Battery Backup Sizing Calculator

UPS Sizing Guide: Choosing the Right Battery Backup

An uninterruptible power supply (UPS) protects your equipment from power outages, voltage sags, and electrical noise. But choosing the right size can be confusing. Too small, and it won't provide enough runtime or may overload. Too large, and you've wasted money. This guide helps you find the sweet spot.

Understanding UPS Ratings: VA vs. Watts

UPS units are rated in two ways:

• VA (Volt-Amps): Apparent power - the maximum the UPS can deliver before overloading
• Watts: Real power - the actual power your devices consume

These aren't the same due to something called "power factor." Most consumer UPS units have a power factor of 0.6, meaning a 1000VA UPS delivers only 600W of real power. Higher-end units may have 0.8 or even 0.9 power factor.

Example: A 1500VA UPS with 0.6 power factor = 900W of usable power

The UPS Sizing Formula

To size a UPS, follow these steps:

1. Add up your device wattages - Check power supply labels or use our calculator
2. Add 25-30% headroom - For startup surges and future expansion
3. Convert to VA - Divide by power factor (usually 0.6)

Example: A 300W load needs (300 x 1.3) / 0.6 = 650VA minimum. An 850VA UPS provides comfortable headroom.

Runtime Expectations

Runtime decreases dramatically as load increases. A UPS at 50% load lasts far longer than at 100% load. Here's a general guide:

Key insight: If you need longer runtime, it's often more cost-effective to get a larger UPS than to run your current UPS at near-maximum capacity.

Pure Sine Wave vs. Simulated Sine Wave

UPS units produce two types of output waveforms:

Pure Sine Wave

Produces a smooth, clean waveform identical to utility power. Required for:

• Computers with Active PFC power supplies (most modern PCs built after 2010)
• Sensitive audio/video equipment
• Medical devices
• Variable speed motors

Simulated (Stepped/Modified) Sine Wave

Produces a stepped approximation of a sine wave. Acceptable for:

• Basic computers with older power supplies
• Simple electronics (routers, modems, LED lights)
• Devices that don't require clean power

When in doubt, choose pure sine wave. The price difference is modest, and it ensures compatibility with all equipment.

What NOT to Connect to a UPS

Some devices should never be connected to the battery-backed outlets:

• Laser printers: Draw 1000-1500W when printing, can overload UPS
• Space heaters: High continuous draw depletes battery instantly
• Vacuum cleaners: Motor startup can overload inverter
• Coffee makers: Heating elements draw too much power
• Hair dryers: High wattage, not worth protecting

Most UPS units have "surge only" outlets for these devices—they get surge protection but no battery backup.

UPS Types Explained

Standby (Offline) UPS

Basic protection. Switches to battery when power fails, with a 5-12ms transfer time. Suitable for: Home computers, basic equipment.

Line-Interactive UPS

Mid-range protection. Regulates voltage without battery drain, 2-4ms transfer time. Suitable for: Home offices, small servers, gaming PCs.

Online (Double-Conversion) UPS

Premium protection. Always runs from battery while continuously charging. Zero transfer time. Suitable for: Servers, critical equipment, medical devices.

For most home and office use, line-interactive UPS provides the best balance of protection and value.

UPS Maintenance Tips

• Test monthly: Use the self-test button to verify battery health
• Replace batteries every 3-5 years: Even if the UPS still works, capacity degrades
• Don't overload: Running above 80% capacity shortens battery life
• Keep cool: Heat is the enemy of batteries. Don't enclose in cabinets
• Update firmware: Newer UPS models often receive updates for better performance