UPS and Backup Battery

How to use

1. Pick the job. UPS for a rack or equipment sizes a sealed or serviceable unit from the connected load. Battery for a panel or power supply sizes the 12 or 24 VDC battery that holds a panel through a power loss.
2. On the equipment job, add the devices, the runtime wanted, and whether the unit is sealed or serviceable. Headroom, power factor, efficiency, depth of discharge, and derate stay at their defaults unless Advanced is opened.
3. On the panel job, enter system voltage, the total current with everything active, the runtime the AHJ, the specification, or the listing requires, and the power supply amp rating. Installed amp-hours are optional.
4. Read the headline as the minimum watt rating, or as required amp-hours at the 20-hour rating. Standard sizes list the runtime each one supports at the entered current. None of them is marked as the answer.

Assumptions

• Sealed lead-acid capacity is taken from a rate-capacity table. The labeled amp-hours are the 20-hour rating. Shorter pulls deliver a smaller fraction of that rating. The table is a typical curve; a datasheet override is in Advanced.
• The battery is treated as at its rated capacity and near room temperature unless the derate is changed. Capacity falls with age and cold.
• Inverter efficiency is treated as constant. The equipment job defaults to 85 percent. The panel job has no inverter.
• A sealed UPS is limited by its watt rating and by the manufacturer's runtime chart. The optional chart point is a straight-line estimate from one published load.
• Scope is NVR and monitor, PoE switches, access control panels, and intercom head ends. Not data-center plant sizing.
• Results are estimates. The battery datasheet, the UPS manufacturer's chart, the listing, the specification, and the AHJ govern.

FAQ

Why does a 7 Ah battery not deliver 7 amps for an hour?

Amp-hours are rated at a long, slow discharge, commonly 20 hours. A 7 Ah battery is 7 Ah over 20 hours; over one hour it gives closer to 4 Ah. That is why a panel that calculates to two hours runs for about one.

Do two 12 V 7 Ah batteries make 14 Ah at 24 V?

No. Batteries in series add voltage, not capacity. Two 12 V 7 Ah batteries are a 24 V 7 Ah bank.

Why does the manufacturer runtime chart govern a sealed UPS?

A sealed UPS with a fixed internal battery is characterized by the vendor at that inverter, that cutoff voltage, and that load curve. The optional chart point is a straight-line estimate from one published load. Runtime falls off faster than linearly at higher loads. The chart governs.

What does one published chart point actually tell you?

Usable watt-hours implied by that point equal the published load times the published hours. Dividing that figure by the entered load is a straight-line estimate. It is not a substitute for the curve at the actual load.

Who sets the standby hours on an access-control supply?

The listing, the project specification, and the AHJ. The hours on this page are whatever is entered. Locks fail safe or fail secure on power loss, and that behavior decides what the battery actually has to hold up.

What does the recharge time mean?

Current left for charging is the power supply rating minus the load current. Recharge hours are the battery amp-hours times 1.2, divided by that leftover current. When the leftover is zero or negative, no current is left for charging.