Power Station Discharge Rate and Usable Capacity

Volume I  ·  May 2026  ·  461 words

A power station's advertised watt-hour capacity — 1,024 Wh, 2,048 Wh, 3,600 Wh — is measured at a specific discharge rate, typically C/5 (a full discharge over 5 hours). At higher discharge rates, the usable capacity is lower. A power station rated for 1,024 Wh at C/5 may deliver only 880–940 Wh when discharged at C/2 (over 2 hours), and the shortfall widens further as the load increases. This reduction is not a defect; it is the predictable consequence of internal cell resistance, DC-DC converter losses, and inverter efficiency curves — and it must be accounted for when sizing a power station for a known load.

C-rate and cell chemistry. The C-rate expresses the discharge current as a multiple of the battery's rated capacity. A 1,024 Wh LFP battery discharged at 500 watts is operating at approximately C/2. LFP cells maintain 95–97% of their rated capacity at C/2, dropping to 90–93% at 1C (a full discharge in 1 hour). NMC cells, used in older and some compact power stations, are more affected by discharge rate, delivering 88–92% at C/2 and 80–85% at 1C. The EcoFlow Delta 2 (1,024 Wh LFP) tested at a 500W continuous load delivers approximately 920–960 Wh of AC output, representing 90–94% of the rated DC capacity after inverter losses. The Bluetti AC200L (2,048 Wh LFP) at 1,000W delivers approximately 1,800–1,880 Wh of AC output.

Peukert effect in lithium. The Peukert exponent describes how a battery's capacity declines as the discharge rate increases. Lead-acid batteries have a Peukert exponent of 1.10–1.25, meaning capacity drops sharply at high loads. LFP lithium cells have a Peukert exponent of 1.00–1.02 — the effect is nearly negligible below 1C. For practical purposes, a user can ignore the Peukert effect when sizing an LFP power station for loads below 1C. The dominant capacity reduction in LFP power stations comes from inverter efficiency, not cell chemistry.

Inverter efficiency curve. A power station's inverter achieves peak efficiency (88–93%) at 30–70% of its rated output. At very low loads (under 10% of rated output), the inverter's no-load power consumption — typically 8–15 watts — becomes a significant fraction of the total load, reducing effective efficiency to 50–70%. At loads approaching the inverter's maximum, efficiency drops 3–5 percentage points below peak due to increased switching losses and thermal derating. For maximum usable capacity, size the power station so the expected load falls within the 20–80% range of the inverter's continuous rating.