UPS Size Calculator
Size a UPS, estimate battery runtime, or calculate battery capacity for any load. Covers lead-acid and lithium, three UPS types, and 20+ preset devices — free, no signup.
UPS Size Calculator for Solar
Add your loads and desired runtime. Get total VA, recommended UPS size, battery runtime, and the solar panels needed to recharge the system.
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What This Tool Covers
The UPS Size Calculator determines the correct uninterruptible power supply for solar backup and battery systems. Add your loads, set the runtime you need, and it returns the required VA rating, battery count, and number of solar panels needed to recharge the system in a target window.
Inputs Accepted
- • Load list: device name, watts, VA, and quantity
- • Desired runtime in minutes
- • Battery type (AGM, lithium, gel)
- • Battery voltage and capacity (Ah)
- • Solar recharge target (hours to full recharge)
Outputs Generated
- • Total load in watts and VA
- • Calculated power factor
- • Required UPS VA rating
- • Recommended UPS size (with safety margin)
- • Battery runtime at connected load
- • Number of batteries required
- • Solar panels needed to recharge in target hours
Features
Load List Builder
Add multiple devices with individual watt and VA values. The calculator sums true power (W) and apparent power (VA) separately to derive actual power factor.
Battery Configuration
Select battery chemistry and enter voltage and Ah capacity. The tool calculates how many batteries you need in series or parallel to deliver the required runtime.
Solar Recharge Sizing
Enter a recharge window (e.g. 6 hours of sun) and the calculator tells you how many solar panels are needed to fully restore the battery bank from a discharge event.
How It Works
List All Loads
Add each device you want the UPS to protect. Enter the device name, power draw in watts, VA rating (from the device label or spec sheet), and quantity. The total W and VA are calculated automatically.
Set Runtime and Battery Type
Enter the minimum runtime in minutes you need during a power outage. Select your battery chemistry - lithium batteries have higher usable depth of discharge (80–90%) than AGM (50%) or gel (60%).
Get UPS VA Rating and Size
The calculator determines the minimum VA rating required and adds a 20–25% safety margin to arrive at the recommended UPS size. This margin prevents the UPS from running at full load continuously.
Review Battery Requirements
Based on your runtime, load, and battery specs, the tool returns the number of batteries required. It accounts for depth of discharge limits so the battery bank isn't damaged by full discharge.
Size Solar Recharge Panels
Set the number of hours available for solar recharging. The calculator returns the minimum panel wattage and count needed to fully recharge the depleted battery bank within that window.
Use Cases
Solar + Battery Backup Systems
Size the UPS component of a solar-plus-storage installation. Ensure the inverter and battery bank can handle critical loads (refrigerator, lights, router) for a defined minimum runtime.
Server Room and IT Equipment
Data centers and home server rooms need UPS protection sized to the VA rating of all connected equipment, not just watts - this tool handles both dimensions correctly.
Off-Grid System Design
Off-grid designers use this tool to size the battery bank and solar array together - ensuring the system can run critical loads through the night and recharge fully by midday the next day.
Calculation Methodology
Power Factor
PF = Total Watts ÷ Total VA
Required UPS VA
UPS VA = Total VA × 1.25 (safety margin)
Battery Energy Required
Wh = (Total W × Runtime min ÷ 60) ÷ DoD
Solar Panels to Recharge
Panels = Wh ÷ (Panel W × Recharge Hours × 0.85)
Pro Tips
Never size a UPS at 100% load. Running a UPS at full rated load reduces efficiency, generates excess heat, and shortens battery life. The calculator adds a 25% margin automatically - don't override it downward.
Use VA from the device nameplate, not watts. Motors, compressors, and power supplies have a power factor below 1.0 - their VA is higher than watts. Using only watts underestimates the UPS requirement.
Limit AGM depth of discharge to 50%. Discharging AGM batteries below 50% SOC significantly shortens cycle life. Lithium (LFP) can safely discharge to 80–90% DoD, making them more efficient per Ah of rated capacity.
Account for inverter efficiency in recharge calculations. Solar panels charge batteries through a charge controller and potentially an inverter. The 85% efficiency factor in the recharge formula covers these conversion losses - don't remove it.
Frequently Asked Questions
What is the difference between watts and VA for UPS sizing?
Watts (W) measure real power - the actual work done. VA (volt-amperes) measure apparent power, which includes reactive power drawn by motors and power supplies. UPS units are rated in VA. If your load draws 800W at a 0.8 power factor, the UPS sees 1,000 VA and must be rated at least that high.
What depth of discharge should I use for lithium vs AGM?
Lithium iron phosphate (LFP) batteries support 80–90% DoD with minimal cycle life impact. AGM batteries should be kept above 50% SOC (50% DoD maximum) to maintain longevity. Gel batteries fall between these at around 60% DoD. The calculator applies these defaults by battery type.
Can I use this for a whole-home backup system?
Yes, but whole-home backup typically means using a battery inverter system (like a Powerwall or Sol-Ark), not a traditional UPS. The calculation method is the same - total load, runtime, battery capacity, and solar recharge. Size the battery bank for the loads you actually want to back up, not every circuit in the home.
How do I find the VA rating of my devices?
Check the device nameplate or the power supply label. Many devices list both watts and VA. If only watts are listed, divide by an assumed power factor (0.7–0.8 for electronics, 0.95+ for resistive loads like heaters) to estimate VA.
What runtime is realistic for a home backup UPS?
Traditional UPS units (for IT equipment) target 5–30 minutes - just enough to save work and shut down cleanly. Solar battery backup systems target 8–24 hours for critical loads. The calculator handles both scenarios - just change the runtime input.
Does this calculator work for three-phase UPS systems?
This tool is designed for single-phase systems, which cover the vast majority of residential and small commercial applications. Three-phase UPS sizing involves per-phase load balancing and different VA calculations - use a commercial-grade sizing tool for three-phase installations.
Why does the recommended UPS size seem larger than I expected?
The 25% safety margin is intentional. Running a UPS at 80% of rated capacity rather than 100% significantly extends battery and capacitor life. It also leaves headroom for inrush current spikes when motors or compressors start, which can be 3–6x steady-state current.
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