If a solar panel is rated at 300W, how much power will it actually produce?

  • If the Solar Panel is rated 300 watts. Does it mean it will generate 300 watts per hour all day long while the sun is shining?

There are several factors that determine the amount of electricity a solar panel will produce.  

Solar panels don't generate the same amount of electricity  throughout the entire day - in the mornings and evenings (when the sun is low in the sky) they will generate less power than in the afternoon (when the sun is shining on them directly). This is why you'll sometimes hear 'watts peak' (Wp) used instead of watts when someone is talking about a solar panel's capacity. 300 watts (300W or 300Wp) refers to the panel's peak capacity.   Besides time of day, there are other factors that affect solar panels electrical production. 

All of these factors combine to determine the amount of direct sun on the solar panel. These include angle to the sun, weather, shade from trees/buildings, dust/dirt/grease on the panel, line loss (the amount of electricity the electrical wire "absorbs" when transmitting the energy),  etc.   When solar panels are tested and rated, they use optimal "in lab" testing which can never really be duplicated in the real world.  Practical experience has shown that in optimal conditions, a  high quality 300 watt solar panel will produce about 285 watts of electricity per hour.  (in other words 90%-95% of the rated power.)  Poor quality solar panels... will produce much less, sometimes only half of their rated power. 

AS A GENERAL RULE OF THUMB, IF YOU AVERAGE THE DIRECT SUN ON A SOLAR PANEL FOR A YEAR (SUMMER, FALL, WINTER, SPRING)... IT IS ABOUT  6 HOURS IN THE UNITED STATES.  Of course it is much higher during the summer (8-10 hrs) and less during the winter (3-5 hours).  This can also vary according to Latitude (how far north or south), and the weather for the area. (Lot's more sun in Phoenix than in Seattle because of weather and latitude.) 

Besides, Latitude, weather, shade trees, shade from buildings, dust/dirt, the angle of the solar panel in relation to the sun/horizon is also important.  For best conditions, during the summer the angle should be around 25°  when the sun is almost directly overhead. And during the winter around 56° with the sun is low on the horizon. If you cannot adjust the angle of the panels for the seasons, then the best compromise is usually around 40º...or whatever angle your roof is to the southern horizon.   (For example in Hawaii, because space is limited, you will see solar panels mounted on the north, east, west  parts of the roof besides the south.)

Understand that there is also a difference between watts and watt hours.  (or kilowatts and kilowatt hours).  However, for general purposes, a device that produces 300 watts... will produce 300 watts per hour.  Or a device that uses 300 watts... will use 300 watts per hour.  (Note: a kilowatt is 1000 watts. a megawatt is 1,000,000 watts)

 ​For example, a 4,000 watt (or 4 kilowatt, 4kW) vacuum cleaner uses 4kW of electricity when you have it turned on. If you run it for an hour, you will have used 4,000 watt-hours (or 4 kilowatt-hours, 4kWh) by the end of that hour. Similarly, if a 300W (0.3kW) solar panel generates power in full sunshine for an hour, by the end of that hour it will have generated 300 watt-hours (or 0.3 kilowatt-hours, 0.3kWh) of electricity.


Power Management (How much "stuff" will your solar system run?)

The big difference between grid power and solar power... is that unless you have a massive solar system, with solar you have a limited supply of electricity whereas with grid you basically have an unlimited supply of electricity.  
Most people try to match their solar system with their electrical needs.  They try to get the smallest (and least expensive)  system they can get to run the most equipment they can.  However, experience has shown that most people underestimate the amount of power requirements they have.  They guess on how much energy their well pump takes.  They guess how much energy their 20 year old refrigerator/freezer uses.  They guess how much energy their air conditioner uses.  

WE RECOMMEND USING A "KILL-A-WATT" METER.  One good way to actually know how much energy your appliance is really using, is by using a Kill-A-Watt meter. (approx. $20-$29 online, or at Lowes, Harbor Freight, Home Depot). Basically, you plug your appliance into this meter, and then plug the meter into the wall... and it will tell you how much energy (in watts or amps) it is actually using.  Importantly, you can see how much energy an appliance takes on the initial start or "surge" of the appliance.   This meter can then help you determine what your "actual"  power usage is.  



Our goal is to have your unit delivered to you safely and securely so that you will be able to use it immediately. The following information will help us achieve that goal:

Most of our solar units are shipped via large commercial freight trucks because of their size and weight. The units are shipped in heavy duty, reinforced, sealed shipping crates , including solar panels. 

These trucks have tailgate lifts where they can lower the unit to the ground so they can be unboxed and then moved. All units are shipped with insurance. If there are any questions, please do not hesitate to call customer service. 208-745-7100.

1. DELIVERY LOCATION: Customers, when making an order, should make sure they include any special shipping problems that might be encountered. (such as delivering to an apartment or a location where a large truck cannot make a delivery. Customer Service can help in this process. Please do not hesitate to call them.

2. For SPECIAL ORDERS, and units shipped out of the Continental US, a shipping quote will need to be obtained. It takes about 1-3 days for the shipping companies to give us a quote on unique shipping items.

3. WILL CALL. Units can be ordered and picked up by the Customer in Rigby, Idaho. Please call ahead to schedule a pickup when your order is ready.

4. UNITS DAMAGED DURING SHIPPING. Unfortunately, we have had units that have been dropped, crushed, and have had forklifts run through them. The good news is that this doesn’t happen very often. The bad news is that it does happen occasionally. Therefore, WHEN YOU RECEIVE YOUR UNIT… CHECK IT OVER FOR ANY DAMAGE THAT MIGHT HAVE HAPPENED DURING SHIPPING! If there is damage…DO NOT SIGN ANYTHING, until you call us and talk to customer service.

Take lots of pictures. Outside, Inside, etc. We will then instruct you to either receive the shipment…wherein we will work with you in repairing or replacing the damaged part, send a technician out to fix it, OR, have the shipper send it back (refuse shipment) , and/or send out a new unit. Please note that if a unit is damaged, the insurance will pay for it to be returned. If you refuse a shipment and there is no damage, then you will have to pay for it’s return.