The result is a planning estimate. It is not an electrical design, a medical-device recommendation, or a safety certification. It cannot tell you whether a specific battery or generator will run a specific appliance.
The formula
For each load, the calculator multiplies its watts by the hours it is used per day, by its duty-cycle percentage, and by the number of outage days in your planning window. It then adds those values together:
required usable watt-hours = the sum of watts × hours per day × duty-cycle percentage × outage days
Outage days are the planning duration divided by 24: 0.5 days for 12 hours, 1 for 24, 2 for 48, and 3 for 72. That sum is shown as the base energy.
Two buffers are then applied, in this order:
with loss buffer = base energy × 1.15
estimated requirement = with loss buffer × 1.20
Together the buffers multiply the base energy by 1.38. The calculator shows the result in watt-hours (Wh) and kilowatt-hours (kWh), where 1 kWh equals 1,000 Wh.
Worked example
A refrigerator at 150 watts, 24 hours per day, 33% duty cycle, plus a modem and router at 15 watts, 24 hours per day, 100% duty cycle, for a 24 hour plan:
- Refrigerator: 150 × 24 × 0.33 × 1 = 1,188 Wh
- Modem and router: 15 × 24 × 1.00 × 1 = 360 Wh
- Base energy: 1,548 Wh
- With 15% loss buffer: 1,548 × 1.15 = 1,780 Wh (rounded)
- With 20% reserve: 1,780.2 × 1.20 = 2,136 Wh, or about 2.14 kWh
That lands in the 2 to 4 kWh category: several essential loads.
Why the buffers exist
15% inverter and system-loss buffer
Most battery power stations store energy as direct current (DC) and use an inverter to produce the alternating current (AC) your appliances use. That conversion, along with wiring and standby consumption, loses some energy as heat. The real loss depends on the equipment and how heavily it is loaded. The calculator uses a flat 15% as a reasonable planning allowance rather than a measured value.
20% battery reserve
Planning to run a battery all the way to empty leaves no margin for a longer outage, a colder night, or loads that run more than expected. Usable capacity can also be lower than the rated figure, and it varies with battery chemistry, age, and temperature. The 20% reserve adds headroom so the plan does not depend on every assumption being exactly right.
Continuous watts
The calculator also adds up the watts of every selected load. That is the power needed if everything runs at the same moment, and it is a useful check against a backup source’s continuous output rating. It does not include starting surge. Motors in pumps, compressors, and blowers can briefly draw several times their running watts when they start.
Backup categories
| Estimated requirement | Plain-English category |
|---|---|
| Under 500 Wh | Small electronics and lighting |
| 500 Wh to 1 kWh | Communications and modest low-power loads |
| 1 to 2 kWh | A refrigerator or similar intermittent essential load |
| 2 to 4 kWh | Several essential loads |
| Over 4 kWh | A larger battery system, generator, or a narrower load plan may be necessary |
A value exactly on a boundary falls into the lower category. For example, exactly 1,000 Wh is in the 500 Wh to 1 kWh category, and anything above 1,000 Wh is in the 1 to 2 kWh category.
The categories describe the general scale of a plan. They are not product classes and do not imply that any particular product in that range will work for you.
Why labels, manuals, and real duty cycles matter
Appliance wattage varies widely between models, sizes, and ages. The calculator’s defaults are rounded starting points chosen to make the tool usable, not measurements of any product. The most reliable figures for your home come from:
- The appliance label or nameplate. It usually lists watts, or volts and amps. Watts are roughly volts multiplied by amps. Nameplate figures are often maximums, so actual average use can be lower.
- The owner’s or installation manual. It may list running watts, starting requirements, and guidance on backup power.
- Real-world duty cycles. A refrigerator, freezer, furnace blower, or pump does not run all the time. The U.S. Department of Energy suggests that, for a rough estimate, a refrigerator’s running time can be approximated as about one third of the time it is plugged in, which is why the default refrigerator duty cycle is 33%. Your appliance, room temperature, and how often doors open will change that.
Default starting values
| Preset | Watts | Hours/day | Duty |
|---|---|---|---|
| Refrigerator | 150 | 24 | 33% |
| Freezer | 100 | 24 | 33% |
| Modem and router | 15 | 24 | 100% |
| Phone charging (per phone) | 10 | 3 | 100% |
| LED lighting (per bulb) | 9 | 6 | 100% |
| CPAP machine | 50 | 8 | 100% |
| Sump pump | 800 | 24 | 10% |
| Gas furnace blower | 500 | 24 | 40% |
| Television | 100 | 4 | 100% |
| Laptop | 60 | 4 | 100% |
| Well pump | 1,000 | 24 | 10% |
Every value is editable in the calculator. Replace them with your own figures whenever you can.
Important limitations
- Starting surge is not modeled. Motors can need several times their running watts to start. A backup source can have enough energy and still fail to start a pump or compressor.
- Battery chemistry and temperature. Usable capacity, surge capability, and charging behavior differ by battery type and drop in cold conditions.
- Extension cords and outlets. Cords must be rated for the load. Undersized cords can overheat. Some appliances, such as space heaters, should be plugged directly into an outlet according to their manuals.
- Home wiring and transfer switches. Hardwired loads, such as many furnaces and well pumps, cannot simply be plugged in. Connecting backup power to home circuits requires a properly installed transfer switch or interlock and a licensed electrician. Never backfeed power through an outlet.
- Generators, ventilation, and fuel. Generator runtime depends on fuel supply and load. Generators must run outdoors only, far from doors, windows, and vents.
- Device-specific requirements. Medical devices and some electronics may have requirements that a generic estimate cannot capture, including sensitivity to the type of power an inverter produces.
- Recharging is not modeled. The estimate assumes no recharging during the planning window.
Privacy of your entries
The calculator runs entirely in your browser. Your loads and settings are saved in your browser’s local storage so they are there when you return. They are never sent to a server. You can clear them at any time with the Reset all button. See the privacy policy.
Sources
- Estimating Appliance and Home Electronic Energy Use (external, U.S. Department of Energy): watts times hours as the basis for energy estimates, and the refrigerator cycling rule of thumb.
- Three Ways to Keep Food Safe When You Lose Power (external, FoodSafety.gov): refrigerator and freezer hold times.
- Be Safe During a Power Outage (external, FEMA): generator placement and outage safety.
- Avoiding Carbon Monoxide Poisoning (external, CDC): generator and fuel-burning equipment safety.
- CPSC safety tips to prevent carbon monoxide poisoning and fires (external, CPSC): generator distance and carbon monoxide alarms.
- Home Use Devices: How to Prepare for and Handle Power Outages for Medical Devices that Require Electricity (external, FDA, PDF): medical-device outage planning.
The 15% loss buffer, 20% reserve, category thresholds, and default appliance values are Outage.info planning conventions, not figures taken from these sources.