How Large Does Your Solar Power System Need to Be?

7 of 7 in Series: The Essentials of Planning Your Solar Power System

Switching to solar power can require a substantial outlay of money. At a minimum, you need to determine the following in order to get a good idea how much your standalone photovoltaic (PV) solar power system will cost:

  • The total watt-hours per day of energy you'll need: Compile a list of all your appliances and devices and how many hours per day each will be run.

  • Define your peak instantaneous power output, measured in watts: Determine which appliances you'll be running at the same time; add their power draws, in watts.

  • Figure out the duty cycle: For example, a weekend cabin used for two days has a duty cycle of 2/7, or about 28 percent. A system used every day has a duty cycle of 100 percent.

  • Estimate how many hours of good sunlight a day you can expect: Sunlight is difficult to estimate with much accuracy because it depends on the weather and on the time of year during which you're interested in using your system.

Here is an example of some of the calculations for an off-grid home in the mountains of Northern California. Here's what you need to keep in mind about this scenario:

  • The duty cycle is 100 percent, and the house is used year-round.

  • Worst-case expected sunlight per day is around four hours.

  • A two-day power reserve is required because the backup generator is 20 years old and may or may not start, depending on its cantankerous mood.

  • The system must output 120VAC.

And in case you're curious, here's how the owners are reducing their energy requirements:

  • For heat, they use a wood-burning stove exclusively.

  • Residents don't need electric-powered water heating because they use a solar water heater.

  • Both the cooking stove and refrigerator work with bottled propane.

  • The house is extremely efficient, with well-designed window overhangs, a sunroom on the southern front, and a modular design that enables the living area to be closed off from the rest of the house on the cruelest winter days.

  • A solar attic vent fan is installed in the attic space, and a large solar-powered ceiling fan in the great room keeps the comfort level on the hottest summer days tolerable.

The following table shows the sample power load for the California cabin.

Energy Consumption in an Off-Grid Home
AC Device Watts Hours/Day Watt-Hours/Day
Kitchen lights 120 6 720
Family room light 120 4 480
TV 70 3 210
Coffee pot 200 0.5 100
Clock radio 1 24 24
Table fan 15 6 90
Computer 100 7 700
Miscellaneous appliances 400 0.5 200

With this chart, you can do some of the following calculations for your load analysis. To calculate the last column, simply multiply the first two columns.

Calculation Answer
Total energy needs in one day 2.5 kWh/day
Adjustment for inefficiency (10 percent) 2.8 kWh/day
Maximum instantaneous load 700 W
Duty cycle 100%
Total energy needs in one week 19.4 kWh/week
Power needed from PV panels (@4hrs/day) 700 W

Next, calculate battery size, which is specified in terms of amp-hours (Ah). Most batteries are 12VDC, but other sizes are also available. For this example, assume a 12VDC system.

  1. Take the total kWh/day, multiply this by 1,000 to get kWh/day, and then divide this value by the battery voltage.

  2. The generator is old, so triple the result from Step 1 to account for the two reserve days.

    To cover the two reserve days, the cabin owners need batteries that can hold three days' worth of charge.

  3. Multiply the minimum battery capacity from Step 2 by a factor of two.

    Batteries last much longer when they're not drained of more than about 70 percent of their available energy.

Finally, they're going to need a larger PV module capacity in order to get the three-day reserve. It's okay to go without power a few times, and there's a backup generator, so if they double the size of the PV module capacity, they should be safe. Hence, the owners need 1,400 watts of PV.

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The Essentials of Planning Your Solar Power System

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