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Solar Utility Grid Tie Backup Systems

Design Considerations for Grid Tie
Systems with Battery Backup

Battery Backup Systems run Critical Loads when Utility Power is down.

The most important design consideration for a Backup System is isolating the Critical Loads in a subpanel that is separate from the main service panel. This requires wiring that isolates each critical device from the non-critical ones in circuits fed by the subpanel. The non-critical loads are wired to the main panel. This is easily done if its new construction, but might be problematic if existing wiring includes circuits with both critical and non critical loads.

Backup System Size:

A) Critical Loads (The devices that you don't want to lose power - Refrigerator, TV, Lights etc)

  • Add up the total wattage of your Critical Loads and divide that by 0.9 for the inverter inefficiency.
  • Decide how many hours you will need backup power
  • Multiply the two and divide by the DC Voltage of the Battery Bank (12, 24, or 48). This gives you the Amp Hours your Critical Loads will need available from the Bank.
    For Example: If the critical loads need 1800 watts for 8 hours from a 24 vdc bank, the AH would be 1800 Watts divided by .9 = 2000 Adjusted Watts x 8Hrs = 16000Whr divided by 24vdc = 667AmpHours

B) Sizing the Battery Bank

  • Multiply the Critical Load AH by 1.5 (for 80% discharge and 85% battery inefficiency). This is the minimum size of your Battery Bank in Amp Hours.
  • You should also derate for cold temperatures if the batteries are not in a heated area. (1% for each degree Celsius below 20 degrees)
  • Batteries in Series (+ to -) doubles the voltage but not the Amp Hours. Calculate the number of Batteries required in each Series by dividing the Battery Bank DC Voltage by the Battery Voltage eg 48vdc/6vdc=8 Batteries in each Series String.
  • Batteries (or series of batteries) in Parallel (+ to +) doubles the Amp Hours but not the Voltage. The Battery Bank's Amp Hours divided by the Battery's Rated AH Capacity, rounded up, gives the number of Parallel Series Strings.
  • Multiply Series by Parallel for the Total Number of Batteries required.
    For example: From above the minimum size of the Battery Bank would be 667AH x 1.5 = 1000AH. Using L-16 6vdc batteries with a capacity of 350AH, there would be 4 batteries in series to give 24vdc. 3 parallel strings (of 4 in series) would give 3 x 350AH = 1050AH or 8.4hrs of backup power from 12 batteries. (2 parallel strings of 4 would give 5.6hrs using 8 batteries) Assumes 20degC (68degF).

C) Backup Inverters

  • The older and less expensive models take the PV array power thru the same DC circuitry that an off-grid system does (Charge Controller, DC Disconnect) and that results in a net power loss in the cycling of the battery bank of around 15% and they wear out batteries faster. They also require a separate Grid Tie unit for synchronizing. These units also must detect when the Grid has a power outage and prevent the System from sending power to the Grid that might endanger repair personnel.
  • Newer and more expensive backup inverters have two systems, a dc converter/controller that keeps the bank charged and the AC Grid Tie/Subpanel circuit with a sensing switch that changes to battery power when the grid goes down feeding only the subpanel of Critical Loads. The have built in DC/AC Disconnects, Surge Protection, Ground Fault Detection Interrupts and Charge Control. They are more efficient and easier on the batteries and use low voltage PV arrays.
  • Sunny Island from SMA converts any AC input (PV, Grid, Generator) to DC only when the batteries need charging. A pass thru AC circuit allows the Grid to be used for battery charging at night if required while high voltage PV strings connected to Sunny Boy Grid Tie Inverter(s) and controlled by the Sunny Island allow PV battery daytime charging during a power outage. This system does not cycle down the batteries except when the grid goes down so the battery bank can discharge 80% of capacity without harm. That means fewer batteries and longer battery life. Sunny Island is rated at 4000W, 35A for Critical Loads. They can be stacked if more backup power is required.

Backup system components and installation costs in a Grid Tie System must be deducted from the Total System Cost in calculating the 30% Federal Tax Credit. For Information on California's Solar Rebate Program and Federal Tax Credits see the Solar Incentives Page or the California Solar Initiative Web Site.

Utility Interface System with Backup

For more information call (707) 462-2427 or E-mail Collins By Design
Cal Contractors Lic #B557124

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