Measure the load
Add the operating watts of equipment that must run simultaneously.
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Backup Power Guides
Learn how to size batteries, understand watt-hours, choose an inverter and estimate real runtime without relying on optimistic marketing numbers.
Battery basicsAppliance loadsInverter sizingRuntime planning
Plan backup power in four steps
The order matters: identify the load first, then stored energy, output limits and finally runtime.
Find stored energy
Use watt-hours directly or calculate volts × amp-hours for a battery bank.
Check output limits
Continuous and startup watts must both fit the inverter or UPS.
Estimate real runtime
Apply usable capacity, conversion efficiency and idle consumption.
Foundation
Watts and watt-hours are different
Watts describe the rate at which a device uses power at a moment. Watt-hours describe energy accumulated over time. A steady 100 W device running for five hours consumes about 500 Wh before conversion losses. Start with the home appliance wattage chart when you do not know the load.
Energy used (Wh) = Power (W) × Time (hours)
Stored energy
Convert battery labels correctly
Portable power stations often state capacity in Wh. Individual batteries commonly state voltage and amp-hours. Multiply bank voltage by total bank Ah to estimate nominal energy. Batteries wired in series increase voltage but do not add amp-hours; parallel strings add amp-hours while voltage stays the same.
Nominal battery energy (Wh) = Bank voltage (V) × Bank capacity (Ah)
- Lead-acid systems often use a more conservative usable percentage to protect cycle life.
- LiFePO4 batteries typically allow a larger usable share, but follow the battery-management-system limits.
- Capacity falls with age, temperature and high discharge rates.
Power delivery
Size the inverter in watts—not runtime
The inverter rating answers whether it can power the load; the battery capacity answers for how long. Keep these questions separate. The continuous rating should exceed the simultaneous operating load with reasonable headroom. VA is not stored energy and should never be entered as Wh.
Do not assume that a large battery can compensate for an undersized inverter. An inverter can overload immediately even when the battery is full.
Motor loads
Startup surge can decide compatibility
Refrigerators, freezers, pumps, fans and air conditioners may briefly demand much more power when starting. Check the equipment documentation or use an appropriate meter with surge capture. The backup system must satisfy both continuous output and short-duration surge requirements.
- Resistive devices such as heaters usually have little motor surge but very high continuous demand.
- Electronic power supplies can have brief inrush current even when their average load is modest.
- Avoid planning around marketing peak ratings without checking their allowed duration.
Real-world estimate
Account for usable capacity and efficiency
Nameplate capacity is not fully delivered to the appliance. Depth-of-discharge limits, inverter losses and the inverter's own idle draw reduce runtime. Variable loads such as refrigerators need average consumption or a realistic duty cycle instead of continuous running watts.
Runtime = Battery Wh × Usable fraction × Efficiency ÷ (Average load W + Idle draw W)
Verification
Treat the result as a planning estimate
Measure critical equipment, confirm manufacturer specifications and leave headroom. Battery wiring, fusing, ventilation, conductor sizing and protective devices require appropriate technical judgment. For medical, safety or business-critical loads, use qualified professional advice and test the complete system before an outage.
Turn the guide into a calculation
Choose the tool matching the specification label you already have.
Capacity shown in Wh
Power Station Runtime Calculator
Estimate portable power station runtime with appliance load, inverter losses and idle draw.
Open calculator →Battery labeled V and AhBattery Backup Time Calculator
Calculate runtime for 12 V, 24 V or 48 V lead-acid, AGM and LiFePO4 banks.
Open calculator →Cycling compressor loadRefrigerator Backup Calculator
Use daily kWh or running watts and duty cycle with startup-surge validation.
Open calculator →Multiple office devicesHome Office UPS Runtime Calculator
Build a complete workstation load and check continuous and peak power.
Open calculator →Compact refrigerator planningMini Fridge Wattage Guide
Compare running watts, startup surge, daily energy use and a worked 500 Wh runtime example.
Read guide →Running watts, surge and kWhHow Many Watts Does a Refrigerator Use?
Separate compressor running power from startup surge and daily energy, then work through a realistic battery-backup example.
Read guide →kWh, Ah, DoD and backup daysHow to Size a Solar Battery Bank
Calculate nameplate kWh, amp-hours, battery count and supporting solar power with a complete worked example.
Read guide →PowerRuntime learning topics
The guides are organized into focused clusters so each future article answers one real planning question and links directly to a useful tool.
Capacity, chemistry and wiring
Watt-hours, amp-hours, series versus parallel, depth of discharge, battery aging and cold-weather capacity.
Core education clusterWatts, cycling and startup
Refrigerators, home offices, pumps, fans, CPAP equipment and other practical backup loads.
High-intent traffic clusterUPS, inverter and power stations
Continuous rating, surge limits, efficiency, idle draw and choosing capacity for a target runtime.
Commercial-intent clusterBackup power questions
How much battery backup does a home need?
It depends on which loads must operate and for how long. Add the required appliance watts, then calculate energy for the target hours with efficiency losses.
Is a power station rating accurate?
Nameplate Wh is useful, but delivered AC energy is lower because of usable-capacity limits, inverter conversion and idle draw.
What should I calculate first?
Start with the simultaneous running load and the largest expected startup demand. Then select output capacity and battery energy.
Can solar panels guarantee longer runtime?
No. Solar input varies with weather, orientation, season and charge-controller limits. Calculate battery-only runtime first, then model solar separately.