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Buying Guide

Last Updated: February 2, 2018

Utility Grid-Tie PV System Design

A grid-connected PV system consists of PV modules, output cables, a module mounting structure, AC and DC disconnect switches, inverter(s), grounding equipment, and a metering system, as shown in the diagram below. The Grid-Tie System Worksheet is designed to help size a PV array to offset a site’s electrical usage with the largest system that would be cost-effective to install. A smaller system can reduce part of the electric bill, and in locations with tiered or progressive rates, it may have a faster financial payback. Compare the worksheet result with the amount of space available to mount the PV array in order to get a rough idea of the maximum PV array size.

Below is a diagram of a typical grid-tie system (utility intertie) without energy storage. Many grid-tie inverters have built-in DC disconnect switches, while some have both a DC and an AC disconnect. Many models also contain a PV array string combiner so a separate one may not be necessary. Separate overcurrent protection for each series string of modules in a PV array (typically provided in the array combiner box) is required only if there are three or more series strings of modules connected to a single inverter input. Inverters with multiple MPPT input channels can have one or two series strings per channel without individual string fusing.

Utility Grid-Tie PV System Design

Worksheet: Grid-Tie PV System Design

Determine PV array size for a grid-tied system (no energy storage)

Step 1: Determine the daily average electricity usage from the electric bills.

  • This will be in kilowatt-hours (kWh). Due to air conditioning, heating, and other seasonal usage, it is a good idea to add up all the kWh for the year and then divide by 365 to find the average daily usage.

Step 2: Find the location's average peak sun-hours per day.

  • For example, the average for Central California is 5 sun-hours. NREL’s PVWatts online sizing program ( can provide this data as well as monthly and yearly expected AC production totals. It can also account for array tilt-angle and azimuth to get more accurate results.

Step 3: Calculate the system size (AC watts) needed to offset the average usage.

  • Divide the daily average electricity use by average sun-hours per day. For example, if the daily average electricity use is 30 kWh and the site is in Central California, system size would be: 30 kWh / 5 h = 6 kW AC. Multiply kW by 1,000 to get AC watts.

Step 4: Calculate total required nameplate power of the PV array.

  • Divide the AC watts from step 3 by the system derate factor. Use a derate factor of 0.82 for most systems (this is the standard derate used by PVWatts). For example, if an array size of 6,000 WAC is calculated in Step 3, divide 6,000 WAC by 0.82 to get 7,318 WDC based on the module’s STC rating.

NOTE: System derating factors

The overall system derating factor represents losses in the system due to the difference between the PV module’s nameplate DC ratings, and actual expected output in real-world conditions, module mismatch, losses in diodes, connections and wiring, module soiling, array shading, tracking error, system aging, and the inverter efficiency at maximum power. The default derate typically used is 0.82, but specific site conditions and equipment used may cause variations. The 0.82 derate is based on 14% systemic losses and 96% inverter efficiency.

Step 5: Calculate the number of PV modules required for this system.

  • Divide the system DC wattage in Step 4 by the nameplate rating of the chosen modules to calculate the number of PV modules needed to provide the desired AC output.

Step 6: Select the inverter/module combination available from Solar Gear Supply that will work with the desired system size, and system AC voltage and phase.

  • Generally in most locations the nameplate solar array size can be up to 1.25 times larger than the maximum inverter capacity. Solar radiation is rarely the full 1000 WsqM that is the standard condition. Also it has to be very cold for the PV cells to be operating at 25°C where they are rated for full power. At some locations, particularly at high altitude these conditions may be possible and oversizing the array may not be advised. Most inverters now have two or more MPPT channels, some of which are limited to one string of modules, so it is best to use series strings at the highest voltage possible so long as the maximum voltage is never exceeded even in the coldest conditions. Because PV modules have the potential to have a voltage output +-10% from the rating, and there is voltage drop in the wiring and connections, and there is about 0.1% drop in power per year, it is best to size strings so that they are at least 10% higher than the minimum input voltage for the inverter in the hottest conditions. Most inverter manufacturers have online calculators for sizing arrays and strings with their inverters. For assistance in sizing an inverter, you are also welcome to contact Solar Gear Supply so we can connect you with our Technical Support Team. Other factors, such as high or low temperatures, shading, array orientation, roof pitch, and dirt on the modules, will affect the system’s actual output.

Grid-Tie with Battery Backup

Many solar customers are surprised and disappointed to learn that their typical grid-tie solar PV system will not power their home during a utility outage. In areas where blackouts and extended weather-related outages are common, a battery backup system, like the one shown in the diagram below, can add substantial value.

Sizing and designing a grid-tie system with battery backup is more complex than designing a typical system without energy storage. They perform two separate functions: offsetting the power purchased from the electric utility, just like a standard grid-tie system, and providing emergency backup power during utility outages. Both of these functions require separate design considerations and calculations.

The “grid-tie” part of the system is designed to offset kilowatt-hour energy consumption using the average peak sun-hours available where the PV array is located.

The “battery backup” part of the system is designed to meet the power draw of the critical loads that need to operate during a grid outage for however long the outage is expected to occur. These systems are generally designed to run only specific circuits located in a separate sub-panel. They are not designed to power the whole house; although this can be done, it adds considerable cost and complexity.

Battery backup systems require specialized inverters and other components and must be carefully sized, so be sure to contact Solar Gear Supply so we can connect you with our Technical Support Team for assistance if you’re unfamiliar with this type of system.

Grid-Tie with Battery Backup

Inverters for Grid-Tie with Battery Backup

OutBack Power GFX and FXR inverters and switch gear, as well as OutBack Power Radian inverters, can power loads individually from 2 to 8 kW and multiple inverters can be combined in a single system up to 80 kW in size.

The Schneider Electric Conext XW+ series of inverters offers grid-tie inverters with battery backup capability in 4 kW, 5.5 kW, and 6.8 kW increments. Up to four units can be paralleled for battery backup systems up to 27.2 kW.

The SMA Sunny Island inverters, in conjunction with a SMA Sunny Boy inverter and PV array, can be used to provide backup power in a grid-tied home or business using AC coupling. Backup systems can be configured with up to 24 kW single-phase output using up to four Sunny Island inverters or up to 72 kW of three-phase output with up to 12 Sunny Island inverters and a Multi-Cluster Box.

See Battery-Based Inverters for more information on these inverters.

Follow steps 1-6 on the Grid-Tie PV System Design Worksheet (above) to determine the size of the PV array required to provide the desired percentage of total power, then calculate the inverter size and battery capacity needed using the worksheet below.

Worksheet: Inverter and Battery Sizing for Grid-Tie with Backup System

Determine energy storage requirement for backup system.

Step 1: Find the power requirements (watts) for the appliances that need power during an outage.

  • Make a list of the loads and appliances that need power during an outage, such as refrigerators, safety lighting, etc. You can use the Load Analysis template below or create your own. Only list the essential items, since the system size (and cost) will vary widely with power needed. The wattage of individual appliances can usually be found on the back of the appliance or in the owner’s manual. If an appliance is rated in amps, multiply amps by the operating voltage (usually 120 or 240 VAC) to find watts. Add up the wattage of all the items on the list that may need to run simultaneously to arrive at the total amount of watts. This is the “peak wattage” inverter requirement and will determine the minimum size of the dual-function inverter that you will need. If the PV array total wattage is larger than the peak wattage required to run the chosen loads, then ensure that the inverter capacity is equal to or greater than the PV array nameplate capacity.

Step 2: Define how long of an outage the system must accommodate.

  • Power outages last from a few minutes, to a day or more. This decision will greatly affect the system size and cost, so the desired length of time should be traded against the total loads supported. If the system needs to provide power for an indefinite period of time, use the array and battery bank sizing instructions for an off-grid system on the following pages.

Step 3: Determine the amount of energy (kWh or watt-hours) that would need to be consumed during the length of the expected outage.

  • Multiply the power requirements (in step 1) by duration in hours (in step 2). The result will be watt-hours. For example, powering a 350 W refrigerator, a 150 W computer, and a 500 W lighting system for 2 hours would require 2,000 watt-hours (or 2 kWh) of energy storage.

Step 4: Factor in the inverter losses.

  • Multiply the total watt-hours or kWh to be supplied to the loads by 0.87 to account for inverter losses.

Step 5: Calculate the minimum energy storage needed.

  • Divide the Step 3 result by 0.8 (batteries should not be discharged past 80%). For example, if the battery bank needs to supply 2 kWh of energy, at least 2 kWh ÷ 0.8 = 2.4 kWh of nominal battery energy storage is needed.

Step 6: Calculate battery capacity needed.

  • Divide the energy storage requirement from step 4 by the DC voltage of the system (usually 48 VDC, but sometimes 24 VDC) to get battery amp-hour (Ah) capacity. Most backup systems use sealed batteries due to their reduced maintenance requirements and because they can be more easily placed in enclosed battery compartments. Flooded batteries are not recommended for backup or standby applications.

AC-Coupled Systems

An AC-coupled power system is another form of battery-based system. It can be used either in a grid-tie system with a battery backup application, or in a completely off-grid system. Instead of using a battery charge controller with the PV array, these systems utilize standard grid-tie inverters that produce AC power (usually 240 VAC), which can be “sold” to the utility grid when the grid is connected or can be used by a separate battery-based inverter to charge a battery bank during a grid outage.

Along with the standard grid-tie inverter, a second, bidirectional, battery-based inverter is used with a battery bank to provide AC power during a grid outage. Both the AC output of the grid-tie inverter and the AC output of the battery inverter are connected in the protected loads sub-panel. During normal operation when the grid is “up”, the power from the PV array and grid-tie inverter simply passes through the sub-panel and the battery inverter’s built-in AC transfer switch and on to the utility main panel. From there it is either consumed by house loads connected there or exported to the grid. If a grid outage occurs, the grid-tie inverter will automatically shut off. At the same time, the battery-based inverter will automatically switch off the grid connection and begin to power the loads in the protected loads panel using energy drawn from the battery bank. Since the grid-tie inverter is connected in this sub-panel, it detects the AC power from the battery inverter and, (after a 5-minute delay) will turn back on. The power output from the array and grid-tie inverter will then be used directly by the protected loads connected to the sub-panel or be used to charge the batteries via the battery-based inverter/charger.

The SMA Sunny Island battery inverters are designed to work with SMA Sunny Boy inverters and will communicate with each other to control the battery charging process. Other brands of battery-based inverters, such as OutBack Power, Schneider Electric XW+, and Magnum Energy MS models can be used with most grid-tie inverters in an AC-coupled system; most however have no built-in way to control battery charging from the grid-tie inverter. A relay can be placed in the AC connection to the grid-tie inverter, controlled by a battery voltage activated switch (such as the AUX relay built into many inverters) to disconnect the grid-tie inverter when the battery voltage rises to the full-charge voltage, ending the charge cycle. Alternatively, a diversion controller connected to the battery, can be used with an AC or DC diversion load to consume the excess power and keep the batteries from being overcharged.

AC-Coupled Systems AEE Solar

Off-Grid System 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).

Off-grid power systems are designed differently than grid-tie systems. With a typical grid-tie system, sizing calculations are based on the yearly average peak sun-hours available at the site, and are used to offset the annual power consumption drawn from the utility grid. With an off-grid system design, the calculations are usually based on the peak sun-hour figures for the darkest month of the year, rather than the yearly average, in order to provide sufficient on-site power year-round. In locations where it is not practical to install a PV power system that will provide 100% autonomy during the darkest time of the year, a generator may be used to help run loads and charge the battery bank, or if site conditions allow, other energy producing systems, such as wind or micro-hydroelectric turbines can be used to supplement the PV array.

Off-grid power system design is complex, and these systems require specialized inverters, charge controllers, and battery banks. Please contact Solar Gear Supply so we can connect you with our Technical Support Team for system design assistance.

Off-grid solar PV system Off-Grid System Sizing Information

Efficiency and Energy Conservation

Energy-efficient appliances and lighting, and non-electric alternatives, can help to reduce the cost of producing and storing energy in off-grid systems. Every watt that doesn’t need to be used is a watt that doesn’t have to be produced or stored. The information below pertains mostly to off-grid systems, but can also help to reduce the size and cost of grid-tied PV systems, with or without battery backup capability.

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 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 cabin, RV, or boat, low voltage DC lighting with LEDs is sometimes 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 with many lights, using an inverter to supply AC power for conventional lighting is more cost-effective. AC LED lights are now common and very efficient, but it is a good idea to have a DC-powered light in the same room as the inverter and batteries in case of an inverter fault. Finally, AC light dimmers will only function properly with inverters that have true sine-wave output.


Gas powered absorption refrigerators can work well in small systems when bottled gas is available. Modern absorption refrigerators consume approximately 5-10 gallons of LP gas per 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.

Major Appliances

Standard AC electric motors in washing machines, larger shop machinery and tools, swamp coolers, and pumps, are usually ¼ to ¾ horsepower and consume relatively large amounts of electricity, thus requiring a large inverter. These electric motors can also be hard to start on inverter power, due to the large surge of power they need for starting, which can be as much as three-times or more of the power as they draw while running. Variable-frequency drives can be used with large motors to provide a “soft-start”, reducing the surge load on the inverter system. A standard top-loading washing machine uses between 300 and 500 watt-hours per load, but new front-loading models can use less than half the energy per load. 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.

Small Appliances

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 inverter system and batteries, they will operate, but the user may need to schedule those activities with respect to the battery charging cycle. For example, by ironing in the morning, the PV system can then recharge the battery bank during the day. Or, if these loads can be run during a sunny day, the energy from the PV array can supply the power to run the appliance without needing to draw energy from the battery bank.

Electronic equipment, such as stereos, televisions, DVD players and computers, draw less power than appliances with heating elements, but these loads can add up, so opt for more efficient models when possible, such as an LED or LCD TV instead of a plasma or CRT design.

Phantom Loads

Many appliances, especially ones with wireless remote controls, draw power even when turned “off”. While each load may be small, the energy consumption of multiple appliances over a 24 hr period can add up and be quite large. Placing these loads on a switchable outlet or plug strip can save a considerable amount of energy.

Worksheet: Off-Grid Load Analysis

Determine the total kilowatt-hours (kWh) per day used by the AC and DC loads.

Step 1: 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. 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. lists annual Wh consumption for Energy Star electrical appliances; divide this number by 52 to get watt-hours per week.

Calculate AC loads (If there are no AC loads, skip to Step 3)

 Description of AC loads run by inverter




























































                                                                                 Total watt-hours per week:  


Step 2: Convert to DC watt-hours per week.

  • Multiply the result of Step 1 by 1.13 to correct for inverter loss.

Step 3: 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 (if applicable)

Description of AC loads run by inverter




































                                                                    Total watt-hours per week:


Step 4: Calculate total DC watt-hours per week.

  • Add the total DC watt-hours per week used by AC loads from Step 2 to the watt-hours per week used by DC loads from Step 3 to get the total DC watt-hours per week used by all loads.

Step 5: Calculate your total watt-hours per day consumption.

  • Divide the total DC watt-hours per week from Step 4 by 7 days to get the total average watt-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 energy (kWh) the solar array must produce to size the PV array and determine the total number of solar modules required for the system.

Step 1: List the total average watt-hours per day needed to power the electrical loads.

  • Obtain this number from the Off-Grid Loads Worksheet on the previous page.

Step 2: Calculate the minimum watt-hours needed per day.

  • Multiply the watt-hours per day needed by 1.25 to compensate for PV array and battery charge/discharge losses. This is the minimum total watt-hours that the PV array needs to produce, on average, each day. However, increasing the array size further will allow the system to provide some additional charging during cloudy weather and catch up more quickly after a cloudy period. Increasing the array size can also allow for reduced battery storage requirements.

Step 3: List the average sun-hours per day at the system’s location.

  • 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 winter insolation. If you want 100% autonomy only in summer, use the higher summer insolation. If you have a utility grid-tie system with net metering, use the yearly average figure.

Step 4: Determine the minimum nameplate capacity.

  • Divide the result of Step 2 by the average sun-hours per day from Step 3 to determine the minimum nameplate capacity of the PV array.

NOTE: Sizing Solar Arrays with PWM or MPPT Charge Controllers If you are planning a small low-cost system with a PWM charge controller, with 12 or 24 VDC “nominal” PV modules (36 or 72 cells), continue to Step 5 below. If you are planning a 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: Calculate peak amps.

  • Divide the total solar array wattage required from Step 4 by the system’s DC battery voltage (usually 12, 24, or 48 VDC) to get the total peak amps (A) that the PV array must produce.

Step 6: Find the peak-power current (Imp) of the module you will be using from its specifications or Data Sheet.

Step 7: Calculate the number of parallel strings.

  • Divide the result of Step 5 by the result of Step 6. Round up to the next whole number. This is the total number of parallel module strings required to produce the total array current needed.

Step 8: Use the table below to determine the number of modules in each series string needed to match the DC battery voltage of the power system.

     Nominal System Voltage

        Number of Series Connected Modules per String


                12 V module

                24 V module










Step 9: Calculate the minimum number of solar modules.

  • Multiply the number of strings from Step 7 by the number of modules per string from Step 8 to get the total minimum number of solar modules required with a PWM charge controller.

Step 10: Calculate minimum PWM charge controller rating.

  • Multiply the number of strings from Step 7 by the module’s short-circuit current (ISC) and then by a 1.25 Code-required safety factor. The current rating of the selected PWM charge controller must exceed this number.

Sizing Solar Arrays with MPPT Charge Controllers

Step 5: Note the minimum solar array nameplate capacity required from Step 4.

Step 6: Enter the nameplate power (in watts) of the PV module you plan to use.

Step 7: Determine the minimum number of modules needed.

  • Divide the PV array capacity from Step 5 by the module nameplate power from Step 6 to determine the minimum number of modules needed. Round up to the nearest whole number. (NOTE: This number may need to be adjusted in Step 10).

Step 8: Determine the number of modules in each series string.

  • Use the table below to determine the number of modules needed in each series string based on the system’s battery voltage and PV charge controller used.

Step 9: Calculate the number of series strings needed.

  • Divide the total number of modules from Step 7 by the number of modules per series string from Step 8. Round up to a whole number. This is the total number of array series strings needed.

Step 10: Determine the total number of modules needed.

  • Multiply the number of module strings from Step 9 by the number of modules per string from Step 8 to determine the total number of modules needed.

Step 11: Find the total number of chosen controllers needed.

  • Multiply the total number of modules needed (from Step 10) by the rated wattage of the module being used. This is the adjusted total PV array nameplate capacity. Using the chart below, find a controller rated for the total array wattage (or more). If the total array wattage is more than a single controller can handle, either use a larger controller or use multiple controllers in parallel. NOTE: Most charge controllers must have their own separate PV array, so larger arrays need to be divided into sub-arrays for each charge controller.

Max Array Wattage Per Controller  Size

Battery Voltage

Controller rated Output Amps

15 A

20 A

30 A

45 A

60 A

75 A

80 A

95 A

100 A