What battery do I need for my solar system?

The battery is the most expensive piece of a modern solar setup — and the one people get most wrong. Here's the sizing formula and key picks for chemistry and voltage.

Capacity formula

“Usable capacity (kWh) = Nighttime consumption + reserve (days autonomy × daily total) ÷ depth of discharge”

— Off-grid and hybrid sizing model

Example 1: average grid-tied home

Typical night use: 4-5 kWh. No reserve needed (grid covers). Usable target: 5 kWh. With LiFePO4 90% DOD: 5.5-6 kWh nominal. A Tesla Powerwall 3 (13.5 kWh) is overkill — a Pylontech US5000 or Enphase IQ Battery 5P fits perfectly.

Example 2: average off-grid home

10 kWh/day × 2 days autonomy = 20 kWh usable ÷ 0.9 DOD = 22-23 kWh nominal. Stack 4-5 × 5 kWh batteries or use a BYD Premium HVM 22 wall.

2026 chemistries

• LiFePO4: the 2026 standard. Safe, 6,000-10,000 cycles, 12-15 years
• NMC: higher density but heat-sensitive. Phasing out of residential
• AGM or gel lead-acid: cheap but heavy, 1,500-3,000 cycles. Cabins / light use only
• Vanadium flow / molten salt: utility-scale only, not residential

Voltage: low (12-48 V) vs high (>200 V)

Small residential runs at 24 or 48 V. Modern modular hybrids run at 200-400 V DC (Tesla, BYD, Huawei, Enphase): higher efficiency, less copper. If you'll expand, pick a high-voltage modular system.

Compare chemistries in LiFePO4 vs lead-acid, learn lifespan in how long a solar battery lasts, and see how to add a battery to an existing system.