After completing your site survey and tallying the home's annual kilowatt hour usage, you'll be ready to size your solar electric system. If you're contracting with a solar company or contractor, you can discuss and choose a system that works best for your circumstances and budget.
In solar power parlance, system sizing refers to the calculations that determine the correct size of the photovoltaic array, inverter and other components that will make up the electrical circuit. You'll be measuring or estimating power, voltage, current, area dimensions and a few other variables. You'll also have to compile order quantities, wire gauges and ampacity ratings. None of this is difficult (except maybe the wire sizing). But there's a lot of number crunching to do. Steps 5-9 of this tutorial help you to perform these calculations.
(Note: TheSolarPlanner.com provides free sizing worksheets that you can download here.)
Initial System Sizing and Cost Estimate
System sizing starts with determining the total array power output (in watts) needed to power the appliances, lights and other electrical loads used in your home. Many utility companies stipulate that the maximum solar electric power capacity they'll allow in a home must not exceed the kilowatts consumed by the household in the previous year. (That way they don't have to pay you a profit for excess production.) Consequently, the system size is based in part on annual consumption. So two pieces of data are used to estimate an annual power output for your PV array:
The first number was deduced during your energy audit back in Step 3 of this guide. The second number can be either a general insolation figure for your local area (see map below), or a more carefully calculated figure using a Solar Pathfinder, Solmetric Suneye, or other data gathering devices during the site survey.
Solar installation companies rely on a Pathfinder or Suneye report because it takes into account potential shading around your house and the azimuth/tilt angles of your proposed array. (Also, the report is required by most state rebate programs.) In our photovoltaic tutorial, we examined how this is done and offered suggestions to help you crunch the shading and sun hour numbers manually. For a refresher, see Optimum Array Orientation and Placement .
This solar output map of the United States lists baseline sun hours for different regions. A more accurate figure is calculated when the array's compass orientation and tilt are determined, along with the shading impact. Sun path data is provided free in the form of NREL Data Sets and online caculators. Go to our Calculators, Tools and App page for the links.
Once the daily insolation figure is known, you can estimate your annual power generation. For example, if you have an average daily insolation amount of 4.5 hours, then you would multiply that number by 365 to get an annual projection of 1,643 sun hours. Next, divide your annual kilowatt hour usage by the result to determine how many watts your PV array should ideally generate. Here's the general formula:
System Size = Annual kWh / Annual PSH
For instance, if your annual electrical consumption is 7,500 kWh, the math looks like this: 7,500 kWh / 1,643 h = 4.56 kilowatts. Notice how the "h" (for hours) neatly cancels out in the first part of the equation. So now you're dealing solely with watts, not time.
Of course, the system size you get from this simple equation represents AC watts used in grid-delivered electricity. A solar array produces power in DC watts, however. That juice will be converted to AC when it reaches the inverter. So the second calculation involves some of the power loss that take place between the array and kWh meter near the home's service main panel. This is known as the derate factor. Derating accounts for the inevitable losses when converting an array's DC power generation into AC electricity and moving it downstream.
There are actually multiple derate factors, not just a couple. Each contributes to the final loss of watts. For instance, occasional soiling on the module glass reduces insolation. As the modules age they become slightly less efficient. Each year there is some PV system downtime as a result of grid outages (unless you use battery backup). Shading may also be factored into the derate factor, but only if the sun hour data used in the first equation does not take it into consideration. At any rate, here's the complete list of derate factors and a sum calculator provided by NREL.
In general, you can expect about 75% of your array DC watts to reach the utility net meter and get recorded for credit on your bill. This translates to a .75 derate factor used in the formula below. So here's the second calculation, which essentially replaces the first one above:
System Size = Annual kWh / Annual PSH / derate factor
Thus, (7,500 KWh / 1,643 h) / .75 = 6.08 kilowatts. That's over a thousand watts more than what the original equation produced. This system size should deliver 100% of your home electricity consumption and meet the upper limit your utility company will allow you to install.
It turns out, however, that few solar homeowners install a system that offsets 100% of their grid electricity. By opting for an offset between 30% and 75%, you pay a lot less for your PV sytem purchase, and that means a quicker payback time on your investment. So the solar power you produce will mostly cancel out your higher-tiered-rate charges each month (e.g. 20 to 50 cents per kWh), leaving you to pay just the baseline rate (e.g. 10 to 15 cents). From an earnings perspective, the lower offset is always the way to go. And that means adding one more variable into our system size calculation.
System Size = Annual kWh / Annual PSH / derate factor X grid offset
Here's the equation for a homeowner that opts for a 50% offset: (7,500 Wh / 1,643 h) / .75 X .50 = 3.04 kilowatts. Therefore, a 3K PV system is the right size to buy if you want to keep your utility bill in check and start turning a profit within a few years.
The tricky part about these calculations, of course, is knowing when to divide, and when to multiply. Here's a tip in case you ever get confused:
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