In this segment, we're going to look at correlating electrical usage to PV requirements, so we can go from energy load to photovoltaic supply. The goals of this lesson are to define the difference between calculating for annualized energy, and the percentage-based energy planning using solar. Use a solar insolation map to identify daily and annual average solar energy levels from a given location. Calculate the number of photovoltaic panels required to serve an annual energy demand using a simplified efficiency limitation of solar PV systems, and finally perform a basic calculation defining the solar power requirements to serve an energy demand. Let's go to energy demand. In the previous video, we looked at several ways to determine energy demand of individual appliances. These included using a power meter, looking at product labels, and online resources like the Energy Star website to determine instantaneous power when in use. We then had to estimate the amount of time the appliance will be in use during the year to calculate the annual energy demand. A simpler way to determine energy demand when dealing with a grid-tied building is to use a utility bill. Remember that energy demand to use is an annual event, but it does vary day to day, and possibly month to month or seasonally. So while we could calculate daily energy use, we typically plan solar for annual demand. The only time daily variations in energy is critical, is if you're working off grid, or need to make sure that a battery or a photovoltaic system can support the maximum daily energy demand. Let's consider this example of an annual usage of 12,000 kilowatt hours. An energy demand that's a little higher than the average United States Customer. One way to look at energy demand and photovoltaic systems design is to plan on replacing 100 percent of our energy from a traditional utility with solar. So, if we use and pay for 12,000 kilowatt hours per year from our utility, then we would want to produce 12,000 kilowatt hours of electricity from the PV system during the year, so we come out even. Now that's 100 percent of our annual demand. Because of net metering, it does not mean that we have to produce 100 percent of the demand each day, just annually. So, we'll discuss net metering in a future lesson, but it's essentially a financial system that allows you to sell excess electricity to the utility when production exceeds need, but purchase that electricity when the PV system is not meeting the electrical load demand. In other cases, we might want to look at installing a system that's a fraction of the total electrical need. This is going to be dictated sometimes by cost limitations. Maybe we can't afford a system that's large enough to supplement 100 percent of our energy, or space limitations. Maybe the rooftop or the ground area is too small to accommodate a system that large. There's also oftentimes both financial and electrical limitations for producing more than 100 percent of the electrical need. You can't produce more power than your electrical system can accept, and several states limit that payback on production till no more than maybe 100 or 110 percent of your annual electric load. Otherwise, you end up being what's called an independent power producer. So, review your federal state and local rules to know your maximum allowable photovoltaic electricity production. Now that we know how to calculate annual energy demand, we're going to move forward, and look at sizing the whole photovoltaic system. Solar energy insulation or incident solar radiation varies across the globe, and varies due to differences in latitude, as well as, climate. Oftentimes these are connected though. Places that are hot and sunny tend to be in lower latitudes closer to the equator, and places that are cooler and less sunny tend to be further from the equator at higher latitudes. However, there can still be a lot of variation because of local geography. We're going to need some mapping tool to help us understand that total insulation value. Several resources are available, but at this point, we're going to look at just one simple online map that I've downloaded from the United States National Renewable Energy Lab or NREL. This map of the United States is color-coded to show what areas have high amounts of sunlight, which are red and orange, and low amounts which are green and blue. We'll come back to this map in a few moments. But first, let's take a look at how to use the map of sunlight, and begin to connect it to the photovoltaic module. So how do we size for photovoltaic power? Meaning how many modules do we need to plan for? Well, photovoltaic power output depends on the sunlight level. That means that the more power that comes out of the panel will occur if it's sunny out, and less power comes out when it's cloudy or night-time. Photovoltaic energy which is a time of use measurement combines the hours of sunlight, and the intensity. So, you could have a low intensity for a long amount of time, or have a high intensity for a short amount of time, or any variation on that. It's very much like the hair dryer and the light bulb analogy that we've previously discussed, and why we're again going to focus on energy. Photovoltaic modules are rated by power, and that value is the power output under standard test conditions. As we discussed previously that corresponds to a sunlight intensity of 1,000 watts per square meter at 25 degrees Celsius. This means is that a PV module in a sunny place with lots of daylight hours produces more annual energy than the same panel in a less sunny location. So, more sunlight power and multiplied by a specific amount of time means more solar energy, and solar electricity. Things are not as simple as just looking at the energy of sunlight though, or the panel power, because we also need to think about efficiency of the system. Photovoltaics have an efficiency of about ten to 20 percent. A concept in calculation that we reviewed in module one. That means that only 10 or 20 percent of the light that hits the panel is actually converted into usable electricity. Luckily that's already faction to the power rating on the panel. Aside from their internal efficiency of the solar panel, there are losses from things such as wiring mismatches from junctions and differences and wire sizing, the inverter which converts the direct current to alternating current, as well as, simple things like dust that falls on the panel surface, which lowers the amount of sunlight available to be converted. Another efficiency limitation is temperature mismatch. Panels are rated under test conditions; 25 degrees Celsius about 77 degrees Fahrenheit. In reality, when it's sunny outside the actual temperature is usually much higher. What's interesting about photovoltaic panels is that when the temperature goes up, the efficiency goes down. So, overall between the wiring mismatch, dust temperature losses, a module loses about 10 to 20 percent from the nameplate or panel rating itself. So the true power that's delivered by a panel is only about 80 to 85 percent of what's listed. So we need to plan for that as we size our system. So let's look at an example where a photovoltaic system produces 10,000 watts of electrical power. Because of the external efficiency limitations, only 80 percent of that power can be collected, or 8,000 watts. So we always need to oversize a system to overcome that external efficiency limit. So if we truly wanted 10,000 watts to be delivered to our home or our building, we would divide by 80 percent. We see we actually need a system that's rated at 12,500 watts, a little bit larger. So, what's the next step? Well, we need to go from the electricity load which is how much PV electricity we need to supply, to the sunlight energy that's available, and then calculate the size of the photovoltaic system. We're going to look at just the very basic approaches to this at this point, and look much more in depth in subsequent courses.