Three calculators in one: size your UPS in VA, estimate runtime in minutes, or reverse-engineer the battery capacity you need for any target backup duration.
An Uninterruptible Power Supply (UPS) is a battery backup device that provides emergency power when the main power supply fails. UPS units are rated in VA (Volt-Amperes), not Watts — but most devices are labeled in Watts. This disconnect confuses buyers and leads to undersized (dangerous) or oversized (wasteful) UPS purchases.
This free UPS size calculator bridges that gap. Enter your devices in watts, and we calculate the VA rating you need, recommend the right UPS tier, and optionally estimate how many minutes you will run on battery — or reverse-engineer the battery capacity needed for any target runtime.
For solar professionals, the Solar Bridge panel shows how to replace short-term UPS backup with a permanent solar + battery system — the right solution when days of autonomy matter more than minutes.
UPS Sizer gives you the VA recommendation. Runtime Estimator tells you how long you will last. Battery Sizer shows what you need for a target duration. All three share the same device load table.
One-click add for desktops, servers, NAS systems, IP cameras, CPAP machines, and more. All preset wattages are sourced from real product specifications and datasheets.
Calculations update instantly as you type — no submit button required. The Solar Bridge panel updates across all three modes to show what a permanent backup system would look like.
Protecting network closets, small data centers, and rack equipment. Use Mode 1 for initial sizing and Mode 2 to verify runtime for a graceful shutdown before generator pickup — typically 5–15 minutes.
Avoid losing work during brief outages with a properly sized UPS. Solar professionals use Mode 3 to reverse-size batteries for critical loads, then use the Solar Bridge panel to design a permanent backup system.
NVR/DVR systems need 4–8 hours of runtime to capture outage footage. CPAP machines and oxygen concentrators require uninterrupted power. Use Mode 3 for the exact battery size you need.
Choose UPS Sizer to get a VA recommendation, Runtime Estimator to find out how long your current UPS will last, or Battery Sizer to reverse-engineer the battery you need for a target runtime. You can switch modes without losing your device list.
Click a category (Computers, Networking, Security, Medical, Business), then click a device chip to add it to your load table. Each click adds a row. Adjust the quantity for multiple units of the same device. Use “Add Custom Device” for anything not in the presets.
For UPS Sizer: set your load power factor (0.95 for modern PCs) and the maximum load percentage (80% is standard). For Runtime Estimator: enter battery voltage, Ah rating, number of batteries, and UPS type. For Battery Sizer: set your target runtime, battery voltage, and depth of discharge.
Mode 1 shows your recommended VA tier and UPS type. Mode 2 displays estimated runtime with a color-coded status badge (Critical / Marginal / Good / Extended). Mode 3 presents a battery options comparison table with the most practical option highlighted.
Scroll below the results to see the Solar Bridge panel. It shows your critical load, the battery capacity needed for 4 hours of backup, and the solar array size required to recharge it daily — your starting point for permanent solar + battery backup design with SurgePV.
The next standard VA tier above your calculated minimum. Standard tiers: 350, 450, 600, 750, 850, 1,000, 1,500, 2,000, 3,000, 5,000, 10,000, 20,000 VA.
Your total watts divided by your load power factor. This is the theoretical minimum VA before applying the headroom buffer.
How much of the recommended UPS capacity your devices actually use. Staying below 80% is the industry standard — it provides headroom for startup surges and extends UPS life.
Standby for home loads under 500W; Line-Interactive for most IT and office environments (500W–3kW); Online/Double-Conversion for critical or high-load environments above 3kW.
Minutes and seconds of backup available from your battery. Calculated as: (Battery Wh × UPS Efficiency) ÷ Load Watts × 60.
Critical (<5 min): not enough for graceful shutdown. Marginal (5–15 min): save work and shut down. Good (15–60 min): extended work or generator transfer. Extended (60+ min): medical or critical loads.
Gross battery capacity needed, including losses from UPS efficiency and depth of discharge. This is what you need to shop for.
Shows batteries needed for each common Ah size (9, 18, 35, 75, 100 Ah) at your selected voltage. The highlighted row is the most practical option (4 or fewer batteries).
All calculations in this tool follow established industry standards from IEEE, IEC, and leading UPS manufacturers including APC, Eaton, and Schneider Electric.
IEEE Std 446 — Recommended Practice for Emergency and Standby Power Systems for Commercial and Industrial Applications.
IEC 62040-3 — UPS performance requirements and testing methodology.
NFPA 111 — Standard on Stored Electrical Energy Emergency and Standby Power Systems. The 80% maximum load recommendation is consistent across APC, Eaton, and Schneider Electric application notes.
Worked example: A server room with 2 servers (400W each), network switch (50W), firewall (30W). Total load: 880W. Target runtime: 15 minutes. UPS capacity: 880W × (15/60) = 220 Wh, plus 20% margin = 264 Wh. VA rating: 880W / 0.8 power factor = 1,100VA. Select a 1,500VA / 900W UPS with at least 300 Wh battery capacity. Lead-acid degrades ~20% at 25°C — upsize by 1.25× for warm server rooms.
Calculations sourced from SurgePV’s UPS Calculator — surgepv.com/tools/ups-calculator/
Use this table as a starting point. The UPS Size Calculator above will give you a precise recommendation based on your actual devices.
| UPS VA Tier | Typical Use Case | Max Watts (80% rule) | Typical Runtime (50% load) | UPS Type |
|---|---|---|---|---|
| 350–450 VA | Single PC or NAS | ~280–360 W | 5–10 min | Standby |
| 600–750 VA | PC + monitor + router | ~480–600 W | 8–15 min | Standby / Line-Int. |
| 850–1000 VA | Small server or workstation | ~680–800 W | 10–20 min | Line-Interactive |
| 1500 VA | Mid-range server + switch | ~1,200 W | 12–25 min | Line-Interactive |
| 2000–3000 VA | Small server room (2–4 servers) | ~1,600–2,400 W | 15–30 min | Line-Int. / Online |
| 5000–10000 VA | Mid-size data center row | ~4,000–8,000 W | 10–20 min | Online Double-Conv. |
| 20000 VA+ | Enterprise / critical infrastructure | 16,000 W+ | Variable (EBM) | Online Double-Conv. |
A UPS loaded at 100% of its VA rating runs hot, shuts down faster on battery, and leaves no headroom for motor startup surges. The industry standard is to keep actual load at or below 80% of rated VA. For server rooms with unpredictable growth, size to 60%.
Every 8°C above 25°C (77°F) cuts lead-acid battery life in half. If your UPS is in a hot closet or machine room, expect to replace batteries every 1–2 years instead of 3–5. Ensure adequate ventilation and avoid placing the UPS directly on the floor.
Standby and Line-Interactive UPS types have a transfer time (2–8ms) when switching to battery. Modern computers handle this fine, but older PLCs, industrial equipment, and some medical devices cannot tolerate any interruption. Online/Double-Conversion has zero transfer time.
Every watt on the UPS reduces runtime for critical loads. Separate your loads: protect the server, the monitor, and the network switch. Connect non-critical items (desk lamps, phone chargers, speakers) to a separate power strip. Smaller load = longer runtime.
VA (Volt-Amperes) is apparent power — it accounts for the phase difference between voltage and current in AC circuits. Watts is real power (what devices actually consume). VA = Watts / Power Factor. For modern computers with active PFC, power factor is ~0.95–1.0, so VA and watts are nearly equal. Older equipment and mixed loads may have power factors of 0.70–0.85, meaning VA is significantly higher than watts.
Runtime depends on your actual load and battery capacity, not just VA rating. A 1000VA UPS with a typical 9Ah/12V internal battery (108Wh): at 500W load (~50% capacity) = about 10–12 minutes at 90% efficiency. At 200W load = about 30 minutes. Use the Runtime Estimator tab above with your actual battery specs for a precise answer.
Line-Interactive is the best value for most users — it handles power fluctuations with AVR (automatic voltage regulation) and provides clean power from battery during brief outages. Online/Double-Conversion is best for sensitive electronics or always-critical loads (medical, data centers) — zero transfer time, but more expensive and generates more heat.
Yes, many UPS models (especially rack-mount and tower units in the 1500VA+ range) support external battery modules (EBMs). Check your specific UPS model's specifications. Mode 3 (Battery Sizer) in this calculator shows how many batteries you would need for any target runtime.
A UPS provides short-term backup (minutes to hours) from a pre-charged internal battery, designed to bridge power outages until grid power returns or a generator starts. Solar battery backup recharges from the sun and can provide days of autonomy — a permanent, self-sustaining system. SurgePV helps you design solar + battery systems for long-term critical load backup.
Never connect: laser printers (spike on startup, can overload UPS instantly), space heaters or resistive heating elements (too high wattage), power tools, or vacuum cleaners. These devices have high startup currents that exceed most UPS output ratings and can damage the UPS.
Lead-acid UPS batteries typically last 3–5 years depending on temperature and discharge cycles. Signs it's time: UPS beeps continuously on battery, runtime has dropped significantly, battery indicator shows fault. Most UPS manufacturers sell replacement battery cartridges that are user-installable.
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