricity bills over time.
For this step of the calculation, decide what percentage of your monthly household electricity consumption you’d like to offset with solar.
For example, if you want to cut your bill in half, your Offset Goal is 50%.
Feel free to plug in different numbers.
Your Electricity Bill Offset helps determine the size, type, and cost of the solar power system you need to achieve your goals.
Once you price out a system, you can estimate your solar payback period — the time it takes before it pays for itself through savings on electricity bills.
The average solar payback period in the US is between 5-12 years…
High-quality solar panels last 25-30 years before needing to be replaced, giving you ample time to recoup your initial investment.
When determining your upfront costs, don’t forget to factor in government incentives.
For example, the 30% Federal Solar Tax Credit can save US taxpayers 30% of the total purchase and installation costs of an eligible solar + storage system, like EcoFlow DELTA Pro Ultra + solar panels.
Once you’ve completed the subsequent steps, play around with different Offset Percentages to get a better idea of the goals you can reasonably achieve.
4. System Efficiency (Derate Factor)
It’s a scientific impossibility for any electrical system to achieve 100% efficiency — and residential photovoltaic systems are no different.
Solar professionals use a Derate Factor to estimate power losses due to system inefficiencies, including:
- Solar inverter and other balance of system component inefficiencies
- Wiring losses (tends to increase over long cable runs)
- Impact of temperature on solar panel efficiency
- PV module degradation (typically less than 1% efficiency loss per year)
- Dirt and partial shade obstruction of PV modules
The actual Derate Factor of any system depends on numerous factors, not least the quality of manufacturing.
For our ballpark estimate, let’s use a System Efficiency rate of 80% (0.8).
That’s likely on the high side for high-quality systems, but it’s better than not accounting for power losses at all.
Now, input your data from steps 1 – 4 and estimate the total PV generation potential and number of solar panels you need to meet your electricity offset goals.
Plug in the rated power of the PV module type you’re considering.
Most residential rooftop arrays use multiple identical solar panels with a rated power output of 300W – 450W.
Press the blue button for the minimum combined output wattage of your solar panel array and the number of modules needed to meet your electricity offset goals.
There you have it!
Final Thoughts
Installing a residential solar power system is a smart investment for most homeowners looking to save money on electricity bills and reduce dependence on aging infrastructure and the utility grid.
EcoFlow’s DELTA Series Whole Home Generators provide multiple charging options, including grid and solar power, providing convenience, flexibility, and home energy security during power outages.
Compared to grid-tied and off-grid solar systems, hybrid solutions offer more options for achieving your electricity offset and home backup goals.
EcoFlow has a wide variety of portable and mountable solar panels and off-grid and hybrid solar power systems to meet your needs — from road trips and camping to powering your whole home.
Check out our selection today.