The size of battery backup you need depends on two things: how much power your devices draw and how long you need them to run during an outage. A small 400–650VA unit handles a modem and router for hours, while a desktop gaming PC might need 1500VA or more just to shut down safely. The key is adding up your devices’ wattage, then choosing a unit with enough headroom to handle the load without strain.
How to Calculate Your Total Load
Start by listing every device you plan to plug into the battery backup and noting its wattage. You can usually find this on a label on the power brick or the back of the device. Here are typical ranges for common equipment:
- Laptop: 45 watts
- Desktop computer with LCD monitor: 100–300 watts
- Wi-Fi router and modem: 15–30 watts combined
- External hard drive: 10–25 watts
- Standard CPAP machine: 30–60 watts (70–120 watts with a heated humidifier)
Once you have your total wattage, multiply it by 1.2 to 1.25. This 20–25% buffer is the industry-standard safety margin. Running a UPS at 100% load constantly stresses the battery and electronics, shortening the unit’s life and reducing reliability. So if your devices total 400 watts, you should shop for a unit rated for at least 480–500 watts.
VA Ratings vs. Watts
Most battery backups are sold by their VA (volt-amp) rating, not their wattage. These two numbers aren’t the same. Watts measure the actual power your devices consume, while VA measures the total electrical load the UPS can handle. The relationship between them is the power factor: watts equal VA multiplied by the power factor. For most home electronics, the power factor is around 0.9, meaning a 1000VA UPS delivers roughly 900 watts of usable power.
When comparing units, always check both the VA and watt ratings listed in the specs. A 1500VA unit, for example, typically delivers around 900–1000 watts of real power. That’s the number you match against your total device load.
Common Setups and Recommended Sizes
Internet and Networking Gear
If you just want to keep your internet running during a power outage, a small 400–650VA unit is plenty. A router and modem together pull around 15–30 watts. At that light load, even a budget 650VA UPS can keep your connection alive for 2–3 hours or longer. This is one of the most cost-effective uses for a battery backup, especially if you work from home or rely on security cameras.
Home Office With a Desktop
A desktop computer with a monitor draws 100–300 watts depending on the hardware. Add a router and an external drive, and you’re looking at 150–350 watts total. A 1000VA unit (typically around 600 watts capacity) covers a modest office setup with room to spare. At half load, you can expect roughly 15–20 minutes of runtime, which is enough to save your work and shut down gracefully.
Gaming PCs
Gaming systems are the most power-hungry home setups. A mid-range gaming PC draws 300–500 watts under load, while a high-end build with a top-tier graphics card and processor can pull 800–1000 watts during peak moments. For a gaming rig, a 1500VA UPS is the starting point. Using runtime data from a typical 1500VA unit: at a 500-watt load, you get about 19 minutes of backup. At nearly full capacity (980 watts), that drops to just 7 minutes. The goal here isn’t usually to keep gaming through an outage. It’s to avoid a hard crash that corrupts save files or damages components.
CPAP Machines
If you use a CPAP for sleep, sizing depends heavily on whether you run a heated humidifier. A standard CPAP without humidification draws 30–60 watts. A portable power station with 300 watt-hours of capacity can run a basic CPAP for one to two full nights. With a humidifier, consumption jumps to 70–120 watts, and you’ll want 500 watt-hours or more to get through a single night. Turning off the humidifier during outages is the simplest way to extend runtime dramatically. BiPAP machines sit in between at 60–90 watts.
How Runtime Changes With Load
Runtime isn’t linear. Halving your load doesn’t double your runtime; it often triples or quadruples it. Data from a standard 1500VA unit illustrates this clearly: at a 200-watt load (about 20% capacity), it runs for nearly an hour. At 500 watts, that drops to 19 minutes. At full load, you get just 7 minutes.
This is why prioritizing which devices actually need backup power matters so much. Plugging in only your computer and monitor, while leaving printers and speakers on a regular power strip, can add significant runtime to the devices that count.
Pure Sine Wave vs. Simulated Sine Wave
Battery backups produce power in different waveform shapes, and this matters more than most buyers realize. A pure sine wave UPS produces smooth, clean power identical to what comes from your wall outlet. A simulated sine wave (sometimes called “modified” or “stepped”) approximates that shape but with small jumps that create electrical noise.
Simulated sine wave units work fine for most standard electronics: routers, monitors, basic desktops, and phone chargers. But sensitive equipment can behave unpredictably on simulated power. If you’re powering a CPAP, audio equipment, or a computer with a high-efficiency power supply, a pure sine wave unit is the safer choice. The price difference has narrowed significantly in recent years, making pure sine wave the better default unless budget is very tight.
Lead-Acid vs. Lithium-Ion Batteries
Most affordable battery backups use sealed lead-acid batteries, which last 3–5 years before needing replacement. They’re heavier, charge slowly, and lose capacity faster as they age. Lithium-ion units cost more upfront but last 10–15 years, charge significantly faster, weigh less, and handle deeper discharge cycles without degrading as quickly.
For a unit that sits under your desk protecting a single computer, lead-acid is perfectly adequate and keeps costs low. Replacement batteries typically run $20–40 for smaller units. For a CPAP backup you’ll rely on nightly, or a setup where fast recharging between outages matters, lithium-ion pays for itself over time by avoiding multiple battery swaps. Once a lead-acid UPS is more than 8 years old and needs a new battery, replacing the entire unit is often the smarter move, since the electronics themselves degrade over time too.
Quick Sizing Formula
Here’s the simplest approach to get the right size:
- Step 1: Add up the wattage of every device you’ll connect.
- Step 2: Multiply that total by 1.25 for your safety margin.
- Step 3: Divide by 0.9 to convert watts to VA (since most electronics have a power factor around 0.9).
- Step 4: Choose a UPS with a VA rating at or above that number.
For example, a desktop (200W) plus monitor (40W) plus router (20W) totals 260 watts. With the 25% buffer, that’s 325 watts. Divided by 0.9, you need roughly 360VA minimum. A 500–650VA unit gives you comfortable headroom and decent runtime. If you want 30+ minutes of backup instead of 10, step up to 1000VA or higher.