PowerUsageSizing a Home Battery
Picking kWh capacity and kW power output for backup, solar self-consumption, and peak shaving — with the round-trip and depth-of-discharge math.
Home batteries are spec'd by two numbers: energy (kWh) and power (kW). You need enough of both, and they're not interchangeable. A 20 kWh battery that can only deliver 5 kW won't run your AC. A 10 kW battery that only holds 4 kWh will run anything but only for minutes.
Sizing comes down to one question: what is the battery for? The answer drives both numbers.
Three reasons to buy a battery
| Use case | Drives kWh sizing | Drives kW sizing |
|---|---|---|
| Backup power | Outage duration × critical load | Largest simultaneous load |
| Solar self-consumption | Evening household consumption | Peak evening draw |
| Peak shaving / arbitrage | Peak-period kWh consumed | Peak-period max draw |
Most people buy for a combination, with backup usually the primary driver. Size for backup first, then check if the same battery covers the other two needs.
Step 1: Decode the spec sheet
Manufacturers love nameplate numbers. Real-world numbers are smaller.
Nameplate kWh vs usable kWh. Lithium batteries can't safely discharge to 0%. The usable window is typically 90–100% of nameplate for LiFePO4, 80–90% for NMC.
| Battery | Nameplate | Usable | Power (continuous) | Chemistry |
|---|---|---|---|---|
| Tesla Powerwall 3 | 13.5 kWh | 13.5 kWh | 11.5 kW | NMC |
| Enphase IQ Battery 10C | 10.08 kWh | 10.08 kWh | 7.68 kW | LFP |
| Franklin aPower 2 | 15.0 kWh | 15.0 kWh | 10.0 kW | LFP |
| FranklinWH aGate + 1× aPower | 13.6 kWh | 13.6 kWh | 5.0 kW | LFP |
| EcoFlow DELTA Pro Ultra | 6.0 kWh (per unit) | 6.0 kWh | 7.2 kW | LFP |
Always size on usable kWh, never nameplate.
Round-trip efficiency. A kWh charged into the battery comes back out as 0.85–0.92 kWh. The rest is lost to inverter, BMS, and heat. For peak-shaving math, every stored kWh loses ~10% of its value.
Cycle life. LFP chemistries are warranted for 10 years / ~10,000 cycles. NMC is shorter — typically 10 years / 4,000 cycles. Daily peak-shaving cycling burns through warranty faster than backup-only use.
Step 2: Size for backup
This is usually the dominant constraint. Decide three things:
1. What's the critical load list?
Not "everything in the house." Pick what must stay on during an outage:
| Load | Power | Daily kWh | Notes |
|---|---|---|---|
| Fridge | 150W avg | 3.6 | Cycles, peak ~600W on compressor start |
| Internet / router / ONT | 30W | 0.7 | Critical for remote work |
| Lights (LED, partial house) | 50W | 0.5 | Evenings only |
| One small window AC or mini-split | 800W | 6.0 | Comfort, climate-dependent |
| Furnace fan (gas heat) | 400W | 1.5 | Critical in winter |
| Home lab server rack | 200–600W | 5–14 | Highly variable |
| Phones / laptops / misc | 100W | 1.0 | Charging only |
| Typical critical load total | — | 12–25 kWh/day |
2. How long is your typical outage?
| Region pattern | Plan for |
|---|---|
| Urban grid, occasional storms | 8–24 hours |
| Suburban, weather-prone | 1–3 days |
| Wildfire PSPS zones (CA) | 2–5 days |
| Hurricane corridors | 3–7 days |
| Rural / extreme | 7+ days (consider generator) |
3. Multiply, then add headroom.
Battery kWh = critical load kWh/day × outage days × 1.2 (efficiency + buffer)
Example: 15 kWh/day critical load, 2-day target outage = 36 kWh usable. That's ~3 Powerwalls or ~2.5 Franklin aPowers. Expensive.
Most households compromise: size for 12–24 hours of full critical load, accept that a multi-day outage means shedding non-essential loads (AC, server rack) and stretching what's left.
Step 3: Check the kW number
Backup sizing kWh is meaningless if the battery can't deliver enough power for your loads to run.
Add up your largest simultaneous loads. Compressor start surges are the killer — they pull 2–5× steady-state for a few seconds.
| Load | Steady | Surge |
|---|---|---|
| Central AC (3-ton) | 3,500W | 10,000–15,000W |
| Heat pump (3-ton) | 3,000W | 8,000–12,000W |
| Well pump (1 HP) | 1,000W | 4,000–6,000W |
| Fridge compressor | 150W avg | 600–1,000W |
| Microwave | 1,200W | 1,500W |
A single Powerwall 3 (11.5 kW continuous, 30 kW peak for 10s) handles most homes. A single Enphase IQ 10C (7.68 kW) cannot start a central AC reliably. Mini-splits and inverter-driven appliances are far easier on batteries than fixed-speed motors.
If your critical loads include a central AC and you want it to run during outages, you'll either need a higher-power battery (Powerwall 3, Franklin aPower) or you'll need to start the AC on grid/generator and let the battery carry the steady-state load.
Step 4: Size for solar self-consumption (if applicable)
If you have solar and are on NBT/NEM 3.0, the goal is to store midday production and use it during evening peak (export rates are pennies; peak retail rates are 40–50¢).
Battery kWh = average evening (4–9 PM) household consumption
For most households, that's 8–15 kWh. A single 10–14 kWh battery covers it. Adding a second battery only helps if you also have backup needs.
Step 5: Size for peak shaving (if applicable)
See Peak Shaving with a Battery. Short version: don't buy a battery just for this. If you're buying for backup or solar, size up enough to also cover your peak-period draw and the peak shaving becomes a free bonus.
Putting it together: example households
Small home, urban, no solar, occasional outages:
- Critical load: 10 kWh/day, target 12 hours = 5 kWh
- Peak load: <5 kW
- Recommendation: 1× 10 kWh battery (10 kWh usable, ~5–8 kW), backup focus. ~$10–12K installed.
Suburban home with solar, NBT, occasional PSPS events:
- Critical load: 18 kWh/day, target 2 days = 43 kWh
- Solar self-consumption need: 12 kWh evening
- Peak load: 8 kW (one AC unit)
- Recommendation: 2–3× battery system (~27–40 kWh usable, 16+ kW). ~$25–40K installed.
Off-grid-curious rural home, frequent outages:
- Critical load: 25 kWh/day, target 4 days = 120 kWh
- Peak load: 10 kW (well pump, heat pump)
- Recommendation: Battery alone is impractical at this scale (~$80K+). Pair smaller battery (20–30 kWh) with auto-start generator for multi-day events.
What to avoid
- Oversizing for peak shaving alone. Math rarely works. Buy for backup or solar first.
- Undersizing kW because kWh looks fine. A 20 kWh battery that throttles at 5 kW cannot run a central AC. Check both numbers.
- Mixing chemistries / brands in one system. Stick with one vendor. Tesla + Enphase + Franklin all use different control protocols.
- Skipping the critical-load panel. Whole-home backup needs a much larger battery and inverter than a critical-load subpanel. If budget is tight, isolate the critical loads on their own panel.
- Ignoring degradation. Plan for 70–80% of nameplate capacity at year 10. Size for that, not day-one capacity.
How to use PowerUsage for battery sizing
- Use the calculator to total your evening (4–9 PM) consumption from your tracked devices.
- Subtract anything you'd shed during an outage (electric range, AC, hot tub, server rack if not essential).
- The remainder is your critical evening load — sets minimum usable kWh for self-consumption.
- Multiply by your target outage duration for backup sizing.
- Pick the largest simultaneous load from your device list for the minimum kW spec.