Sighten now offers battery modeling! You can add DC and AC batteries to projects and model storage behavior to estimate overall homeowner savings for the project.

Battery Modeling Overview

Battery toolset allows users to add storage to a project with charge/discharge behavior, solar-supply, and grid-supply power taken into account. 

• The daily usage chart on the Project page allows users to see how the battery will meet usage needs.
• Admins have control over what batteries are available under Settings and can add any batteries they're using themselves without needing to wait for Sighten to make updates.

Battery modeling – there are three ways to model storage charge and discharge schedules through Sighten: PV Self-Consumption, Advanced TOU Self-Consumption, and Back-up Power.

1. PV Self-Consumption: whenever the PV system generates more kWh than is needed for the load, then all excess energy is sent directly to the battery. Once the battery has reached capacity, all access energy will be sent to the grid. When the PV system does not meet the need of the load, then the battery will discharge the exact amount of kWh required to meet the load. The battery will discharge until the minimum state of charge bound is reached.
2. Advanced TOU Self-Consumption: Production is used to fully charge the battery during all off-peak hours. The home is 100% powered by grid during this time until the battery is 100% charged.  As soon as on-peak or rates occur, the battery discharges to serve 100% of the homeowner’s consumption during the peak window(s). All production will go directly back to the grid during this time.
   
   1. If battery cannot meet 100% of consumption needs, production will be applied to offset additional consumption to get to 100%.  
   2. If battery meets 100% of consumption needs, production will sent back to the grid.
3. Back-up Power: The battery will charge from excess production but there will be no discharge from the battery.  This will ensure that the battery is always available to discharge during unpredictable blackouts or for other circumstances.

Stringing – All DC batteries will require an inverter. It can be strung with its own inverter (AC-coupled) or strung together with an existing project inverter (DC-coupled).  

• Storage - AC - no inverter selection required. It is assumed that the inverter is included in the battery, so there is no option to add an inverter for AC batteries.
• Storage - DC - includes inverter selection in the "add battery" section or you can use an inverter added in the "inverter" section.

Battery chart - daily - the daily usage chart shows consumption, solar production, battery charging, and battery discharging behavior. Add a battery, click Calculate, and then use this chart to determine how much of the consumption is met by the discharging battery vs the grid.

Battery sizing - generally with residential, 1 or 2 batteries will meet the needs of most homes when the goal is PV self-consumption (storing the daytime solar production for use in the evening). The daily chart can be used to compare what usage will be with different numbers of batteries - add a 2nd battery if there is a lot of usage in the evening / at night that cannot be met by the battery supply. You will then be able to see how much of the evening usage is covered by the discharge of the additional battery vs pulled from the grid. If there is no noticeable difference with the additional battery, it is likely that the PV system must be larger to charge the additional battery.

Multiple batteries – when the battery count is incremented a battery of the same manufacturer and model are added to the project, then they are considered to be stacked.  

• If two AC-coupled battery systems are added to the project of different manufacturer or model, the more efficient battery will be charged first; the same applies to DC-coupled battery systems.  
• If DC-coupled and AC-coupled battery systems are both added to a project, then the DC-coupled battery system will be charged first, and the AC-coupled battery system will be discharged first.

State of charge - there are bounds on the state of charge to ensure a longer life for the battery as this is used in real-world applications to prevent degradation. The bounds are default percentages for charging and discharging the battery, and a battery's usable capacity automatically factors this in.

Losses – each battery has its own round-trip efficiency that is applied when discharging the battery. Inverter efficiency and loss is applied to DC-coupled DC batteries on discharge, applied to AC-coupled DC batteries on charge and discharge, and not applied to AC batteries a it is assumed to be included in the AC battery’s round trip efficiency.  

• For DC-coupled batteries we reverse the impact of the inverter losses on the AC solar generation to approximate DC excess solar power flowing to the battery directly from the solar system when charging.
• There is no thermal modeling, so batteries stored indoors and outdoors will be modeled identically.

Degradation – this is not currently modeled. All calculations assume the battery does not degrade over time, and thus the capacity remains constant through the years. In real-world applications, batteries degrade over time implying a decrease in savings and more production being delivered to the grid.

Chemistry Agnostic – batteries of different chemistries, ie Lithium-ion and Lead Acid, are modeled identically for charge/discharge, capacity, and losses.

Methodology & Limitations

• Backup power & "islanding" - use cases for 2+ batteries can arise if the homeowner has a big consumption load (much higher than typical) or want to isolate loads from the grid ("islanding" or off-grid applications). For off-grid or large backup power applications, there are limitations both for the battery technologies and our modeling, which is designed for grid-tied systems.
• No battery degradation is modeled. The tool does not model battery degradation - it assumes capacity stays the same over the years. In reality, batteries degrade and the rate at which capacity degrades depends on charge and discharge behavior, including depth of discharge. Sighten modeling is based on best-practice cycling and depth of discharge assumptions, which protect battery life over time when followed.
• No battery thermal modeling - this is the temperature's impact on battery performance. The tool has no thermal model - batteries stored indoors and outdoors are modeled identically.
• No difference in battery modeling based on battery chemistry/battery type. The tool is chemistry-agnostic, meaning batteries of different chemistries, ie Lithium-ion and Lead Acid, are modeled identically for charge/discharge, capacity, and losses.

Related Articles

You might also want to check out these related articles:

• Getting Started
• How do I add battery storage?
• What does the proposal look like?