Estimate only. The watts are typical values and the hours are starting guesses; your nameplates and your habits come first. A licensed electrician should confirm the loads and the connection before you buy anything.
Pick what you want to keep running
Your backup estimate
Battery or generator, side by side
Load by load
Prefer the full contact form? Open it with this filled in.
How the math works
- Running wattsEverything you checked, added up as if it all ran at once. That’s the continuous output to look for.
- SurgeRunning watts plus the largest single motor start, assuming motors start one at a time.
- Energy a dayEach load’s watts times its hours, added up, in kWh.
- Battery sizeEnergy a day times the days, divided by inverter efficiency and the usable share. Both assumptions are shown and yours to change.
- GeneratorAbout 20% above running watts, rounded up, with a starting rating at least the surge, the same as our generator sizing estimate.
Code notes, in plain English
Our own summary of the 2023 NEC as adopted in California, not the code text.
- Keep it off the utility linesA backup source needs transfer equipment, a listed interlock or a backup panel so it can’t feed the utility’s wires while crews are working (Article 702 for optional standby systems).
- Batteries are their own articleHome battery systems follow Article 706 and the listing and installation instructions, with a permit and inspection.
- Load managementAn energy management system (220.70 and Article 750) can shed big loads so a smaller battery or generator covers what matters.
- The 2026 NECWe use the 2023 edition. The 2026 edition renumbers Article 220 to 120 and changes several values; your city or county decides which edition applies.
What this sizer doesn’t do
- Pick a productWe don’t recommend brands. Compare the continuous output, surge rating and usable kWh on any spec sheet against these numbers.
- Solar recharging or cold weatherSolar can stretch a battery across days, and cold can shrink it. Neither is modeled here.
- Fuel use or pricesGenerator fuel use depends on the model and the load, so we leave it out.
How IES helps
- We size it with youWe look at your panel and the loads that matter most, and help you pick the right size.
- Battery, generator inlet or standby, installed rightTransfer equipment, permit and inspection, start to finish.
- Free estimatesCall (530) 401-7419, answered 24/7, or send your list below.
Common questions
How big a battery do I need?
Add up the energy your essential loads use in a day (watts times hours), divide by the inverter’s efficiency and by the share of the battery you can use, then multiply by the days you want to cover. The sizer shows each step.
What are depth of discharge and inverter efficiency?
Depth of discharge is the share of a battery’s rated energy you can actually use. Inverter efficiency is how much of the stored energy reaches your outlets. We start both at 90% as round assumptions; check the spec sheet for the battery you’re looking at.
Why does the surge number matter?
Motors like fridges, pumps and air conditioners draw extra power for a moment when they start. The inverter or generator has to handle that surge on top of everything already running.
Are these watts right for my fridge?
They’re typical starting values. Your appliance’s nameplate or a plug-in meter is better, so please change them. A fridge runs on and off, which is why its hours a day start below 24.
Battery or generator?
A battery is quiet and switches over on its own but holds a set amount of energy, so it suits essential loads and shorter outages, longer with solar. A generator runs as long as it has fuel. Many homes use one, the other or both.
Do I need a special panel or switch?
Yes. Any backup source has to be kept from feeding the utility lines, through transfer equipment, a listed interlock or a backup panel, installed with a permit. We can help pick the right one.
Send your backup list for a free estimate
Free estimates, no pressure. Tell us a little about the job and we’ll get back to you. Or call (530) 401-7419, answered 24/7.
The starting watts are the same typical values as our generator sizing estimate. Sources and code notes checked November 2026. We don’t sell or endorse any brand.