Each solar panel, from the industrial-sized units used in power plants to the ones that fit in your pocket, has a wattage associated with it. The wattage indicates the amount of power it can generate. Likewise, each appliance has a particular wattage, or amount of power it needs to work. Using these figures, and accounting for factors like intermittent sun cover and inefficiency, you can calculate what size solar panel you'll need to run a given appliance.
A watt is a unit of electrical power use, equal to one joule per second. Appliances vary greatly in the wattage they need. A small LED lamp might need only 7 watts, while a larger one might need 60 watts. A small hand-held blender needs around 200 watts, but a larger food processor might need around 450 watts. On the higher end of the scale, even single-burner hotplates can use 1200 or more watts. Check the documentation on your appliance to see how many watts it needs to run.
Solar panels put out power in direct current (DC). To power an alternating current (AC) appliance, you need to use a device called an inverter. Unfortunately, inverters are never 100 percent efficient as some energy is lost in the conversion from DC to AC. Modern inverters have efficiencies of around 85 percent to 95 percent, so you should choose a panel with a wattage at least 15 percent higher than your appliance needs to cover this loss.
Panel Size and Power Storage
If an appliance does not need to be running at all times, you don't necessarily need to match the panel's output to the appliance's requirements. Instead, you can set up your solar panel to charge a battery. For example, imagine your solar set-up generates 30 watts after accounting for inverter efficiency. If you leave it charging a battery for two hours, you could run a 60-watt appliance for one hour. Likewise, if your solar panels puts out 240 watts, that one-hour charging time would run a 60-watt appliance for four hours; 15 minutes of charging would run it for an hour.
Sub-Optimal Power Output
Wattages listed on solar panels assume optimal conditions, but the sun doesn't shine at all times. Cloud cover, time of day and the angle of the sun's rays in relation to the panel all affect output. These factors are hard to quantify because they vary by location, but unless you live in a desert, you'll probably have to increase either the wattage of the panels or the amount of time you leave your battery charging to run your appliance. This is especially relevant if the panel will power an appliance that must run continuously, like a small refrigerator.
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