Sources and method
Every number in the calculator, where it came from, and what is our assumption. Checked October 28, 2026.
Method
Daily watt-hours = watts × hours per day × quantity (Virginia Tech BSE-301: wattage × hours used per day ÷ 1000 = daily kWh). Average load = daily Wh ÷ 24. Runtime = battery Wh × usable share ÷ average load. Battery counts = average load × outage hours ÷ usable Wh, rounded up.
What is our assumption, not a source: the 85% usable share; the default hours per day; which appliances are ticked for summer or winter; the four outage lengths; the rule that the biggest motor starts while everything else runs. All are editable on the calculator.
Starting watts: for each ticked motor the extra load to start is its starting watts minus its running watts (3 times running unless you set it; central AC defaults to 15,000 W, Generac's upper figure; Champion lists 5,000-6,000 W, so use a soft-start figure if you have one). The estimate is everything running plus the largest extra. Custom loads count the same way.
Verdict and checks: a battery shows "load fits" only when running watts are within its continuous rating and estimated starting watts are within its surge rating (Powerwall 3: 185 A motor-start rating) or, where no surge rating is verified, within its continuous rating. The Anker F3800's 120 V outlets are limited to 2,400 W; above that it shows "needs 240 V / transfer switch". Base Power never shows "fits" because no kW rating is published on the pages we read.
Generator fuel: Generac Guardian 22 kW spec sheet: propane 2.53 gal/h at half load, 3.90 gal/h at full load; natural gas 228 and 327 ft3/h. Loads up to 11,000 W use the half-load figure as a ceiling (lighter loads burn less). A 500-gallon tank at 80% (400 gal) lasts about 158 h at half load and 103 h at full load, our arithmetic. Portable generator fuel use is not covered.
Input rules: every number must be a number above zero inside its range (outage 1-720 h; usable 50-100%; watts 1-50,000; hours per day up to 24; quantity 1-100). Blank, zero, negative, text and out-of-range entries show a red message and block the result; nothing is replaced silently.
What we leave out on purpose: prices (we cannot source current ones), solar recharging, solar-charging times, and whole-home wiring. A licensed electrician must size anything wired into your panel.
Appliance defaults
| Appliance | Default watts | Default h/day | Basis | Source |
|---|---|---|---|---|
| Refrigerator, side-mount with icemaker | 250 W | 8 h | EWEB range 210-250 W while running. Compressor cycles; VT says to divide plugged-in time by 3, so 24 h becomes 8 h. | EWEB |
| Freezer, upright, auto-defrost | 240 W | 8 h | EWEB range 200-240 W (ENERGY STAR) to 240-273 W (non-ENERGY STAR). Same cycling rule as the fridge. | EWEB |
| Microwave oven, 1.25 cu ft | 1,500 W | 0.3 h | EWEB: 1,500 W. About 20 minutes a day assumed. | EWEB |
| Coffee maker | 1,300 W | 0.3 h | EWEB range 1,100-1,500 W. | EWEB |
| LED bulbs, 60 W equivalent (each) | 9 W | 5 h | EWEB: 9 W each. Six bulbs for five hours assumed. | EWEB |
| Wi-Fi router | 7 W | 24 h | EWEB: 6-7 W, always on. | EWEB |
| Cable or DSL modem | 6 W | 24 h | EWEB: 6 W, always on. | EWEB |
| Laptop computer | 50 W | 6 h | VT: 50 W. | VT |
| Central AC, 3-ton (unticked: very large) | 3,500 W | 8 h | Generac: about 3.5-5 kW running and up to 15-18 kW starting for 3 tons without a soft start. Champion lists 3-3.5 kW running and 5-6 kW starting, so sources disagree: we use the higher start. With a soft-start kit enter a lower starting watts. | GENSIZE |
| TV, LED or LCD 45-inch | 160 W | 3 h | EWEB range 110-213 W; midpoint used. | EWEB |
| CPAP, ResMed AirSense 11 | 56 W | 8 h | ResMed: typical 56.1 W, peak 73.2 W. Humidifier or heated tube raise it. Check your own device label. | RESMED |
| Ceiling fan | 120 W | 10 h | VT range 65-175 W; midpoint used. | VT |
| Room (window) air conditioner | 1,200 W | 8 h | EWEB range 1,000-1,400 W. | EWEB |
| Furnace blower fan, standard motor | 500 W | 8 h | EWEB: 500 W standard motor (VT lists 750 W). A gas furnace still needs its blower. Electric heat strips are far larger; enter them as a custom load. | EWEB |
| Portable space heater | 1,500 W | 6 h | EWEB range 1,000-1,500 W. Draining a battery fast; shown so you can see it. | EWEB |
| Electric blanket (single or double) | 100 W | 8 h | EWEB: 100 W. VT lists 60 W single, 100 W double. | EWEB |
| Well pump, deep well | 1,100 W | 1 h | VT range 250-1,100 W; the top of the range is used so you are not caught short. Starting demand can be about 3 times higher. | VT |
| Electric water heater, 50 gal | 5,000 W | 3 h | EWEB range 4,500-5,500 W. Heavy: most power stations cannot run it. | EWEB |
Battery specifications
| Unit | Capacity | Continuous | Surge | Source |
|---|---|---|---|---|
| EcoFlow DELTA 2 | 1,024 Wh | 1,800 W | none verified; compared with continuous | ECOFLOW2 |
| EcoFlow DELTA Pro | 3,600 Wh | 3,600 W | none verified; compared with continuous | ECOFLOWP |
| Anker SOLIX F3800 | 3,840 Wh | 6,000 W | none verified; 120 V outlets limited to 2,400 W (20 A); full 6,000 W only on 240 V | ANKER |
| Tesla Powerwall 3 (installed) | 13,500 Wh | 11,500 W | 185 A motor-start (LRA) rating | TESLA |
| Base Power (Texas battery service) | 39,200 Wh | not published | kW rating not published on pages we read | BASE |
Specifications are the makers' published numbers on the pages below, read October 28, 2026. The Anker row cites the F3800 page; Anker's F3800 Plus page carries the same spec text. Surge ratings quoted on some retail pages (for example 7,200 W for the DELTA Pro) could not be verified on the maker's page for a single unit, so we do not use them. Base Power's capacity and fees come from its help page and CoServ's page; Base's help pages differ by utility area and promotion. Makers revise products; check the page before you buy.
Source list
| Key | Document | Link | Checked |
|---|---|---|---|
| EWEB | EWEB, Typical Residential Electricity Use and Cost (rates as of Feb 1, 2026) | https://www.eweb.org/documents/energy-efficiency/cost-of-use-common-household-appliances-and-electronics.pdf | October 28, 2026 |
| VT | Virginia Cooperative Extension BSE-301, Estimating Appliance and Home Electronic Energy Use (2020) | https://www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/2901/2901-9014/BSE-301.pdf | October 28, 2026 |
| RESMED | ResMed AirSense 11 user guide, technical specifications | https://document.resmed.com/documents/products/machine/airsense-11/user-guide/airsense11_user-guide_amer_mul.pdf | October 28, 2026 |
| ECOFLOW2 | EcoFlow DELTA 2 product page | https://www.ecoflow.com/us/delta-2-portable-power-station | October 28, 2026 |
| ECOFLOWP | EcoFlow DELTA Pro product page | https://www.ecoflow.com/us/delta-pro-portable-power-station | October 28, 2026 |
| ANKER | Anker SOLIX F3800 product page | https://www.ankersolix.com/products/f3800 | October 28, 2026 |
| TESLA | Tesla Powerwall 3 datasheet | https://energylibrary.tesla.com/docs/Public/EnergyStorage/Powerwall/3/Datasheet/en-us/Powerwall-3-Datasheet.pdf | October 28, 2026 |
| CPSC | CPSC, What to Know About Generators and Carbon Monoxide (CO) | https://www.cpsc.gov/s3fs-public/468-WhattoKnowGenerators_2022.pdf | October 28, 2026 |
| GENSIZE | Generac, What Size Generator Do I Need? (3-ton central AC: about 3.5-5 kW running, up to 15-18 kW starting) | https://www.generac.com/resources/home-management/what-size-generator-do-i-need/ | October 28, 2026 |
| CHAMP | Champion Power Equipment, Generator Wattage Chart (3-ton central AC: 3,000-3,500 W running, 5,000-6,000 W starting) | https://www.championpowerequipment.com/generator-wattage-chart/ | October 28, 2026 |
| GENAC | Generac Guardian 16/20/22 kW standby generator spec sheet (22 kW: propane 2.53 gal/h at half load, 3.90 gal/h at full load; natural gas 228 / 327 ft3/h) | https://www.generac.com/globalassets/products/residential/standby-generators/spec-sheets/16-22kw-guardian-standby-generator-specsheet.pdf | October 28, 2026 |
| BASE | Base Power help center: battery cost (39.2 kWh system; $50 refundable deposit; installation and monthly fee vary by utility area; page read Oct 28, 2026) | https://help.basepowercompany.com/en/articles/10194945 | October 28, 2026 |
| COSERV | CoServ, PeakTime Perks: Base Power ($695 installation and $19 a month after the founding promotion; up to 36 hours of backup with reduced use) | https://www.coserv.com/energy-solutions/peaktime-perks/ptp-base-power/ | October 28, 2026 |
| EIA | U.S. EIA, Electric Power Annual, Table 11.3 Reliability Metrics by State, 2025 and 2024 (Texas SAIDI minutes per customer per year: 2024 with major events 1,270.6, without 129.4; 2025 with 223.5, without 128.7) | https://www.eia.gov/electricity/annual/html/epa_11_03.html | October 28, 2026 |
| FOOD | FoodSafety.gov, Food Safety During Power Outage (reviewed Aug 8, 2024) | https://www.foodsafety.gov/food-safety-charts/food-safety-during-power-outage | October 28, 2026 |
Corrections
Spot a wrong number? Email [email protected] with the link to the better source and we will check and fix it.