OFF-GRID SIZING INFORMATION
Off-grid solar PV systems, like the one shown in the diagram below, are one of the most economical ways to provide electricity in the absence of an electrical power grid. Off-grid systems are useful for remote homes and cabins, RVs and boats, and even for industrial applications like remote telemetry, cathodic protection and telecommunications. The size of an off-grid solar electric system depends on the amount of power that is required (watts), the amount of time it is used (hours) and the amount of energy available from the sun in a particular area (sun-hours per day).
Efficiency and Energy Conservation
The use of energy-efficient appliances and lighting, as well as non-electric alternatives, can make solar electricity a cost-competitive alternative to gasoline generators and, in some cases, utility power. Outlined below is information on typical energy consumption for various appliances and lighting.
Cooking, Heating and Cooling
Each burner on an electric range uses about 1,500 W, which is why bottled propane or natural gas is a popular alternative for cooking. A microwave oven has about the same power draw, but since food cooks more quickly in a microwave oven, the amount of kilowatt hours used is typically lower. Propane, wood or solar-heated water are generally better alternatives for space heating than electric baseboards. Good passive solar design and proper insulation can also reduce the need for winter heating. Evaporative cooling is a more reasonable load than air conditioning and in locations with low humidity, it's a great alternative.
Lighting requires careful study since type, size, voltage and placement can all significantly impact the power required. In a small home, an RV, or a boat, low voltage DC lighting with LEDs is often the best choice. DC wiring runs can be kept short, allowing the use of fairly small gauge wire. Since an inverter is not required, the system cost is lower. In a large installation or one with many lights, using an inverter to supply AC power for conventional lighting is often more cost-effective. AC compact fluorescent lights are common and efficient, but it is a good idea to have a DC-powered light in the room where the inverter and batteries are in case of an inverter fault. Also, AC light dimmers will only function properly on AC power from inverters that have sine wave output.
Gas powered absorption refrigerators can work well in small systems if bottled gas is available. Modern absorption refrigerators consume 5-10 gallons of LP gas/month. If an electric refrigerator will be used in a standalone system, it should be a high-efficiency type. High-efficiency DC refrigerators are also available and can offer significant energy savings.
Standard AC electric motors in washing machines, larger shop machinery and tools, swamp coolers, pumps, etc. (usually 1/4 to 3/4 horsepower) consume relatively large amounts of electricity and require a large inverter. Often, a 2,000 watt or larger inverter will be required. These electric motors can also be hard to start on inverter power, due to large surge loads at start-up, and they are very wasteful compared to high-efficiency motors, which use 50% to 75% less electricity. A standard washing machine uses between 300 and 500 watt-hours per load, but new front-loading models use less than 1/2 as much power. If the appliance is used more than a few hours per week, it is often more economical to pay more for a high-efficiency appliance rather than make the electrical system larger to support a low efficiency load. Vacuum cleaners usually consume 600 to 1,000 watts, depending on how powerful they are, but most vacuum cleaners will operate on inverters as small as 1,000 watts since they have low-surge motors.
Many small appliances with heating elements such as irons, toasters and hair dryers consume a very large amount of power when they are used but, by their nature, require only short or infrequent use. With a sufficiently large system inverter and batteries, they will operate, but the user may need to schedule those activities with respect to the battery charging cycle – for example, ironing in the morning so that the PV system can recharge the battery bank during the day. Electronic equipment, such as stereos, televisions, VCRs and computers, draw less power than appliances with heating elements, but these loads can add up as well, so opt for more efficient models, such as an LCD TV instead of a plasma or CRT design.
Worksheet: Off-Grid Load
Determine the total amp-hours per day used by the AC and DC loads.
|List all AC loads, wattage and hours of use per week in the table below. (If there are no AC
loads, skip to Step 5.)
Multiply watts by hours/week to get AC watt-hours per week (Wh/Wk). Add up all the watt hours per week to determine total AC watt-hours per week.
NOTE: Wattage of appliances can usually be determined from tags on the back of the appliance or from the owner's manual. If an appliance is rated in amps, multiply amps by operating voltage (120 or 240 VAC) to find watts.
Calculate AC loads (If there are no AC loads, skip to Step 5)
| Step 2:
|Convert to DC watt-hours per week by multiplying the result of Step 1 by 1.15 to correct for inverter loss.
| Step 3:
|List the inverter DC input voltage; usually 12, 24 or 48 VDC. This is DC system voltage.
| Step 4:
|Divide the DC Watt-hours per week by the DC system voltage to get the total DC amp-hours per week used by the AC loads.
| Step 5:
|List all DC loads, wattage and hours of use per week in the table below. Multiply watts by hours/week to get DC watt-hours per week (Wh/Wk). Add up all the watt hours per week to determine total DC watt-hours per week.
Calculate DC loads
| Step 6:
|List DC system voltage. Usually 12, 24, or 48 VDC.
| Step 7:
|Divide the total watt-hours per week by the DC system voltage to find total amp-hours per week used by DC loads.
| Step 8:
|Add the total DC amp-hours per week used by AC loads from Step 4 to the amp-hours used by DC loads from Step 7 to get the total DC amp-hours per week used by all loads.
| Step 9:
|Calculate your amp-hours per day. Divide the total DC amp-hours per week from Step 8 by 7 days to get the total average amp-hours per day that needs to be supplied by the battery. You will need this number to begin sizing the PV array and battery bank. Note that the Solar Array Sizing Worksheet in this section, as well as the Battery Sizing Worksheet in the Batteries Section both begin with this number in their Step 1.
Worksheet: Off-Grid Solar Array Sizing
Determine How Much Current the Solar Array Must Produce to identify the total number of solar modules required for the system.
| Step 1:
|List the total average amp-hours per day needed. Obtain this number from the Off-Grid Loads Worksheet.
| Step 2:
|Multiply the amp-hours per day needed by 1.2 to compensate for battery charge/discharge losses.
| Step 3:
|List the average sun-hours per day in the system's area.
Check local weather data, look at the map below, or find a city on the Solar Insolation Table in the Reference
Section that has similar latitude and weather to your location. If you want year-round autonomy, use the lower of the two figures. If you want 100% autonomy only in summer, use the higher figure. If you have a utility grid-tie system with net metering, use the yearly average figure.
| Step 4:
|Divide the result of Step 2 by the average sun-hours per day from Step 3 to get the total solar
array amps required.
Sizing Solar Arrays with PWM Charge Controllers
If you are planning a small low-cost system with a PWM charge controller, continue to Step 5 below. If
you are planning a larger system with an MPPT charge controller, go to Step 5 in "Sizing Solar Arrays
with MPPT Charge Controllers." Information on the different types of PV charge controllers can be found
in the Charge Controller section.
| Step 5:
|Find the peak amperage of the module you will be using from its specifications or Data Sheet. We provide the peak power current of our most popular modules in the Solar Module Section.
| Step 6:
|Divide the total solar array amps required from Step 4 to get the total number of parallel strings of modules required. Round up to the nearest whole number.
| Step 7:
Use the table below to determine the number of modules in each series string needed to provide DC battery voltage.
Sizing Solar Arrays with MPPT Charge Controllers