To calculate how many hours a battery will last, divide its energy capacity in watt-hours (Wh) by the power draw of the connected load in watts (W). For example, a 10,000 Wh home battery powering a 1,000 W load will last approximately 10 hours. The actual runtime is always somewhat lower once you factor in efficiency losses and depth of discharge limits.

This calculation applies to any battery system, from small portable units to large residential and commercial energy storage installations. The sections below walk through each variable in the formula, explain common sources of runtime loss, and help you build a more accurate picture of real-world battery performance.

What formula is used to calculate battery runtime?

The core formula for calculating battery runtime is: Runtime (hours) = Battery Capacity (Wh) / Power Consumption (W). This gives you the theoretical maximum number of hours a fully charged battery can sustain a given load. For a 5,000 Wh battery powering a 500 W load, the result is 10 hours.

In practice, you will also want to apply an efficiency factor and a usable capacity factor to get a realistic estimate. A more complete version of the formula looks like this:

1. Determine the battery’s total capacity in watt-hours (e.g. 10,000 Wh)
2. Multiply by the usable depth of discharge (e.g. 90% for a lithium battery = 9,000 Wh usable)
3. Multiply by the system efficiency factor (typically 90-95% for modern inverters)
4. Divide the result by the connected load in watts

Using a battery calculator simplifies this process considerably, especially when loads vary throughout the day or multiple appliances are running simultaneously.

What are watt-hours and why do they matter for battery life?

Watt-hours (Wh) is the unit that measures how much energy a battery can store and deliver over time. One watt-hour means the battery can supply one watt of power for one hour. It is the single most important specification when evaluating how long a battery will last, because it directly determines the total energy available before the battery needs recharging.

Battery capacity is sometimes listed in amp-hours (Ah) rather than watt-hours. To convert, multiply the amp-hour rating by the battery’s voltage: a 200 Ah battery at 48 V has a capacity of 9,600 Wh. For home battery systems paired with solar panels, capacities typically range from around 5,000 Wh to 20,000 Wh or more for larger installations.

Understanding watt-hours matters not just for runtime calculations but also for sizing decisions. If a household or commercial facility has a daily energy demand of 15,000 Wh, a single 5,000 Wh battery will not cover it without recharging mid-cycle. Accurate watt-hour figures are therefore the foundation of any reliable energy storage design.

How does power consumption affect how long a battery lasts?

Power consumption has a direct and proportional effect on battery runtime. The higher the load in watts, the faster the battery depletes. Doubling the power draw cuts the runtime in half. This is why understanding the actual consumption of connected devices is just as important as knowing the battery’s capacity.

In residential and commercial settings, power consumption is rarely constant. Appliances cycle on and off, and peak loads can be significantly higher than average loads. To calculate a realistic runtime, it helps to estimate an average load rather than relying on the peak wattage of individual devices. You can do this by listing all connected loads, noting their wattage and the number of hours they run per day, and summing the totals.

For installation companies designing battery systems for clients, accurately modelling consumption patterns is a critical step. Underestimating load leads to undersized systems and dissatisfied customers. Overestimating leads to unnecessary costs. Precise load analysis sits at the heart of every well-executed battery installation project.

Does battery depth of discharge change the runtime calculation?

Yes, depth of discharge (DoD) directly changes the usable energy available and therefore the runtime. Depth of discharge refers to the percentage of a battery’s total capacity that can be safely used before recharging. A battery with a 90% DoD rating means only 90% of its stated capacity is available for use; discharging beyond that threshold risks accelerating degradation or triggering the battery management system’s protection cutoff.

Different battery chemistries have different recommended DoD limits. Lithium iron phosphate (LFP) batteries, which are common in modern home storage systems, typically support a DoD of 80 to 100%. Older lead-acid batteries are often limited to 50% to preserve cycle life. When calculating runtime, always use the usable capacity rather than the total rated capacity.

For example, a 10,000 Wh battery with an 80% DoD has 8,000 Wh of usable energy. At a 1,000 W load and 95% inverter efficiency, the realistic runtime would be approximately 7.6 hours, not the 10 hours the raw capacity figure might suggest. Ignoring DoD is one of the most common errors in battery sizing calculations.

How long does a typical home battery last on a full charge?

A typical home battery in 2026 lasts between 8 and 16 hours on a full charge under average household consumption, though this varies significantly based on system size and load. Most residential battery installations range from 5,000 Wh to 15,000 Wh, and average household power consumption during evening hours typically falls between 500 W and 1,500 W.

At the lower end, a 5,000 Wh battery with 90% usable capacity powering a 600 W average load will last roughly 7.5 hours. A 13,500 Wh system under the same conditions would last over 20 hours. For commercial or industrial properties with higher baseline consumption, runtime expectations are naturally lower unless larger battery banks are installed.

It is also worth noting that home batteries are rarely used in isolation. Most systems are paired with solar panels, which recharge the battery during daylight hours and extend its effective coverage. In a well-designed solar-plus-storage system, the battery does not need to cover the entire night on a single charge alone.

What factors reduce a battery’s actual runtime below the calculated value?

Several real-world factors cause actual battery runtime to fall short of the theoretical calculation. The most significant are inverter efficiency losses, temperature effects, battery aging, and inaccurate load estimates. Understanding these factors helps set realistic expectations and design more robust systems.

• Inverter efficiency: Most inverters operate at 90 to 97% efficiency, meaning a portion of stored energy is lost as heat during the conversion from DC to AC power. This loss should always be included in runtime calculations.
• Temperature: Battery performance degrades in both high and low temperatures. Cold conditions in particular reduce available capacity, sometimes by 20% or more in extreme cases. Installations in colder climates need to account for seasonal variation.
• Battery aging: Over time and charge cycles, battery capacity gradually declines. A battery rated at 10,000 Wh at installation may deliver noticeably less after several years of use. Manufacturers typically specify capacity retention over a given number of cycles.
• Self-discharge: Batteries lose a small amount of charge simply by sitting idle. While modern lithium batteries have very low self-discharge rates, this is still a minor factor in long-term storage scenarios.
• Load variability: Sudden spikes in demand from high-draw appliances can cause the battery management system to throttle output, affecting effective runtime in ways a simple average-load calculation does not capture.

For installation companies, accounting for these variables during the design phase leads to more accurate proposals, fewer post-installation complaints, and stronger client relationships.

How OpusFlow Supports Battery System Calculations and Installation Workflows

Accurate battery calculations are only one part of running a successful installation business. Managing the full workflow from initial assessment and system design through to quotation, planning, and aftercare requires tools that keep every step connected. That is where OpusFlow comes in.

OpusFlow is the most complete ERP platform for sustainable installation companies, built specifically for businesses working with solar panels, heat pumps, home batteries, and EV chargers. Here is how we help installation companies work smarter:

• Integrated battery calculator: Our solar battery calculator helps your team produce accurate system sizing and runtime estimates directly within the platform, without switching between separate tools.
• Calculation and quotation module: Translate battery sizing outputs directly into professional, accurate quotes for clients, reducing manual work and pricing errors.
• Project and planning management: Schedule installation crews, track project milestones, and manage procurement for battery systems all within one platform.
• Workflow automation with Toni: Our AI agent Toni automates repetitive steps across the sales and installation pipeline, from creating follow-up tasks when a deal advances to triggering purchase orders when stock runs low.
• Customer portal: Give your clients visibility into their project status and documentation, building trust and reducing inbound support requests.

Whether you are a mid-size installation company looking to scale or a larger enterprise managing dozens of battery projects simultaneously, OpusFlow replaces the fragmented mix of spreadsheets and standalone tools with a single connected platform. Get in touch with our team to see how OpusFlow can streamline your battery installation operations from first calculation to final sign-off.

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