Start with a daily energy list
Before you look at a single panel, you need to know how much energy your system must supply each day. Virginia Cooperative Extension guidance describes this as a load list: write down every appliance, light and device you plan to run, note its wattage and estimate how many hours per day it will be on. Multiply watts by hours for each item to get watt-hours (Wh), then add them together. The total is your daily Wh target. This number drives every other sizing decision, so take time to measure or check nameplate ratings rather than guessing.
Find the solar resource at your location
After you have a daily Wh figure, you need to know how much sunlight is available to meet it. The National Laboratory of the Rockies provides the PVWatts Calculator, which estimates the energy production of grid-connected photovoltaic systems throughout the world; its expected output range is based on 30 years of weather data at the chosen location, and results can be viewed monthly. The calculator’s documentation cautions that its predictions include many inherent assumptions and uncertainties and do not reflect variations between PV technologies or site-specific characteristics beyond the inputs you provide.
The U.S. Energy Information Administration explains that insolation, the amount of solar radiation received on a given surface area during a specific time, varies by latitude, climate and weather patterns. Locations in lower latitudes and arid climates generally receive higher amounts of insolation. Seasonal variations in solar resources increase with increasing distance from the equator. Clouds, dust, pollution and shading from buildings, trees or mountains all affect the insolation that actually reaches a panel. When you use PVWatts, treat the result as a planning estimate, not a precise prediction.
Account for real-world losses
A panel’s rating is not what you will get on a roof or ground mount. The U.S. Department of Energy describes how factors such as temperature, reflection and recombination limit a photovoltaic cell’s conversion efficiency. Higher temperatures cause a much larger decrease in voltage than the slight increase in current, so a hot panel produces less power than its nameplate suggests. The Department of Energy also notes that untreated silicon reflects more than 30% of incident light; anti-reflection coatings help, but some loss always remains.
The Hong Kong Electrical and Mechanical Services Department states that inverter conversion efficiencies are generally in the range of 85% to 95%. That means 5% to 15% of the DC power from the array is lost before it reaches your AC loads or battery bank.
Cable losses add another reduction. Victron Energy’s Wiring Unlimited book explains that cable resistance creates a voltage drop, turning energy into heat. The book recommends aiming for a voltage drop no larger than 2.5% and notes that resistance also comes from fuses, shunts, switches, circuit breakers, terminals and connections. Loose or corroded contacts and poor crimps increase the total resistance further.
Combining these effects gives a loss factor: a planning multiplier you choose, not a measured value, and you should refine it to match your components and layout.
Use the planning formula to size the array
With your daily Wh, local sun hours and a loss factor, you can estimate the array wattage you need. The formula is:
Array watts = daily watt-hours / (equivalent sun hours × loss factor)
As a simple worked example, suppose your load list totals 1,200 Wh per day, you estimate 4 equivalent sun hours for your location in the design month, and you assume a loss factor of 0.7. The calculation is 1,200 / (4 × 0.7) = 429 W. You would plan for roughly 430 W of solar panels. This is a planning estimate; real output will vary with weather, season and component performance.
For a deeper look at the formulas behind this step, see the solar sizing formulas article. If you are still mapping out your loads, the energy needs page walks through the load-list process in detail.
Check the charge controller limits
Once you have a target array wattage, you must confirm that your charge controller can handle the voltage and current the array will produce. Victron Energy’s Wiring Unlimited book advises that the array’s open-circuit voltage (Voc) must not exceed the voltage rating of the MPPT controller. The book gives an example: a 12 V nominal panel may have a Voc around 22 V, so a controller rated for 75 V could accept up to three such panels in series. The same source notes that the controller’s current rating refers to the current going into the battery, and that a 12 V battery bank will receive less power from the same controller than a 24 V bank would.
Always check the specifications of your specific charge controller and panels. The charge controllers section explains how MPPT and PWM controllers differ and what ratings to look for.
How the key inputs affect array size
The table below summarizes the main inputs, where to find them and how they influence the final panel count.
| Input | Where to get it | Effect on array size |
|---|---|---|
| Daily watt-hours | Load list (nameplate ratings and hours of use) | Higher consumption needs more panels |
| Equivalent sun hours | PVWatts or local solar resource data | Fewer sun hours needs more panels |
| Loss factor | Inverter specs, cable layout, temperature expectations | More losses need more panels |
| Charge controller voltage limit | Controller manual | Limits how many panels can be wired in series |
| Charge controller current limit | Controller manual | Limits total array power at a given battery voltage |
Putting the steps in order
Sizing solar panels is a sequence, not a single calculation. Start with a load list, then find local sun hours, pick a realistic loss factor, run the planning formula and finally verify that your charge controller can accept the array you have designed. This approach keeps you from buying panels that are too small to meet your needs or too large for your controller. The solar panel sizing category page collects more resources on this topic, and the broader DIY solar planning section covers system design from start to finish. If you have not yet mapped your whole system on paper, the how to plan a DIY solar system article provides a step-by-step framework.
Connecting an array to home wiring, a grid-tied inverter or a main electrical panel requires a licensed electrician, permits and approval from the local utility. The planning steps above help you estimate equipment needs, but the final installation must follow the electrical code and the manuals for every component in your system.
Frequently asked questions
Why does my panel produce less than its label says?
The U.S. Department of Energy explains that temperature, reflection and recombination all reduce a cell’s output below its rated efficiency. Heat in particular causes a much larger decrease in voltage than the slight increase in current, which lowers total power.
Can I just add more panels if I need more power later?
You can, but you must check whether your charge controller can handle the higher voltage and current. Victron Energy’s Wiring Unlimited book stresses that the array’s open-circuit voltage must stay below the controller’s maximum voltage rating. Exceeding that limit can damage the controller. Also, adding panels changes the cable current, so the existing wiring may need to be upgraded.
What is the biggest mistake beginners make when sizing panels?
Many beginners skip the load list and buy panels based on a rough guess or a neighbor’s system. Without a daily Wh figure, you cannot match the array to your actual consumption. Another common error is ignoring losses from the inverter, cables and heat, which leads to an array that is too small for real-world conditions.
For worksheets and step-by-step planning tools, visit the solar panel sizing category page.
