Getting battery sizing right is one of the most technically demanding parts of any solar-plus-storage installation. Size too small and the homeowner or business runs out of stored energy before sunrise. Size too large and the project becomes unnecessarily expensive, eroding customer satisfaction and margin alike. For installation companies quoting and designing battery systems at scale, a reliable, repeatable sizing methodology is not optional. It is the difference between profitable projects and costly callbacks.

This guide focuses specifically on sizing a battery for overnight use, meaning the battery needs to cover consumption from the point solar generation stops in the evening until it resumes the following morning. Using a structured approach, combined with the right tools such as a solar battery calculator, installation teams can produce accurate capacity recommendations every time.

Calculating your household’s overnight energy consumption

The starting point for any battery sizing exercise is understanding how much energy is actually consumed during the overnight window. This is typically defined as the period between sunset and the next morning when solar generation begins to meaningfully offset demand, often spanning eight to twelve hours depending on season and location.

The most reliable method is to pull interval data from a smart meter or energy monitor, which shows consumption broken down by hour. From this data, isolate the overnight hours and sum the kilowatt-hour (kWh) totals. If interval data is unavailable, a reasonable estimate can be built by identifying which appliances run overnight and multiplying their rated wattage by their expected run time. Key loads to account for include refrigeration, heating and cooling systems, lighting left on overnight, and any devices on standby. In commercial or industrial settings, process loads and security systems add significant baseline demand.

It is worth calculating overnight consumption across different seasons, because winter nights are longer and heating loads are higher, while summer nights are shorter. Sizing to a seasonal average rather than a worst-case scenario often leads to undersizing when it matters most.

Key battery specs that affect overnight capacity

Not all battery capacity is usable capacity. Understanding the specifications that govern real-world performance is essential before translating a consumption figure into a battery size recommendation.

Usable capacity versus nominal capacity

Battery manufacturers advertise nominal capacity in kWh, but usable capacity is what actually matters. Most lithium iron phosphate (LFP) batteries offer a usable capacity of around 90 to 95 percent of nominal, while older lithium nickel manganese cobalt (NMC) chemistries may restrict depth of discharge to protect longevity, effectively reducing usable capacity to 80 percent or less. Always base sizing calculations on the usable figure.

Round-trip efficiency

Energy is lost during both the charging and discharging process. A battery with 95 percent round-trip efficiency means that for every 1 kWh stored, approximately 0.95 kWh is available for use. Over the course of a full overnight discharge cycle, this efficiency loss compounds and should be factored into the required capacity calculation. A common approach is to divide the target overnight consumption by the round-trip efficiency to arrive at the minimum usable capacity needed.

Temperature derating

Battery performance degrades in cold temperatures. In northern European climates particularly, batteries installed in unheated spaces such as garages or outbuildings may operate at reduced effective capacity during winter months. Manufacturers publish derating curves, and these should be consulted when sizing for year-round overnight coverage.

Matching battery size to different overnight use scenarios

Overnight battery requirements vary considerably depending on the type of installation and the goals of the end user. A single sizing formula does not apply universally.

For a typical residential installation aiming to cover average overnight consumption, the calculation is straightforward: take the overnight kWh demand, divide by round-trip efficiency, and add a modest buffer of ten to fifteen percent to account for degradation over the battery’s lifetime. A household consuming 5 kWh overnight, with a battery at 95 percent efficiency, would need approximately 5.5 to 6 kWh of usable capacity as a minimum starting point.

Commercial installations introduce more complexity. A small business running refrigeration, security systems, and HVAC overnight may have a base load of 15 to 30 kWh or more. In these cases, it becomes important to also consider the battery’s continuous power output rating alongside capacity, since some loads require sustained high draw rather than just total energy volume. A battery bank sized purely on kWh may still be unable to support peak overnight demand if the inverter or battery discharge rate is insufficient.

For off-grid or backup-focused scenarios, where the battery must also account for cloudy days reducing the next day’s solar generation, a further day-ahead buffer is typically added. This is a different design brief than pure overnight coverage and requires a separate calculation layer. Tools like a battery capacity calculator built specifically for solar-plus-storage projects make it straightforward to model these scenarios without manual spreadsheet work.

Common sizing mistakes that leave batteries short

Even experienced installation teams make sizing errors that result in systems that underperform expectations. Understanding where these mistakes typically occur helps avoid them systematically.

The most frequent error is using average consumption data without accounting for seasonal variation. A battery sized on summer overnight demand will almost certainly be undersized for winter, when nights are longer and heating loads are higher. A battery runtime calculator that allows seasonal inputs produces far more reliable recommendations than a single annual average figure.

A second common mistake is ignoring future load growth. If a customer plans to add an EV charger or heat pump within the next few years, the overnight load profile will change significantly. Sizing for current consumption without any headroom means the system becomes inadequate shortly after installation, leading to dissatisfied customers and reputational risk for the installer.

Forgetting to account for battery degradation over time is another frequent oversight. Most lithium batteries lose a small percentage of usable capacity each year. A battery that meets overnight requirements at installation may fall short after five years if no degradation buffer was included in the original sizing. A general rule of thumb is to add ten percent to the calculated minimum capacity to account for this.

Finally, some installers size based on the battery product available rather than the actual requirement, leading to either over-specification that increases project cost unnecessarily or under-specification that fails the customer. The sizing calculation should always drive product selection, not the other way around.

How ERP software simplifies battery project sizing

Accurate battery sizing is only one part of delivering a profitable installation project. The challenge for growing installation companies is maintaining consistency across multiple design engineers, sales staff, and project teams, especially when quoting volumes are high and turnaround times are tight.

ERP software purpose-built for installation businesses addresses this by embedding sizing tools directly into the sales and project workflow. Rather than relying on standalone spreadsheets or disconnected calculators, design outputs feed directly into quotation documents, purchase orders, and project planning. This eliminates the manual re-entry of data between systems, which is one of the most common sources of errors in installation project delivery.

When battery sizing is integrated into the broader project management workflow, it also becomes easier to track job costing in real time. If a project requires a larger battery than initially quoted, the cost impact is immediately visible and can be addressed before it erodes margin. This kind of visibility is particularly valuable for companies managing multiple installation crews across different regions simultaneously.

How OpusFlow helps with battery project sizing and management

OpusFlow is the most complete ERP platform for sustainable installation companies, and battery projects are a core part of what we are built to support. For installation businesses scaling their solar-plus-storage operations, we provide an integrated environment where sizing, quoting, planning, and aftercare are all connected in one place.

• Integrated solar battery calculator: Our built-in battery sizing tool allows design teams to calculate required capacity based on actual consumption inputs, efficiency parameters, and seasonal variation, without leaving the platform.
• Quotation and invoicing in one flow: Sizing outputs connect directly to quotation templates, reducing manual data transfer and the errors that come with it.
• Project and crew planning: Once a battery project is sold, it moves seamlessly into planning, with tasks automatically created and assigned to the right installation crew.
• Job costing and margin visibility: Real-time cost tracking ensures that any variation from the original specification is flagged before it impacts profitability.
• Workflow automation with Toni: Our AI agent Toni handles repetitive operational steps, from sending follow-up communications to updating project status, so your team stays focused on the work that requires human expertise.

For installation companies that want to grow their battery and solar business without adding administrative overhead, OpusFlow provides the operational backbone to make that possible. Get in touch with our team to see how we can support your battery project workflow from first calculation to final sign-off.

Related Articles

• How to calculate how many hours a battery will last?
• How does the grid connection type affect which batteries you can install?
• What data do you need to collect during a site visit to size a battery correctly?
• How can installers use role-based access to protect sensitive business data?
• How does ERP software help with warranty tracking for installation work?