1.5.1 The basics
That was the essentials — enough for most homeowners.
The rest is the more technical explanation; keep reading to become a solar-power expert,
or skip ahead to the next section: 1.5.2 Battery form factors →
Before choosing a battery, get the concepts straight. This section covers the 6 key specs — they determine not just how long the battery lasts, but how many devices it can run during an outage.
The 6 specs to actually read
| Spec | Unit | Typical (residential LFP) | Real meaning |
|---|---|---|---|
| Total capacity | kWh | 5–15 (per unit) | Energy stored when fully charged |
| Usable capacity | kWh | Equals total capacity (LFP DoD 100%) | What you can actually discharge |
| Continuous power | kW | 5–12 (per unit) | How much load you can run simultaneously |
| Peak power | kW | 15.4 (10 sec) | Inrush for AC / pump startup |
| Cycle life | cycles | LFP: 6,000+; NMC: 3,000–4,000 | Cycles before capacity falls to 70% |
| Depth of discharge (DoD) | % | LFP: 100%; NMC: 80–90% | What fraction you can pull each cycle |
LFP vs NMC chemistry: 99% of homes should pick LFP
| Dimension | LFP (lithium iron phosphate) | NMC (lithium NMC) |
|---|---|---|
| Safety | Very high, thermal-runaway threshold 270°C+ | Medium, 150–200°C |
| Cycle life | 6,000–10,000 cycles | 3,000–4,000 cycles |
| DoD | 100% | 80–90% |
| Energy density | Medium (160 Wh/kg) | High (240 Wh/kg) |
| Footprint | Slightly larger | Compact |
| Cost | Medium | High |
| Cold weather | Slight degradation < 0°C | Better |
| Residential share | Nearly all mainstream products | Exited residential market |
Why LFP is the default for residential: 2× cycle life, orders-of-magnitude better safety, lower price, 100% DoD = better effective capacity. In residential storage, NMC's energy-density advantage is meaningless — you've got plenty of garage wall.
How capacity translates to outage runtime
| Daily household usage | Off-grid survival | Recommended capacity |
|---|---|---|
| 15 kWh/day (small) | 1 day | 15 kWh |
| 25 kWh/day (typical) | 1 day | 27 kWh |
| 25 kWh/day (typical) | 3 days | 80 kWh |
| 40 kWh/day (large + EV) | 1 day | 40 kWh |
Important: this assumes powering everything. If you only need critical loads (fridge, lights, network, medical), 15 kWh runs 3+ days. Critical Load Panel + right-sized battery = best value.
Continuous power: can it run AC + oven together
Common household high-draw appliance startup / sustained power:
| Appliance | Startup surge (kW) | Sustained (kW) |
|---|---|---|
| 3-ton central AC | 6–8 | 3.5 |
| 5-ton central AC | 10–12 | 5.5 |
| Oven | — | 4 |
| Electric water heater | — | 4.5 |
| Dryer | — | 5 |
| EV charger L2 | — | 7–11 |
| Well pump | 5–6 | 1.5 |
A typical 10–13 kWh mainstream battery (~11 kW continuous): can run 3-ton AC (3.5 kW) + oven (4 kW) + dryer (5 kW) = 12.5 kW — slightly over; needs minor time-staggering.
Two units in parallel: ~22 kW continuous — nearly all loads simultaneous, no problem.
How long does cycle life really last
A typical LFP battery:
- Rated cycle life: 6,000–10,000 cycles (80% DoD)
- One cycle per day → 16–27 years
- Real lifespan bottleneck isn't cycles — it's calendar aging of the cells ≈ 15–20 years
Real-world: residential storage averages 0.7–1.0 cycles/day (daytime charge, evening discharge). LFP cycle life dramatically exceeds your usage. Cycle life should not be the selection bottleneck.
Anti-industry takeaway
Reps like to say "we use NMC because energy density is higher" — density matters for phones and EVs, not for a battery on the wall behind your garage. LFP is safer, more durable, cheaper. If a rep pushes NMC hard, ask why — the honest answer is "the manufacturer pays higher commission"; most reps don't even know that themselves.