Start with your actual load, not a sales pitch
The average home uses about 1,574 kWh/month, above both the state and national average. But that monthly number tells you almost nothing about what your battery needs to do during an outage.
What matters is your sustained draw during an outage. In our planning model, a smaller home is roughly 2.1 kW average load and a medium home is roughly 2.9 kW. A high-comfort summer evening can still push well above that. The gap between those numbers is what drives your sizing decision.
Before you talk to any installer, look at your electric bill and identify your peak month. That's your worst-case scenario. Then decide: do you want full comfort during an outage (AC included) or critical circuit coverage (essentials only)? That decision determines your battery size.
The surge question nobody asks
Your AC draws 3,500W when running. But on startup, it surges to 7,500–9,000W for 10 seconds. Add an air handler starting simultaneously (2,400W surge) and a fridge cycling on (1,200W surge), and you're at 12,000–15,000W for a fraction of a second.
If your battery's inverter can't handle that spike, it trips and your AC never comes on. This is the #1 reason homeowners are disappointed with undersized battery systems. The continuous output rating is not the number that matters. The peak surge rating is.
When comparing systems, always ask: 'What is the peak output for 10 seconds?' Not the continuous output. Not the nominal rating. The 10-second surge number.
How to calculate your runtime
Runtime = usable capacity (kWh) ÷ sustained draw (kW). Simple math, but the draw number is where people get it wrong.
Our plans are sized to deliver: 9 kWh = up to 8 hours, 18 kWh = up to 16 hours, 27 kWh = up to 24 hours, 36 kWh = up to 32 hours. These figures reflect minimal-load operation: fridge, Wi-Fi, lights, and medical devices.
Full power (everything on, summer evening): ~5 kW sustained. A 27 kWh system lasts ~5.5 hours. This is the stress test. Manage load priorities through your smart panel to preserve runtime.
Most homeowners should size for comfort mode and manage load priority through their smart panel for full power scenarios.
The costs nobody tells you about
Battery purchase price is the number everyone compares. But the 10-year total cost of ownership is the number that matters.
A $10,000 battery with $0 annual maintenance costs $10,000 over 10 years. A $6,000 battery with a $200/year service contract costs $8,000 over 10 years. A $8,500 generator with $250/year maintenance plus fuel costs adds up to $12,000–$15,000+ over 10 years.
Questions to ask: What's the warranty length? What components are covered? What's the expected degradation curve? What does a replacement battery module cost if needed after warranty? These questions separate the real cost from the sticker price.
The three questions most buyers forget
1. 'Can this system start my AC?' This is the surge question above. Ask for the 10-second peak rating, not just continuous output.
2. 'What happens when I want more capacity?' Some systems are modular (add battery modules). Others require buying an entirely new unit. If you think your needs might grow, modularity saves thousands.
3. 'How fast does it transfer?' Generator transfer takes 10–30 seconds (your power goes out, then comes back). Battery transfer takes under 20 milliseconds. Most electronics don't even register the switchover. For medical equipment or home office work, this difference matters.