Discharge and Charging Current/Rate - This is the rate you can add energy to and draw energy from a battery, measured in Amperes per

hour (Ah). Batteries are sold based on their maximum one-hour discharge current (105Ah, 20Ah, etc). This means that the battery can

be discharged at this current for one hour before depletion. The rate is expressed as a factor of the charge “C. For a 105Ah lithium battery

example, the following applies:

• 1C means a battery can be discharged at 1 * 105 = 105A
• 0.5C means a battery can be discharged at 0.5 * 105 = 52.5A
• 3C means a battery can be discharged at 3 * 105 = 315A
• (Discharge Current/Rate will affect the amount of energy that can be drawn out of the system on a continuous basis)
• The discharge current has a different outcome depending on the battery type and chemistry. For example, a lead-acid battery discharged at 1C for 30 minutes will be irreparably damaged and will probably not hold another charge. Lithium batteries are highly tolerant and will survive a total discharge event. They can be repeatedly discharged down to 10% for 500 - 10,000 cycles depending upon their chemistry and the rate of discharge nC.
• Lithium Ion batteries are not created equally in this area. A battery with a high energy density (the amount of energy it can store per kilogram) cannot necessarily be discharged at its rated capacity. For example lithium-ion NMC batteries (used in cars and cell phones) cannot be discharged above 1C. Lithium-ion Iron Phosphate batteries can be discharged up to 3C and are used for applications that have high power needs.

Number of Charging and Discharging Cycles - Batteries are rated by the number of times they can be charged and discharged. This detail is very important for the operating capability of your system and is affected by the amount of discharge, time and temperature. For example, a lead acid battery advertised at operating for 10,000 cycles over 20 years means that the battery will stand 10,000 cycles of no more than 10 seconds for each cycle. This works for a UPS or peak shaving application but not for a system that must run a load bank for 18 hours.

Depth-of-discharge - This is the measure of how deeply a battery can be discharged in percent (%). Lithium-ion batteries can be discharged down to 10%. Lead-acid batteries can be discharged down to 50%. This affects the amount of real energy a battery can deliver (true battery size).

• For example, a 105Ah lithium-ion battery can deliver 94.5Ah of real energy. A 105Ah lead-acid battery can deliver only 52.5Ah of real energy. This will determine the amount of battery reserve required to run your loads.

Temperature Stability - Batteries have an optimum operating temperature for charging and discharging. Lithium batteries cannot stand a charging current at freezing temperatures. Whereas, lead-acid batteries are more temperature tolerant.

Life span - Batteries derate over time and have an active life span and a static life span. Lead-acid batteries can have a static life span of 20 years. Lithium-ion batteries have an active life span of 15 years. For an application such as a telecom site where batteries are subjected to extreme temperatures and are rarely used, lead-acid batteries are the best application. However, when large amounts of energy must be charged and discharged repeatedly in systems such as a backup or off-grid system, lithium-ion batteries are superior.

This is a primary consideration and may have already been made for you. If you have a PV system feeding the utility then you’re already grid-tied.

How long can you run your home or

business on a battery? This will depend on the loads you need to run and for

how long. This will probably become a smaller number when you realize the

amount of energy you really consume added to the equation: Runtime in

hours = (((true battery size kWh (see "Number of Charging and Discharging Cycles")* [conversion efficiency/100]) /

24hr consumption in kWh) * 24)

○ How will you manage your loads? Now that you are the generator of the

energy that you consume, you will quickly realize that the lights do not

need to stay on all day long. Your installer/designer may recommend a

smart electrical distribution box for your system or you may manually

turn things off. Make your load list.

○ What is the total amount of battery storage required to run those loads

when solar is not available? = (Minimum panel runtime hours * hourly

loads (kWh) * power conversion efficiency (0.85 - 0.95))

• If you're planning to run non-resistance loads with a high inrush current and a large power needs (air conditioning, motors, pumps) then your inverter must be capable of 2 times the maximum continuous loads for several seconds. Continuous power overhead should be 20%. 10kW loads + 20% = 12kW inverter (24kW surge)
• If you want to power a residential electrical distribution sub-panel, you must provide a 120V/240V split phase output. This is a 1-phase circuit with a center tap 120V|120V for a total of 240V.
• If you only want to run a few critical 120V circuits then you will only need an inverter suitable for 120V 1-phase.
• If you are planning to support a refrigeration system or other 3-phase motor loads, then you will need a 3-phase, 120V/208V Wye setup.

This is important because of the way the devices that convert PV power to battery (DC coupled) or directly to AC voltage (AC coupled - must manage clipping) work. Once the power need is known, an installer will use a device such as Pathfinder to determine the amount of sun irradiation at your site. This will inform your panel layout and design. Remember, your panels will need to harvest enough energy to simultaneously run loads and top off your batteries to a full charge.

Use the following rules-of-thumb when

designing out your DC coupled system:

• Know your system battery voltage. This is important for configuring the panel strings, primarily for the input voltage to the charge controller or DC/DC converter. How many panels will you need to tie in series and or parallel to achieve the effective input voltage
• The charge controller input voltage is the maximum voltage that the charge controller can manage. Your panel string output voltage must be at least 5V or more than the top voltage of the battery bank for an MPPT charge controller. A 48V battery pack has an operating voltage of 51.2V and requires a minimum input voltage of 60V when DC coupled.
• The charge controller current is the maximum constant current the charge controller can send to the battery pack during charging. Ultimately, this will determine the power (watts) available to charge the battery pack.