We love the idea of “backup.” Lights stay on. Fridge keeps humming. You’re the chill house on the block when the grid blinks.
But how big a battery do you actually need? Not the salesy answer. The real one.
Short version: right-size it to your must-run loads and your outage reality. The rest is just math (easy math, promise).
Step 1: Decide what must stay on
- Essentials: fridge, lights, Wi-Fi, a few outlets.
- Nice-to-have: fans, microwave, TV.
- Power hogs (be careful): AC, electric dryer, oven, pool pump.
Not sure what’s guzzling power? Run the numbers in our Appliance Cost Calculator and build a realistic “must-run” list.
Step 2: Estimate daily energy needs (kWh)
- Add up watts for your essentials.
- Multiply by hours you’ll use them.
- Convert to kWh (divide by 1,000).
Or skip the napkin math and let the Battery Sizing Calculator crunch it.
Step 3: Pick your backup window
- 8–12 hours: cover evening peaks + overnight.
- 24 hours: full-day outage security.
- 48+ hours: you live where the grid likes drama.
Longer backup window = bigger battery. (Captain Obvious, but it matters.)
Step 4: Account for depth of discharge & efficiency
Batteries don’t give you 100% of their label every cycle.
- Lithium iron phosphate (LFP): great cycle life, ~90–95% round-trip efficiency.
- Leave a margin. Your future self will thank you.
Step 5: Solar recharge (if you have panels)
If the sun’s out tomorrow, you can get away with a smaller battery. Your array refills the pack by day; you ride the battery by night. That dance can save $$$.
Example (real-world vibe)
- Essentials: fridge (150 W avg), lights (80 W), Wi-Fi + laptop (40 W), TV (60 W)
- Use ~10 hours in an outage → ~3.3 kWh
- Add 20% buffer → ~4 kWh usable
- With LFP and a healthy reserve, you’re looking at a 5 kWh battery module as a sensible starting point.
Going bigger powers more stuff or more hours. But start with needs, not bragging rights.
Financing & ROI angle
Batteries cost real money. Their ROI isn’t just dollars:
- Value: outage protection, time-of-use shifting (if your utility charges more at night).
- Cash vs loan: model both in the Solar ROI Calculator to see your true payback with/without storage.
Common mistakes (skip these)
- Oversizing “just because.” (You’ll pay for capacity you never use.)
- Ignoring starting surge (fridge/AC compressors spike on startup).
- Forgetting future loads (that “maybe later” freezer counts).
Your next move
- List essentials and get a baseline with the Battery Sizing Calculator.
- Check the $$ impact in the Solar ROI Calculator.
- Dial in appliance realities via the Appliance Cost Calculator.
When your numbers feel right, you’ll know. No guesswork. No upsell pressure. Just math and sunlight.