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Batteries · For traders

Charge and discharge rates in system design

Short answer

Capacity is only half the story; power matters too. A 20 kWh battery on a 5 kW inverter can only supply 5 kW, so when the house draws 7 kW, 2 kW still comes from the grid at whatever the rate is. Check the inverter's continuous discharge and charge kW, and the charge time in your cheap window.

Last checked 24 September 2026 · 8 sources

What should I know before deciding?

Example: GivEnergy Gen3 3.6/5 kW hybrid had 3.3 kW max charge and 3.6 kW max discharge (existing product rows). Power Wall 3: 11.04 kW discharge but 5 kW charge (8 kW with expansions) per brief; Fox Evo 10 kW both ways. Big instantaneous loads (showers, ovens, EVs) will import unless the inverter is sized for them.

Can my battery fill up in the cheap overnight window?

Only if its charging power is high enough. Divide usable capacity by charge power: 13.5 kWh at 5 kW takes about 2.7 hours, but 20 kWh at 3.6 kW needs over 5.5 hours. Cheap windows are often 4-7 hours, so check charge power, cold-weather limits and your main fuse.

Octopus Go cheap rate 00:30-05:30 (5 h); Intelligent Octopus Go guarantees 6 h 23:30-05:30. Grid-charging adds import load; keep within cut-out rating (G100 import limitation).

How fast can each battery charge and discharge?

Power, not just kWh, decides whether a battery covers a cooker plus heat pump or fills in a short cheap slot. Powerwall 3 charges at 5 kW (8 kW with Expansions) and discharges up to 11.04 kW. Each SigenStor module handles 3 or 4.6 kW. Huawei LUNA S1 manages 3.5 kW per module (checked 24 Sept 2026).

Powerwall 3: charge 5 kW, 8 kW with 1-3 Expansions; discharge configurable to 11.04 kW. SigenStor BAT 6.0 3.0 kW / BAT 10.0 4.6 kW continuous per module, capped by the EC controller rating. Fox EVO: inverter 5-10 kW; battery charge power not separately stated.

What size battery do I need?

Start from how much electricity you use outside your cheapest hours, not a single rule. EST says a typical home battery is about 10 kWh, and Which? says average homes use 8-10 kWh a day. Many installers size slightly below daily peak-hour use so the battery cycles fully every day, then add modules later if needed.

Installer rule (brief): daily use / 24 x 18 peak hours - assumes flat use; real homes use more in the evening. Also check inverter kW (can it supply your evening peak?) and charge kW (can it refill in the cheap window?). With solar, winter surplus is small, so size for the overnight-tariff role.

What to do next

Record the design assumptions, protection settings and commissioning results. Keep the DNO approval and final settings with the handover pack.

Sources

Last checked: 24 September 2026