A battery energy storage system stores electricity and releases it when it is more useful: to cut a building's peak demand, to keep critical loads running during an outage, or to use more of the building's own solar power. Whether storage makes sense depends on which of those jobs it is meant to do, the utility's rate structure and the building's load pattern. Storage also brings its own fire safety and code questions, which must be settled early in the design.
What storage can do
Peak shaving
Where a building pays demand charges, a battery can discharge during the short periods when the building's load would otherwise set a new peak, and recharge when load is low. This can reduce the demand charge without changing how the building operates. It works best where peaks are short and predictable, and where demand charges are a large share of the bill. See demand charges and power factor.
Backup power
With the right inverter and transfer equipment, a battery can keep selected loads running during a utility outage. For short outages and modest loads, a battery can replace or supplement a generator; for long outages, batteries are often paired with solar PV or a generator. Batteries respond almost instantly, which suits loads that cannot tolerate a break. Note that a battery used for backup must keep enough charge in reserve, which limits how much it can do for peak shaving at the same time. Batteries are generally not a substitute for code-required emergency power unless the design meets the applicable standards. See emergency power vs standby power.
Solar self-consumption
Where exported solar power is credited at a lower rate than imported power, storing midday surplus and using it in the evening can be worth more than exporting it. Where net metering credits exports at the full retail rate, there is little financial reason to store solar for this purpose alone. See net metering and microgeneration in Canada.
Time-of-use and grid services
In regions with time-of-use rates, batteries can charge at low-price hours and discharge at high-price ones. Some utilities and markets also pay for storage that supports the grid. Availability of these programs varies widely across provinces.
The parts of a system
- Battery modules, today mostly lithium-ion. Lithium iron phosphate (LFP) chemistry is common in stationary storage for its stability and cycle life.
- Battery management system (BMS) that monitors cell voltages and temperatures, balances cells and shuts the system down on faults.
- Power conversion, an inverter that charges the battery from AC and discharges back to AC. With solar, the battery may be AC-coupled, with its own inverter, or DC-coupled, sharing a hybrid inverter with the array.
- Controls that decide when to charge and discharge, using meter data, schedules or rate information.
- Enclosure, ventilation, thermal management and fire protection.
Safety and codes
Lithium-ion batteries store a lot of energy, and a failed cell can lead to thermal runaway, which can spread to neighbouring cells, produce flammable and toxic gases, and be difficult to extinguish. Good design addresses this from the start:
- Equipment certified for use in Canada, with system-level safety testing; UL 9540 and the UL 9540A test method are widely referenced for energy storage systems
- Location chosen with the fire safety plan in mind: outdoors, in a dedicated room or in a rated enclosure, with separation from exits and other occupancies
- Ventilation, gas detection or explosion control where required
- Fire detection and suppression appropriate to the system
- Clear labelling and an emergency response plan shared with the local fire department
- Electrical design to the Canadian Electrical Code, with installation by licensed electrical contractors under permit
Building and fire code requirements for energy storage are developing quickly, and municipal fire departments may have their own expectations. Engage the authority having jurisdiction and the fire department early, before equipment is chosen. NFPA 855 is often referenced for installation practices, but check which requirements are adopted where you are.
Sizing
A battery has two ratings that matter: power, in kW, which sets how fast it can charge or discharge, and energy, in kWh, which sets how long it can do so. Peak shaving needs enough power to cover the peak above the target, and enough energy to cover the peak's duration. Backup needs enough power for the critical loads and enough energy for the outage you plan to ride through. Usable energy is less than nameplate, because the system keeps a reserve to protect the battery, and capacity declines with age and cycling.
Questions to settle before choosing equipment
- What is the main job: peak shaving, backup, solar self-consumption or a mix, and which takes priority?
- What does twelve months of interval data show about the size and length of the peaks?
- Which loads must stay on in an outage, and for how long?
- Where can the system go, given fire separation, access and ventilation needs?
- Who will monitor the system, respond to alarms and maintain it?
A typical case
A grocery store has a short, sharp demand peak on summer afternoons when refrigeration and cooling peak together, and a demand charge that makes up a large part of its bill. Interval data shows the peak lasts for a limited time each day. A battery sized to cover the top of that peak for its duration could hold the billed demand down. The analysis also shows a complication: the store's tariff includes a ratchet, so a single day when the battery fails or runs out of charge would set the demand charge for months. The design adds a margin, and the controls are set to forecast the peak from weather and recent load. The owner also asks whether the battery could keep refrigeration running in an outage, which would require reserving charge and adding transfer equipment, changing the economics.
Do not let a battery's purpose drift. A system sized and controlled for peak shaving may have little charge left when an outage hits. If backup matters, design for it explicitly and set the controls to protect the reserve.
How ehZee can help
As part of our solar PV system design work, we assess whether storage fits your building, which job it should do, and how it should be sized and arranged with solar, the grid and any generator. We coordinate the safety design with the authority having jurisdiction from the start.
Common questions
Can a battery let my building go off-grid?
For most commercial buildings in Canada, going fully off-grid would need very large storage and generation to cover long winter periods with little sun. Batteries are usually used alongside the grid rather than instead of it.
How long do batteries last?
It depends on the chemistry, temperature, how deeply and how often they cycle, and how they are managed. Manufacturers usually warrant a number of years or a total energy throughput, with a guaranteed remaining capacity. Plan for capacity to decline over time.
Can batteries be installed indoors?
Sometimes, in a suitable room with the required fire separation, ventilation and protection. Outdoor or separate enclosures are often simpler to approve. The fire department and code requirements will guide the choice.