Power Size Calculator
Estimate the right power supply, inverter, or generator size based on your expected load, startup surge, and safety margin.
Why power sizing matters
Undersized power systems fail when you need them most. Oversized systems cost more, run less efficiently, and can still be configured poorly. A simple power size calculator helps you choose a practical middle ground: enough capacity for normal operation, startup spikes, and real-world inefficiencies.
Whether you are selecting a power supply unit (PSU), inverter, UPS, or portable generator, the same planning logic applies: estimate your true load, allow for surge demand, and include a safety buffer.
What this calculator gives you
- Total running load (W): the steady-state demand from all devices combined.
- Estimated startup/surge load (W): running load plus your surge allowance.
- Recommended output rating (W): surge load plus extra margin for reliability.
- Recommended input/system size (W): adjusted for efficiency losses.
- Current draw (A): expected current at your selected voltage.
- Daily and monthly energy use (kWh): useful for utility cost planning.
How to use the calculator
1) Enter your average load per device
Use label values, meter readings, or manufacturer specs. For motors, compressors, and pumps, use realistic average running wattage rather than only nameplate maximums.
2) Add the number of devices and runtime
If all devices run together, include them all. If not, calculate each operating group separately and size for your highest simultaneous load case.
3) Set surge allowance and safety margin
Startup surge is common for refrigerators, power tools, HVAC equipment, and older motors. Safety margin protects performance as systems age, temperatures rise, and usage varies.
4) Include system efficiency
No conversion system is 100% efficient. Inverters and UPS hardware lose some energy as heat, so required input power is always higher than output power.
Core sizing formulas
- Running Load = Load per Device × Device Count
- Surge Load = Running Load × (1 + Surge %)
- Recommended Output = Surge Load × (1 + Safety Margin %)
- Required Input Size = Recommended Output ÷ Efficiency
- Current (A) = Required Input Size ÷ Voltage
- Energy (kWh/day) = Running Load × Hours ÷ 1000
Practical examples
Home office backup
Suppose you run 2 monitors, a desktop, networking gear, and lights for 6 hours daily. Even if your running load is moderate, a UPS should still include startup and safety headroom so your battery inverter is not constantly at its limits.
Workshop or garage setup
Power tools may draw high inrush current when starting. A generator sized only for running watts can stall or trip under startup load. In that case, increase surge allowance and test with real usage patterns.
Common mistakes to avoid
- Ignoring surge/inrush requirements for motors and compressors.
- Sizing based on one device instead of total simultaneous demand.
- Forgetting efficiency losses in inverters and UPS systems.
- Using no safety margin, leaving zero room for growth.
- Confusing watts (W) and volt-amps (VA) when comparing products.
Final takeaway
A good power size decision balances reliability, efficiency, and cost. Use the calculator to get a fast baseline, then compare your result to manufacturer recommendations and electrical code requirements in your area. For critical systems, always verify design details with a qualified electrician or engineer.