⚡ UPS Tool · Updated 2026
UPS Battery Backup Calculator 2026
Find the exact UPS size and battery capacity you need — click your devices and get an instant recommendation.
🔋 Instant VA + Ah sizing
⏱️ Real runtime estimate
🛡️ 25% headroom built-in
⚡VA + Watt Calc
Converts watts to VA with correct power factor for accurate sizing
⏱️Runtime Estimate
Calculates backup minutes based on load, battery Ah and efficiency
🔋Battery Age Factor
Adjusts for degradation across 1, 2 and 3+ year battery lifespans
🛡️Headroom Buffer
Adds 25% capacity headroom so the UPS never runs at 100% load
What Size UPS Do I Need?
Quick Answer: Add up the wattage of all devices you want to protect. Divide by your power factor (0.8 for most devices) to get VA, then multiply by 1.25 for a 25% headroom buffer. A 300W load needs a minimum 375W / 470VA UPS — the nearest standard size is 650VA or 1000VA.
The most common mistake when buying a UPS is matching watts to VA without accounting for the power factor. A 1000VA UPS typically delivers only 600–700W of real power. Always verify the watt rating on the product spec sheet before purchasing.
For home offices, a 1000VA–1500VA line-interactive UPS covers a PC, two monitors, and a router with 10–15 minutes of runtime. For home servers or NAS arrays, step up to 2000VA–3000VA to handle spin-up surge current.
UPS Size Reference — Load vs Runtime Chart
How to read this table: Find your approximate load in watts on the left, then cross-reference with your target backup time. Values assume a new lead-acid battery and line-interactive efficiency (90%).
| Connected Load | 5 min | 10 min | 30 min | 1 hour |
|---|
| Under 100W (router + hub) | 500VA | 500VA | 650VA | 1000VA |
| 100–200W (PC + router) | 500VA | 650VA | 1000VA | 1500VA |
| 200–400W (desktop + monitor + router) | 650VA | 1000VA | 1500VA | 2000VA+ |
| 400–600W (workstation + NAS) | 1000VA | 1500VA | 2000VA | 3000VA+ |
| 600–1000W (gaming PC + displays) | 1500VA | 2000VA | 3000VA | 5000VA+ |
| 1000W+ (server rack / home lab) | 2000VA | 3000VA | 5000VA+ | EBP needed |
How Is UPS Battery Runtime Calculated?
Runtime formula: Runtime (min) = (Battery Ah × 12V × Efficiency × Condition) ÷ Load Watts × 60. A 9Ah battery at 12V = 108Wh. Running a 200W load at 90% efficiency gives: 108 × 0.9 ÷ 200 × 60 = ~29 minutes.
UPS runtime is not linear — it drops faster at higher loads. A UPS running at 50% capacity lasts more than twice as long as one at 100% load. This is why adding 25% headroom both protects equipment and meaningfully extends runtime.
Battery voltage matters too. Most small UPS units use 12V sealed lead-acid (SLA) batteries. Larger rack-mount units string multiple 12V batteries in series. Online UPS units with extended battery modules (EBMs) can achieve 2–8 hours at moderate loads.
Common Device Wattage Reference
Tip: Check the power label on the back of each device. If only amps are listed, multiply: amps × 120V (US) or amps × 230V (UK/EU) to get watts.
| Device | Typical Wattage | UPS Safe? |
|---|
| 🖥️ Desktop PC (office, no GPU) | 60–120W | ✔ Yes |
| 🎮 Gaming PC (RTX 4070 class) | 300–500W | ✔ Yes |
| 💻 Laptop (charging) | 30–90W | ✔ Yes |
| 🖥 Monitor 24in LED | 20–35W | ✔ Yes |
| 📡 WiFi Router / Modem | 10–25W | ✔ Yes |
| 💾 NAS (2-bay, spinning) | 30–60W | ✔ Yes |
| 📺 Smart TV 55in | 60–130W | ✔ Yes |
| 📷 IP Camera NVR | 10–20W | ✔ Yes |
| 🖨️ Laser Printer | 400–900W | ✘ Avoid |
| ☕ Coffee Maker | 800–1500W | ✘ Avoid |
| 🔌 Space Heater | 1000–1500W | ✘ Avoid |
What Is the Difference Between Standby, Line-Interactive, and Online UPS?
Quick Answer: Standby UPS switches to battery only on outage (4–10ms delay, cheapest). Line-interactive also corrects voltage sags without battery — best for home offices. Online (double-conversion) always runs from battery — zero transfer time, ideal for servers and medical equipment.
For most home users, a line-interactive UPS is the best balance of price and protection. It handles the most common power problems (sags, surges, brownouts) without draining the battery, which significantly extends battery life.
Online UPS units are the right choice for home servers and NAS arrays that cannot tolerate even a 4ms power gap. They provide pure sine wave output that is safe for all equipment types, including those with active power factor correction (APFC) PSUs.
How Often Should You Replace a UPS Battery?
Quick Answer: Replace UPS batteries every 3–5 years. After 3 years, lead-acid batteries lose 20–30% capacity. Signs it is time: the UPS beeps frequently, runtime has dropped, or the self-test LED shows a fault. Most replacement batteries cost $20–$60.
Extend battery life by keeping the UPS in a cool, well-ventilated spot. Heat above 25°C (77°F) can cut battery life by up to 50%. Most UPS units have a battery replacement indicator — do not ignore it.
UPS Battery Backup Calculator — Frequently Asked Questions
What size UPS do I need for a home PC?
A typical desktop PC with monitor draws 300–500W. You need at least a 650VA / 390W UPS for basic protection and a 1000VA / 600W UPS for 10–15 minutes of runtime. Gaming PCs with a dedicated RTX GPU need 1500VA–2000VA.
How long will a UPS last on battery?
Runtime depends on UPS battery capacity and connected load. A 1000VA UPS with a 9Ah battery running a 300W load lasts roughly 15–20 minutes. Reduce the load or add external battery packs to extend runtime beyond 30 minutes.
What is the difference between VA and Watts on a UPS?
VA (Volt-Amperes) is apparent power; Watts is real power. Most devices have a power factor of 0.6–0.8, so a 1000VA UPS delivers 600–800W of real power. Always check the watt rating on the UPS label, not just the VA rating.
How often should I replace UPS batteries?
UPS batteries typically last 3–5 years. After 3 years, capacity drops to 70–80% of original. Replace when runtime drops noticeably or the self-test fails. Most UPS units alert you with a beeping pattern or LED indicator.
Can I add external batteries to extend UPS runtime?
Yes — many line-interactive and online UPS models support external battery expansion packs (EBPs). Connecting one or two EBPs can multiply runtime from 15 minutes to 1–2 hours. Confirm your UPS model supports EBPs before purchasing.
What devices should I connect to a UPS?
Connect only critical devices: PC, monitor, modem, router, NAS drive, and medical equipment. Do NOT connect laser printers, space heaters, or coffee makers — they draw massive surge current that can overload the UPS instantly.
