Backup Power Calculator
Estimate battery or generator runtime, check running and surge loads, and see how long essential appliances could stay powered during an outage.
Estimate Your Backup Power
Fill out your load requirements and power source to see runtime estimates, load safety, surge capacity, and longer-outage battery sizing.
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Runtime Summary
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Running Load
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Starting Surge
Peak power demand during startup
Battery Sizing For Longer Outages
Estimated usable capacity required for extended runtimes at your current -- load:
System Notes
What if I add an appliance?
Select an appliance to see how it affects your runtime. These are editable typical estimates. Warning: Compressors (like refrigerators) have highly variable startup surges.
How the Backup Power Calculator Works
This tool estimates how long your power source will last based on the combined wattage of your selected appliances. The calculations dynamically adjust depending on whether you are using a combustion generator or a battery power station.
Generator Runtime and Load Calculation
For generators, we use the manufacturer's specified 50% load runtime and tank capacity to establish a fuel consumption curve. The engine calculates an interpolated burn rate based on your exact continuous wattage load. It also checks if your generator's starting/surge rating is sufficient for your appliances.
Battery and Portable Power Station Runtime Calculation
For battery backups, the calculator applies an 80% to 85% usable/inverter efficiency factor to the rated watt-hour capacity to account for conversion and system losses. The resulting usable capacity is divided by the continuous load to estimate runtime.
What Size Generator Do I Need?
Finding your ideal generator size requires more than just adding up the continuous running wattage of your appliances. It also depends on:
- The total simultaneous running load.
- The total peak load, including any additional startup/surge demand.
- Your generator's continuous running rating.
- Your generator's peak/surge starting capability.
Merely adding running watts can severely underestimate the required size. For example, if your total running load is 2,000 watts, and one of those appliances requires an additional 1,000 watts to start, your total peak demand is 3,000 watts. The generator's surge/peak rating must exceed this 3,000-watt total without stalling. Use this generator size calculator feature to automatically compare your total peak load requirement against the generator's surge rating.
How Long Will a Battery Backup Last?
Battery runtime depends on your total usable capacity and your continuous appliance load. When calculating power station runtime, remember that not all of the rated watt-hours are usable. The calculator applies an 80% to 85% usable/inverter efficiency factor when estimating battery runtime (85% for premium power station presets and 80% for custom entries).
Actual usable capacity may vary with inverter design, battery condition, temperature, load and manufacturer specifications. For example, if you are running a 200W load on a 1,000Wh battery using an 85% efficiency factor, your usable capacity is 850Wh. Dividing 850Wh by 200W yields an estimated runtime of 4.25 hours.
Tip: If your battery is rated in Ah (amp-hours) rather than Wh, you can convert amp-hours to watt-hours to find its true energy capacity. For generator sizing, you may also need to convert kilowatts to amps or convert VA and kVA.
Running Watts vs Starting / Surge Watts
Running watts (continuous watts) is the steady power an appliance needs to operate. Starting watts (surge watts) is the temporary spike in power required to start electric motors or compressors.
Appliances such as refrigerators, freezers, air conditioners, well pumps, and sump pumps require significant additional startup capability. A standard refrigerator might draw 150 watts while running, but require over 1,200 watts to start the compressor. Your generator or battery's Surge/Peak rating must exceed this combined peak demand, accommodating the simultaneous running load plus the applicable additional startup/surge demand, or the system will overload and shut down.
How Much Backup Power Do Common Appliances Need?
Typical range — check the appliance nameplate/manual for actual wattage.
| Appliance | Typical Running Watts | Typical Starting / Surge Watts | Startup Consideration |
|---|---|---|---|
| Refrigerator | 150 - 400W | 1,200W | High Surge (Compressor) |
| Freezer | 150 - 400W | 1,200W | High Surge (Compressor) |
| Well Pump (1/2 HP) | 1,000W | 3,000W | Very High Surge (Motor) |
| Sump Pump (1/3 HP) | 800W | 1,500W | High Surge (Motor) |
| Air Conditioner (10k BTU) | 1,000W | 3,000W | Very High Surge (Compressor) |
| Furnace Blower (1/2 HP) | 400 - 800W | 1,200 - 2,400W | High Surge (Motor) |
| Microwave | 1,000W | 1,500W | Moderate Surge |
| Television (LED/OLED) | 50 - 150W | 50 - 150W | No Surge |
| Laptop | 50 - 100W | 50 - 100W | No Surge |
| CPAP Machine | 30 - 60W | 30 - 60W | No Surge |
| Wi-Fi Router | 10 - 20W | 10 - 20W | No Surge |
| LED Lighting (per bulb) | 5 - 10W | 5 - 10W | No Surge |
Generator vs Battery Power Station for Backup
Combustion Generators
- Advantages: Potentially long runtime with adequate fuel, suitable for larger sustained loads depending on their rating.
- Limitations: Require fuel storage, routine maintenance, produce noise, and emit exhaust (carbon-monoxide hazards).
Battery Power Stations
- Advantages: Quiet operation, no combustion exhaust during use, suitable for indoor use where manufacturer instructions permit, and convenient for shorter-duration backup.
- Limitations: Capacity-limited runtime, require electricity to recharge, and generally have stricter inverter output and surge limits.
Understanding Your Backup Power Results
Once you've entered your inputs, our calculator outputs several key metrics. Estimated runtime shows how long your system will last at the specified load. The continuous load and surge load meters help you visualize your system suitability and safety margin. For generators, you can estimate fuel usage over time, and for power stations, you can see the required battery usable capacity needed for extended outages.
To dive deeper into your general energy usage when the grid is online, you can analyse your home's electricity usage or calculate appliance electricity cost to see how much they consume. If you're considering charging EVs during outages, calculate EV charging cost to understand the impact on your battery runtime. For long term backup solutions, you might want to estimate solar savings for a resilient home.
Example Calculation
*This is an illustrative example of the calculator's math.
If you have a 1,000Wh battery power station and need to power a 150W refrigerator and a 50W laptop, your total continuous load is 200W. Assuming an 85% inverter efficiency, the usable capacity is 1,000 × 0.85 = 850Wh. Dividing the 850Wh usable capacity by the 200W continuous load yields an estimated runtime of 4.25 hours.
Sources & Calculation Assumptions
Our calculations rely directly on manufacturer specifications where equipment presets exist. Appliance wattages shown are reference estimates—you should always prioritize your specific appliance nameplate or manual values. Any safety guidance references recognized authorities such as the CPSC.
Calculator Formulas
Runtime and sizing estimates are generated from the calculator’s documented mathematical model.
Equipment Specifications
Where equipment presets/models are used, calculations rely on stored manufacturer specification data (such as 50% load runtime for generators or Wh capacity for batteries) available to the tool.
Appliance Wattages
Appliance defaults are estimates and reference values. You should use actual nameplate/manual wattage from your specific appliances whenever available.
Battery Assumptions
The calculator applies an 80% to 85% usable/inverter efficiency factor to simulate real-world thermal and conversion loss when converting DC battery power to AC power.
Generator Assumptions
Fuel consumption is interpolated based on your exact continuous wattage load relative to the manufacturer's rated 50% load curve. A surge check is performed by comparing your peak scenario load against the generator's surge capacity.
Limitations
Actual runtime and fuel consumption can vary because of equipment age, ambient temperature, battery degradation, generator load curve variations, and individual appliance behaviour.
Critical Safety Guidelines
Carbon Monoxide Hazard
NEVER operate a gas, propane, or diesel generator indoors, in a garage, or near open windows. Carbon monoxide is invisible, odorless, and deadly. Operate combustion generators outdoors and follow manufacturer instructions. Only use portable power stations (lithium batteries) indoors. Consult guidelines from authorities like the CPSC.
Medical Equipment Disclaimer
This calculator provides estimations only. For life-supporting or medically necessary equipment, do not rely solely on this calculator. Follow the equipment manufacturer's backup-power requirements and emergency guidance from your healthcare or equipment provider.
Weather Exposure
Do not run standard portable generators in rain or snow without a certified manufacturer-approved tent. Standard portable power stations are generally not waterproof.
Estimate Limitations
Calculations are estimates. Manufacturer specifications can differ. Aging can affect battery capacity, and temperature can affect battery performance. Inverter losses matter, and actual generator fuel consumption varies with load and equipment age. Appliance power consumption can vary significantly from typical averages.
Frequently Asked Questions
What size generator do I need for my house?
To find the right generator size, add up the continuous running watts of all your appliances, then add the single highest surge wattage. Your generator's Surge/Peak rating must exceed this total to start motor-driven appliances like refrigerators or pumps.
How do I calculate how much backup power I need?
Identify which essential appliances you need to run during an outage. Find their continuous running watts and their starting (surge) watts. Total the running watts to find your minimum continuous power requirement, and total the surge requirements to ensure your backup power source can start them.
How long will my generator run during a power outage?
Generator runtime depends on your appliance load and the fuel tank capacity. The higher your continuous running watts, the faster the generator will consume fuel. Our calculator automatically estimates this fuel curve based on your specific appliances.
How long will a battery backup or portable power station last?
Battery runtime is estimated by dividing your total usable battery capacity in Watt-hours (Wh) by your continuous appliance load in Watts, accounting for inverter efficiency losses (typically 10-20%).
How do I calculate battery runtime from watt-hours?
Take the battery's rated capacity in watt-hours (Wh), multiply by the inverter efficiency (often around 0.85), and divide by the total continuous watts drawn by your appliances to get the estimated runtime in hours.
What is the difference between running watts and starting watts?
Running watts (continuous watts) is the steady power an appliance needs to operate. Starting watts (surge watts) is the brief, higher spike of power required to start electric motors, such as those found in air conditioners or well pumps.
How much surge power do appliances such as refrigerators, pumps or air conditioners need?
Motor-driven appliances can require 2 to 3 times their running watts (or more) just to start up. For example, a refrigerator that runs on 150W might temporarily need over 1,200W of surge power to start its compressor.
What size battery backup do I need for a refrigerator?
A standard refrigerator requires a battery backup with an inverter capable of handling 1,200W to 1,500W of starting surge. A battery with a capacity of 1,000Wh to 1,500Wh can typically run a standard refrigerator for 10 to 15 hours, depending on compressor cycling.
Can a portable power station run a refrigerator during an outage?
Yes, if the portable power station's inverter rating can handle the refrigerator's startup surge and its capacity is large enough to sustain the running watts for your desired timeframe.
Should I choose a generator or battery power station for home backup?
Generators can offer prolonged runtime if fuel is available and handle larger loads, but require maintenance and must run outdoors. Battery power stations are quiet, require zero maintenance, and can run safely indoors, but their runtime is limited by their capacity without recharging.
Does inverter efficiency affect battery runtime?
Yes. The inverter within a battery power station loses some energy as heat when converting DC battery power to AC power. This efficiency loss typically reduces real-world runtime by 15% to 20%.
How accurate is a backup power calculator?
Backup power calculators provide educated estimates based on typical appliance wattages, manufacturer fuel curves, and battery capacities. Actual runtime will vary based on appliance age, cycling (like a fridge turning on/off), ambient temperature, and battery degradation over time.
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