Off-Grid & Grid Interactive Battery Based Packages

Plannning Design & Installation Tips

Inverter/chargers are engineered to use a battery bank to store energy. The inverter/charger works in conjunction with a renewable energy source from a solar array, wind turbine, generator or utility grid depending on your application. The size (watts/amps output) of a system depends on the amount of power that is required (usually expressed in watts), the amount of time it is used (hours) and the amount of energy available from the sun and or wind in a particular area (sun-hours per day & wind average MPH). The user has control of the first two variables, while the third depends on the location. You cannot accuratly choose the size of a system without first establishing a daily energy budget.

Off-Grid Design; EVERYTHING Starts With Your Daily Energy Budget

• Determine your loads and establish a daily energy budget. How many watts per day does or will your household consume? The most important challenge in a battery based system is to balance your energy consumption with your supply of energy required. You cannot begin to do that without first knowing how much energy you need or will consume each day. Only you control this budget and only you control your daily consumption of electricity at your house or cabin.

Energy Load Worksheet (Excel)

What is the "Power" required by your electric load?

Appliance & Equipment Load List: List and add up your daily electrical equipment load demand in watt hours (wh). Watt hours (wh) and amp-hours (Ah); Watts = Amps X Volts. Watt hours are the most common measure of electricity usage and are the easiest to understand. Amp Hours = Watt hours / System Voltage. Many professional system designers will use amp hours to size a system because amp hours takes into account real world behavior of solar panels and battery banks. Either method will arrive at the same conclusion if done properly. For our purposes here, we will primarily use watt hours (wh) when sizing the number of solar panels (and inverter) and amp hours (Ah) for our battery selection. IE: 5 - 13 watt light bulbs X 5 hours per day = 65 watts. 18 CF refrigerator @ 5 amps x 120VAC = 600 watts x 6 hours per day = 3,600 watts. THIS IS IMPORTANT: When we say "list, estimate and determine your loads or daily energy budget ", we mean all your loads. From the cell phone chargers to a hair dryer. Need Help? If you download the Excel worksheet you will only have to indicate how much of each piece of equipment you have and how long your run it.

System Voltage or Battery Bank Voltage:

TIP; Higher battery voltage means less resistance which allows equipment to run cooler. Cooler electrical equipment = longer life. We recommend 48 VDC battery bank for systems larger than the micro kits.

Derate Factors
Wire Loss	3%
Panel Mfg. Tolerance	5%
Panel Mismatch	2%
Diodes/Connections	1%
Panel Soiling	5%
Battery Efficiency	15%
Panel PTC (Temp)	12%
Charge Controller	5%
Total	48%

Your system voltage means the nominal voltage you select for your battery bank, charge controller and inverter. Here are some things to consider when choosing your systems voltage:

The DC system voltage is established by the battery bank in off-grid systems. A major factor in making this decision is how much power will be required from the batteries. As power demands increase it is advisable to raise the battery voltage one because of less resistance but a major consideration is you should not put more than 2 banks in parellel of batteries and never more than 3 on one charge controller.

This voltage is important because establishes the type of charge controller and inverter that will be selected. The selection of the battery bank voltage affects the currents. A 1200 watt off-grid system operating at 12 volts draws 100 amps. (1200w / 12v = 100A). The same system draws only 25A at 48 volts. Lower amps reduces the size of conductors, over current protection devices, disconnects and charge controllers. Additionally, since voltage drop and power losses are smaller at lower amps, higher voltage off-grid systems are more efficient. As a rule of thumb, off-grid systems up to 1000 watts use a minimum 12 volt battery bank which limits DC currents to less than 84 amps. For 2000 watt systems, often times a 24 volt battery banks is used. For 5000 watt system a 48 volt battery bank should be selected. NOTE: There is nothing wrong with choosing a 48 VDC system for 6 panels and up, however you need to size the array in increments of 3 panel series because your array voltage needs to be at least 20% higher than your battery bank. IE: 9 panels, 3 wire in series x 3 in parallel for 48VDC battery bank. 8 panels, 4 x 2 in series for 24VDC battery bank.

Daily Off-grid System Charge Requirement in Amp Hours:

Since the energy output to the loads must be balanced by the energy input from your solar panels, we need to calculate your daily charge requirement in amp hours as that number will come in handy later. Take your total daily watts x 48% (derate rule of thumb) to account for power losses in inverter, circuits, wire transfer loss, battery efficiency and module temperature. Now divide by the system voltage you chose based on the previous section and write this number down. This is the charge in amp hours your solar panels will have to provide each day to meet your load requirements you have set. Example 5,000 watts daily load total X 48% = 7,400 watts / 48 volt system = 154 amp hours that will need to be generated. Another way to put it is this means that the daily Ah demand on the batteries will be 154 Ah.

Make the Power; Size Your Solar Panel Array; Input Must Equal Output.

Number of Solar Panels = Daily Watt Requirement (1 - 24 hour period (Energy Budget)) + 48% (Derate Factor) ÷ shortest day sun hours ÷ panel STC rating = number of solar panels needed.

The number of solar panels you will require is a basic off-grid formula that considers your total daily energy requirements X (+) 48% (rule of thumb) to allow for losses in wiring distribution efficiency, panel real world performance Vs. factory STC rating, battery and inverter efficiency. You then take that number and ÷ the lowest solar irradiance available in the area of the system which is the shortest daylight month of the year (December). The last step is to ÷ that number by the STC watts of the solar panel rating. IE: 8,000 watts per day energy budget X 48% = 11,400 Watt-Hours/Day of energy to be generated. 11,400 Watt-Hours per day div the available sun hours in your area (Solar Irradiance) in December which has the shortest day of the year.

Solar Insolation / Lowest Available “Sun-Hours Per Day”

Off-Grid systems use a sizing methodology than is different from grid-tied systems. Since you are on your own with no utility to back you up, you must consider the shortest day of the year or the amount of available sun in December.

Look up the solar irradiance near your area from our lowest peak sun hour resource map. Find the nearest city to your home and write down the lowest daily sun hours. Divide your daily load calculation (Daily energy budget + 48%) ÷ by lowest available sun hours per day from the solar irradiance chart. For example, if the daily load calculation (Daily energy budget + 48%) is 11,400 watts, and the site is near Salt Lake City UT, you would take 11,400 watts / 4 sun hours = 2,850 watt solar array. That means if you choose 300 watt solar panels you would need 9.5 (EA) 300 watt solar panels for the size kit you select (Round Down to 9 EA 300 watt panels.

The “peak sun hours” for a location is a measure of the total insolation (solar radiation or amount of sun) available and is usually expressed as an average daily value. It represents the total sunlight an area typically experiences at a given time of year by converting it to the equivalent number of hours per day of 1000W/m2 of irradiance (kWh/ m2/day). For off-grid calculations, we do not want to use average but lowest since we have to take into account the shortest day of the year in Dec.

One more factor comes into play, for 48V battery bank you will need to connect at a minimum panels in series of 3 to provide enough volts during the hot days of summer to top off your 48 VDC battery bank. Your voltage from your panel series should be a minimum of 20% above the battery bank on the hottest day of the summer because during the hottest days of summer panels produce less energy. (If you have a very long DC run from your array you may want to go as high as 4 or 5 in series to use smaller size wire.) In our example above, you would need to use 12, 9 or 6 panels so you have strings of 3 for a 48VDC battery bank or strings of 2 for a 24VDC battery bank.

Your energy budget is a personal choice and an opportunity to aggressively manage your daily energy usage. It is always cheaper to save energy than to make energy. NOTE: No matter how large your system is, you will need to manage your daily energy consumption as it relates to the amount of available sun. IE: a cloudy stretch of days may only produce 50% of your energy requirements. After many days in a row of heavy cloud cover, you may have to cut back your daily energy consumption until the weather clears a bit.

Evaluating your off-grid site is similar in many ways to grid interactive site evaluations. Key Considerations:

• Check out areas available to install your solar kit. A few things to consider; array dimensions, orientation, tilt and obstructions. For off-grid installations, ground mount systems are the most popular. (Top-of-Pole or Rack Mounted)
• Shading of array location. This is important in any solar installation but a must in off-grid systems. Choose and area as close as possible to your power shed but one that has at least 6 hours of sun (+ -) 3 hours from noon.
• Roof type and age.

TIP; If you plan on re-roofing within 5 years but are not ready yet, some homeowners will replace only the shingles located under the solar array when the panels are installed and leave the rest for later.

TIP; Solar cells are activated by visible light not ultra-violet or infra-red. Solar panels produce electricity when exposed to sun light, but plan to keep your inverter and battery bank indoors.

• Battery Bank and Power Center Location. The battery bank should be no further than 10' from the inverter.

Store The Energy; Battery Sizing and Selection

Battery bank sizing is the part of the off-grid solar wind system that has a higher probability of causing you problems that other parts of your system. Use the battery sizing worksheet to help you through this critical stage. Factors such as your budget may tempt you to look to cheaper battery alternatives but a quality battery will pay off over the years. We recommend you choose a 2VDC or 6VDC battery and connect them in series so that the total DC volatage equals the system voltage. Do NOT put more than 3 banks of batteries on one charge controller.

● Click the Battery Bank Size Tab Above ⇑ to Learn More.

Control The System; Power Center = Inverter, Charge Controller & More

Choosing your charge controller:

A charge controller is an electronic voltage regulator used in off-grid solar and wind systems with battery banks to properly control the charge from the solar panels or wind turbine keep the voltage to the battery bank within acceptable limits. The charge controller automatically tapers, stops, or diverts power when batteries become fully charged. Without a charge controller your solar panels or wind generator would continue to send electricity to the battery bank and eventually destroy your batteries.

Your charge controller will:

• Provide an optimum charge to the batteries.
• Prevent your battery bank for being overcharged from your solar panels or wind turbine.
• Prevent unwanted discharging.
• Provide information on the state of charge of the battery bank.

The simplest charge controllers cut the power when the battery reaches a set voltage, and turn it on when a low voltage set point is reached. Pulse width modulated (PWM) charge controllers turn on and off very rapidly, maintaining the batteries at full charge with whatever power is available. Maximum power point tracking (MPPT) charge controllers optimize the voltage of the solar panels or wind turbine to maximize total power output then convert that to the correct voltage to charge the battery. This process significantly increases the power from a solar array, particularly in low temperatures when battery voltage is significantly below the solar array voltage. Most MPPT charge controllers work with higher array voltages, enabling the use of larger solar panels, which can be more economical on a cost per watt basis. A higher voltage solar array also minimizes the required wire size between the array and the charge controller. While more expensive than PWM controllers, MPPT charge controllers can boost system performance significantly by up to 30%.

Blue Pacific Solar® sells MidNite Solar, Outback and Morningstar charge controllers. All these companies have take much of the work out of the technical calculations required for properly sizing a charge controller with their string calculators. On all our off-grid pre-engineered packages, we have matched the right sized charge controller with the package for you.

Choosing Your Off-Grid Power Center:

Living off the grid means you will be generating, storing and processing every watt your off-grid home or cabin sucks up. If your cabin is going to need AC power, then an off-grid AC inverter is going to be of particular interest to you because you will be depending on it day in and day out.

Off-grid inverters are sold either sine wave or modified sine wave. Sine wave output, which has low total harmonic distortion, will power virtually any type of load, even sensitive audio electronics. Modified sine wave inverters may not run some types of equipment satisfactorily, and some loads won’t run at all.

An off-grid inverter must supply enough power to meet the needs of all the appliances running simultaneously. Before selecting an inverter, you must know the watts your appliances will require and their amp and surge needs. Sizing an inverter for an off-grid system, which is based on instantaneous load, is very different from sizing a grid tied inverter, which is determined by the solar panel array size. A grid tied inverter’s job is simply to convert all the DC electricity from the solar array into AC power, which is fed back into the house electrical system then onto the grid if production exceeds the homes energy consumption. In a grid tied system, the inverter is not responsible for meeting the AC loads, since practically unlimited utility power is available. In the case of an off-grid inverter, the inverter has to provide enough energy to all the AC loads, sometimes at the same time. Say you need to simultaneously power 3,000 Watts from various appliances. For an off-grid system, you’d need an inverter that could supply at least that amount. Note that the solar array size does not enter into this inverter sizing since the inverter pulls its power from the battery bank.

Inverter Surge Capacity

The ability of an off-grid inverter to surge to a higher level than its rated continuous output for a short duration to turn over the locked rotor of large loads like well pumps is critical. The inverter specifications that should be looked at are the Maximum Output Amps and the AC overload capability. If there are large loads a good number to look for is a five second surge capability of at least 1 ½ times the rated output of the inverter. If you have a deep well pump, the minimum requirement may be 3X the continuous run amps.

Blue Pacific Solar® only sells inverters that meet Underwriters Laboratories (UL) 1741 standards. So if you are on the fence between two choices of inverters, you might ask and find out if it is UL listed.

Lightning and Surge Protection

The funny blue glass like bulbs you see on the MidNite power center above are AC and DC surge protectors. Lightning can enter a wiring system by either the DC route or AC or both at the same time. The MidNite Solar Surge Protective Device (MNSPD) is a Type 2 device, designed for indoor and outdoor applications. Engineered for both AC and DC electric systems, it protects both transformer and transformer-less inverters without interfering with the GFP protection circuit, it provides protection to service panels, load centers or where the SPD is directly connected to the electronic device requiring protection.

Connect it All; Wire Size & Electrical Distribution Parts

Combiner / circuit breaker box is a key piece of equipment that begins to bring the pieces of equipment together that allows you to generate electricity. NEC (National Electrical Code) says that each series of strings of panels are to be wired to it's own DC circuit breaker. The combiner box is usually located directly under or near the array

Wire size and breakers are the final items in your off-grid design to consider, but no less important. To have a safe off-grid system, you will need to install breakers and choose the right size wire. If you select one of our pre-wired power systems with your kit, we do all the heavy lifting for you because right size breakers are pre-engineered and pre-wired into each of our power centers. You simply have to hang and connect it following our wire diagram which is supplied with all our kits.

The distance between the combiner box, which is usually located near the solar panels, and the charge controller will be a factor in choosing the best string voltage for the charge controller and battery system. The higher the input voltage the smaller the wire can be for any given amount of power. For example, a system with a 12 volt battery and solar panels consisting of four 6.75 amp 12 volt DC nominal modules located at a distance of 40’ from the batteries could have the modules wired in series, parallel or series and parallel. Input design possibilities in this example are 12, 24, and 48 volts DC. If the panels are configured with the modules wired in parallel, the input voltage would be 12 volts DC with an input current of 26 amps. The same panels wired in series would have an input voltage of 48 volts DC and an input current of 6.5 amps. In this example #1, the 26 amp 12 volts DC panels #1/0 wire, which is prohibitively expensive, would be required to limit voltage drop to 2% which is recommended for 12 volt DC systems. The same panels wired for 48 volts dc would only require a #8 wire. With the #8 AWG wire the 12 volt dc panels would have to be within 7’ of the batteries. The distance that #8 wire can be used is over 5 times greater at 48 volts DC than 12 volts DC.

NOTE: These are just given as examples. At Blue Pacific Solar®®, we would never recommend you use a 12VDC system in your home or cabin. 24VDC are a common choice for micro kits, but 48VDC is a better choice. Higher voltage = smaller wire and less strain on your equipment.

SAFETY WARNING: Danger to life due to high voltages. Risk of death or serious injury due to electric shock. We strongly recommend you employ the services of a licensed local electrician or other properly trained and qualified persons to complete the final connection.

Wire Size

The amount of current (amps) traveling through any electrical circuit depends on the size of the wire (AWG), the voltage of the array or battery bank, and the one way distance of the wire run. Lower AWG gauge wire has less resistance than larger gauge wire. The longer the distance of your wire run while using lower voltage the larger gauge wire you are going to need.

This chart is useful for finding the correct wire size for any voltage, length, or amperage flow in any AC or DC circuit. For most DC circuits, particularly between the solar panels and the batteries, we try to keep the voltage drop to 3% or less.

TIP; NEC 690.8 CALCULATION of MAXIMUM CIRCUIT CURRENT. (A)1 Solar Source Circuit Currents. The maximum current (Amps) shall be the sum of parallel solar panel rated short circuit currents multiplied by 125 percent.

• Amps = Maximum Number of Amps Through the Circuit
• Feet = One Way Distance of Wire
• % Voltage Drop = Percent of Voltage Drop Desired (2 = 2%)
• Voltage = Voltage Carried by the Wire

Voltage Drop Index Chart (VDI)

AWG Wire Size

Copper Wire

Aluminum Wire

	VDI	Ampacity	VDI	Ampacity
4/0	99	230	62	180
3/0	78	200	49	155
2/0	62	175	39	135
1/0	49	150	31	120
2	31	115	20	94
4	20	85	12	65
6	12	65
8	8	50
10	5	30
12	3	20

 

Technical Resources, Solar Consultant & Line Drawing; We Have Your Back

You should not be overly concerned with how or what type of final connection, wire and breaker size because we do the heavy lifting for you. When you make a purchase over $2,000.00 for a kit, your order gets assigned to a Technical Team Captain so your have your own personal solar consultant. Our System Integrators provide polished technical advice on the design and execution of all types of installations. This single point of contact will be there to answer your questions and help walk you through any obstacles you might incur during the process.

When you buy a kit from Blue Pacific Solar®, we supply a custom electrical line drawing that shows you exactly how to connect everything. We will specify on our drawing what size and type of wire and breaker you will need to be NEC code compliant. The line drawing can be used for your permit with the local authority having jurisdiction commonly referred to as AHJ (Building Department). If you need help with your full permit documents for your local AHJ anywhere in the country, our permit document service is available to help take the hassle out of your solar purchase.