Backup Runtime for an Apartment Building: How Many Hours Are Needed

How to decide how many hours are needed

The backup duration must match two things: the expected outage and the priority of the load. For different priority tiers the hours can differ: emergency lighting and low-voltage systems must run for a long time, while the elevator in full mode usually only for a limited time. This differentiation makes the project cheaper and gives residents more honest promises.

The platform lets you compare sets of loads and durations in a range from half an hour to 72 hours, including options of 2, 4, 8, 12 and 24 hours for the “critical only” and “extended” modes. It is a comparison tool, not a forecast.

The formulas behind the calculation

The platform’s methodology follows a sequence of steps. First, the electrical power of each load: for motors it equals shaft power divided by motor efficiency. Then energy as the sum of the products of quantity, power, time and duty factor. Next, the usable battery energy: the energy with reserve, divided by inverter efficiency. Finally, the nominal capacity: the usable energy divided by the permitted depth of discharge, by a factor accounting for temperature losses and by an ageing margin factor.

• Typical default values: reserve 20 percent, inverter efficiency 0.92, permitted depth of discharge 0.8, temperature loss 10 percent, degradation margin 20 percent.
• All of these values are unconfirmed assumptions; they are replaced with data from datasheets and manufacturer recommendations.
• Continuous power equals the sum of electrical powers with the simultaneity factor applied.
• Peak power is taken from the scenario “one motor starts while the others run”, and a starting safety factor is added to it.

Illustrative example: two sets of loads

Below are hypothetical inputs, invented only to illustrate the order of the calculation. Suppose there is emergency lighting of 150 W, an intercom and network node of 115 W, and a pump with an electrical power of 3.5 kW and a duty factor of 0.4. For simplicity the operating times are the same: 8 hours.

For an energy of 13,320 Wh with a 20 percent reserve and an inverter efficiency of 0.92, the usable energy approaches 17.4 kWh. With a depth of discharge of 0.8, a temperature loss of 10 percent and an ageing margin of 20 percent, the nominal capacity is about 30 kWh. If the pump is excluded, 2,120 Wh remain, and the nominal capacity drops roughly sixfold. This contrast shows why the board should not say “just a one-day backup” without choosing what exactly is being backed up.

Why real backup runtime may be lower than calculated

• Cold in the room reduces the available battery capacity, while the calculation includes only an approximate loss.
• Ageing reduces capacity, so years later the system lasts less than on the first day.
• Inrush peaks and simultaneous starting of several motors can shut down the inverter before the energy runs out.
• The load has changed: equipment has been added, the pump mode has changed.
• Unknown inputs are filled in conservatively, but not always in your favor: for example, an unmeasured duty factor may turn out to be higher.
• Uneven distribution across phases limits the power of an individual phase.

Time of day and solar panels

In the scenarios module you can enter load shares for the daytime, evening and nighttime windows and the hour when the outage begins. These shares are specified by the user: the calculator does not invent them. They affect energy but not power, so the peak remains as calculated.

Solar panels in this model do not reduce the required capacity: their output is not added until there is confirmed insolation data, and a grid-tied plant without a hybrid mode does not power the building at all during an outage. In winter and in cloudy weather the contribution of a solar power plant (SPP) is especially limited, so backup runtime cannot be based on the panels without a separate assessment.

Typical mistakes

• Taking a single “hours” figure for all loads instead of different hours by tier.
• Forgetting reserves, efficiency, depth of discharge and temperature, and getting an overstated backup runtime.
• Ignoring peaks and checking only energy.
• Promising residents specific hours although the calculation is approximate.
• Not revisiting the calculation after changes in the building.

What to prepare for a backup runtime discussion

Frequently asked questions

Can a specific number of backup hours be guaranteed?

No, the calculation is approximate. The actual time depends on the load, temperature, battery condition and settings, so promises to residents should be worded carefully and refer to the calculation.

Why does the calculator result show a range rather than a single number?

Input data is often incomplete, and margins and efficiency depend on the equipment. A range shows the uncertainty more honestly and suggests which data to clarify first.

Next steps

Battery capacity calculatorPreliminary backup power calculation