Estimate battery backup energy and runtime from appliance watts and hours. Separate daily kWh, simultaneous power, solar recharge and essential household loads.
Quick answer
Daily energy kWh = sum(watts x quantity x operating hours) / 1,000; usable battery energy / load energy estimates runtime.
Notes updated 2026-10-06
Best for
Homeowners pressure-testing solar proposals before signing.
Inputs needed
electric usage, local rate, system size or quote assumptions
Formula used
Daily energy kWh = sum(watts x quantity x operating hours) / 1,000; usable battery energy / load energy estimates runtime.
Method references
System Advisor Model
Author profile: Marcus Vance . Read the methodology and recheck local tariff, quote, and incentive details before deciding. Attribution is not a record of independent technical review.
Toggle appliances, set quantities, and adjust hours to match your home. Runtime below excludes solar recharge; sizing retains a first-day reserve and assumes later-day recharge. Confirm startup power and nighttime operation separately.
Essentials
Refrigerator
150W each
Daily
3.60 kWh
LED Lighting (per room)
40W each
Daily
0.96 kWh
Internet Router/Modem
15W each
Daily
0.36 kWh
Phone Chargers
25W each
Daily
0.20 kWh
Laptop / Tablet
60W each
Daily
0.36 kWh
Sump Pump
800W each
Daily
0.00 kWh
Well Pump
750W each
Daily
0.00 kWh
Medical
CPAP Machine
30W each
Daily
0.00 kWh
Oxygen Concentrator
150W each
Daily
0.00 kWh
Comfort
TV (55")
100W each
Daily
0.40 kWh
Microwave
1000W each
Daily
0.50 kWh
Coffee Maker
900W each
Daily
0.23 kWh
Window AC Unit
1200W each
Daily
0.00 kWh
Heating/Cooling
Mini Split (1.5 ton)
1500W each
Daily
0.00 kWh
Electric Space Heater
1500W each
Daily
0.00 kWh
EV Charging
EV Charging (Level 1)
1440W each
Daily
0.00 kWh
EV Charging (Level 2)
7200W each
Daily
0.00 kWh
kW your system can generate (reduces battery need)
kW
Wattcrunch Solar Planning Report
Generated 10/23/2026 · Verify assumptions with your utility and installer.
Keep this estimate
Save the scenario or download a branded print/PDF report.
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What should be included in backup loads?
List essential appliances, average running watts, operating hours and quantities. Separate daily energy from simultaneous power. Motor startup, transfer equipment and medical needs need an installer-specific design.
Does solar recharge guarantee continuous backup?
No. Daily solar energy can hide nighttime shortages or a cloudy outage. Check runtime without recharge and size a reserve, then verify whether the inverter can operate the solar array while the grid is down.
How long will a 10 kWh battery last during an outage?
Divide usable delivered energy by average backed-up power. At a steady 500 W load, 10 kWh lasts 20 hours before reserve and conversion losses. At 2 kW it lasts 5 hours. Cycling appliances and solar recharge make actual runtime different.
Runtime for 10 kWh of usable delivered energy
Average backed-up load
Energy available
Idealized runtime
250 W
10 kWh
40 hours
500 W
10 kWh
20 hours
2000 W
10 kWh
5 hours
Illustrative assumptions: Constant average AC load, 10 kWh already usable and delivered, no additional reserve deduction and no solar recharge. If starting from nameplate energy, apply reserve and delivery limits before calculating runtime.
What to check on your bill or installer proposal
Use average cycling power for energy and startup power for inverter sizing.
Include medical equipment, well pumps, refrigeration and communications as needed.
Ask the installer which circuits actually remain powered in an outage.
Example and calculation notes updated . Method references are listed below; example rates are not verified utility tariffs.
How does this battery backup load estimate work?
Daily energy kWh = sum(watts x quantity x operating hours) / 1,000; usable battery energy / load energy estimates runtime.
A worked example with explicit assumptions
A 150 W appliance operating 8 hours uses 1.2 kWh. A 15 W router running 24 hours uses 0.36 kWh. Their combined energy is 1.56 kWh, while simultaneous running power is 165 W.
What should I verify before using the result?
Starting surge, duty cycles and nighttime demand require separate checks. Daily solar balance cannot guarantee continuous backup.
Use your bill or written equipment quote, run a conservative scenario, and compare the result with the same inputs in another model.
Treat the result as a planning range, not a promise. Re-run it with a lower and higher production estimate, your actual utility tariff, and the written installer quote.
Check roof shade, orientation, tilt, and equipment losses.
Separate cash price, financing fees, battery cost, and roof work.
Verify export credits, fixed charges, and interconnection rules.
Confirm incentive eligibility, deadlines, and tax treatment.
Sources behind this estimate
Use the method references below to check the formula and its limitations. Input defaults and illustrative scenarios are not current tariffs, guaranteed output or live installer quotes.
Utility export rates, eligibility rules, and installer pricing can change. Confirm the applicable tariff, interconnection requirements, and tax treatment before making a purchase decision.
Before relying on a backup-load estimate
Energy in kWh determines duration; simultaneous running watts and starting surge determine whether the inverter can power the loads.
Use measured average consumption for cycling appliances. Verify usable AC capacity, reserve and inverter ratings separately.
Check an outage without solar recharge first. Critical medical loads require a professional backup plan, not only this estimate.
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Frequently asked questions
How do I use the battery backup load calculator?+
Start with your actual usage, equipment or quote inputs, keep the displayed units, and change one assumption at a time. Starting surge, duty cycles and nighttime demand require separate checks. Daily solar balance cannot guarantee continuous backup.
What calculation does this tool use?+
Daily energy kWh = sum(watts x quantity x operating hours) / 1,000; usable battery energy / load energy estimates runtime.
Can I check the arithmetic with an example?+
A 150 W appliance operating 8 hours uses 1.2 kWh. A 15 W router running 24 hours uses 0.36 kWh. Their combined energy is 1.56 kWh, while simultaneous running power is 165 W.
What could change the accuracy of my result?+
Starting surge, duty cycles and nighttime demand require separate checks. Daily solar balance cannot guarantee continuous backup.
Can I use the estimate outside the United States?+
Physical kW and kWh calculations use the same units internationally. Dollar results, regional defaults, emissions factors, incentives and utility guidance are US-oriented. Replace rate and production assumptions with local values; do not apply US tax or export rules abroad.
Can I print or save results without registering?+
Use Print / PDF to save this page from your browser without an email address. The solar plan stores supported scenario inputs in your browser; private files should be checked and redacted before uploading for extraction.
Compare the decision, not just the number
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