Quick answer: A 30 kWh/day home backing up for two full days at 80% DoD needs about 75 kWh of installed battery — roughly 3 Powerwall 3 units.
Battery Sizing Calculator
Estimate the installed battery capacity your home needs for backup using either daily usage or an appliance-based planning workflow.
Based on 30.0 kWh/day × 1 day backup ÷ 80% DoD. Usable capacity: 30.0 kWh.
Product comparison (5 options)
| Product | Units | Sized by | Est. cost |
|---|---|---|---|
| BestFranklin Electric aPower2 | 3 | Energy | $37,500 |
| Tesla Powerwall 3 | 3 | Energy | $42,000 |
| Enphase IQ Battery 10T | 4 | Energy | $44,000 |
| LG Energy Solution RESU16H Prime | 3 | Energy | $46,500 |
| Sonnen sonnenBatterie Eco 10 | 4 | Energy | $54,000 |
How we calculate battery capacity
The formula is straightforward: multiply your daily energy use by the number of backup days, then divide by your depth of discharge target to get required installed capacity.
For example, a home using 30 kWh/day that needs 2 days of backup at 80% DoD needs (30 × 2) ÷ 0.80 = 75 kWh of installed capacity. The usable portion is 60 kWh — the rest is held in reserve to protect battery longevity.
Appliance mode uses typical runtime assumptions (a refrigerator runs ~8h/day, not 24) rather than multiplying each appliance's wattage by 24 hours. This tends to produce more accurate estimates than manual kWh math for most homeowners.
Frequently Asked Questions
- How do I calculate how much battery storage I need?
- Multiply your daily electricity usage (kWh) by your desired backup duration (days), then divide by your target depth of discharge. For example: 30 kWh/day × 2 days ÷ 0.80 DoD = 75 kWh of capacity.
- What is a good depth of discharge for home batteries?
- LFP (lithium iron phosphate) batteries can safely discharge to 100%. NMC batteries are typically limited to 80–90% for longevity. The Tesla Powerwall 3, Franklin aPower2, and Enphase IQ batteries are all LFP.
- How many kWh do I need for whole-home backup?
- A typical US home uses 28–35 kWh/day. For one full day of backup at 80% DoD, you need 35–44 kWh of installed capacity — roughly 3 Powerwall 3 units or equivalent.
- Do battery costs include installation?
- Yes. Our cost estimates include hardware, inverter integration, and labor based on regional installer surveys. They do not include the federal 30% ITC, which reduces net cost significantly.
Reviewed April 2026
Methodology and source note
PowerSizing calculators use shared formulas, documented assumptions, and current planning inputs that are summarized on the methodology page. Use these tools for first-pass planning, comparison, and sanity checks, then confirm local code, pricing, utility tariff, and installer specifics before you buy equipment.