Why a 2016Wh Power Station Never Gives You 2016Wh

A post in r/SolarDIY this week put the question bluntly: “Breaking down why the FridgePower only gets 21 hours from 2016Wh.”It’s the most common disappointment in portable power: you buy a station rated for a number, plug in a fridge, and the real-world result lands 15–25% short of the back-of-napkin estimate. The unit isn’t broken and the spec isn’t a lie — it’s measuring something different from what comes out the outlet.

This post breaks down exactly where the watt-hours go, with the four deductions that separate the number on the box from the runtime on your fridge. By the end you’ll have a formula you can apply to any EcoFlow DELTA, Anker SOLIX or Jackery unit, and you’ll stop being surprised by the gap.

Nameplate Wh is stored energy, not delivered energy

The watt-hour figure on the box — 2,016Wh, 3,600Wh, 1,024Wh — describes the energy chemically stored in the battery cells at 100% state of charge. It is a DC measurement taken at the cell terminals. Nothing about that number accounts for the journey the energy takes to reach a 120V appliance: through the battery management system, into the inverter, converted from DC to AC, and out the socket. Every one of those stages costs energy, and the loss is dissipated as heat (which is why power stations get warm under load).

Think of nameplate Wh the way you think of a car’s fuel-tank capacity versus its real range. A 50-litre tank doesn’t promise 50 litres of forward motion — engine efficiency, terrain and driving style decide how far you actually go. A power station is identical: the tank is honest, but the delivered energy depends on the conversion path and the load. Manufacturers quote the tank because it’s the one fixed, repeatable number; the delivered figure changes with every appliance you plug in.

Deduction 1: inverter conversion loss (8–12%)

The inverter turns the battery’s DC into the 120V AC your appliances expect, and that conversion is never free. Modern pure-sine inverters in EcoFlow DELTA-class units run around 88–92% efficient under typical load, meaning 8–12% of every watt-hour is lost as heat before it ever reaches the outlet. Efficiency is worst at the extremes — very light loads (a single phone charger) and loads near the inverter’s rated ceiling both drop a few points below the sweet spot, which sits around 30–70% of rated output.

This is why DC outputs matter for efficiency-critical use. Charging a phone or running a 12V fridge directly off the car port or USB-C bypasses the DC-to-AC conversion entirely and loses only a couple of percent. If you’re running a long off-grid stretch and care about every watt-hour, favouring native DC loads over the AC inverter can claw back 8–10% of your usable capacity — meaningful over a multi-day outage. We cover how to plan around this in the load-sizing worksheet.

Deduction 2: the low-voltage cutoff floor (0–5%)

No well-designed power station drains its cells to literal zero. The battery management system enforces a low-voltage cutoff — typically when the pack hits 0–5% indicated state of charge — to keep the LiFePO4 cells above the voltage where deep-discharge damage and accelerated aging begin. That floor is reserved capacity you paid for but can never use, and it’s baked into the runtime you actually see. On a 2,016Wh unit, a 3% floor quietly removes about 60Wh before any other loss.

LFP chemistry, which EcoFlow uses across the DELTA line, is more forgiving here than the older NMC packs in budget units — its flat discharge curve means you can safely use a larger slice of the pack, so usable capacity sits closer to nameplate. But “closer” isn’t “equal.” The combination of the inverter tax and the discharge floor is why a realistic planning multiplier for AC loads is about 0.85–0.90 of nameplate. For a 2,016Wh station that’s roughly 1,700–1,815 usable Wh at the 120V outlet.

Deduction 3: surge versus running watts

Two different watt numbers govern any motor-driven appliance, and confusing them wrecks runtime math. Surge (or starting) watts is the brief inrush when a compressor or pump motor overcomes inertia — often 3–8× the running figure for a few hundred milliseconds. Running watts is the steady draw once the motor is spinning. A fridge that surges to 1,000W settles to 120–150W; a well pump that surges to 3,500W runs at 800W.

Surge matters for one thing only: whether your inverter can start the device without tripping. An undersized 1,000W inverter will stall on that fridge’s 1,000W surge even though it could easily carry the 150W running load all day. For runtime, you ignore surge entirely and calculate from running watts — the surge lasts too briefly to consume meaningful energy. This is the exact error behind the “Power requirements for a small system”confusion that shows up constantly in solar forums: people size runtime off the scary surge number and then can’t reconcile it with reality.

Deduction 4: duty cycle, the runtime multiplier nobody applies

Here is where most fridge estimates go wrong by a factor of two or three. A refrigerator compressor does not run continuously — once the box is cold, it cycles on and off to hold temperature, running roughly 30–50% of the time depending on ambient heat, how often the door opens, and the unit’s insulation. So a fridge with a 150W running draw averages only 50–75W across an hour. Over 24 hours that’s about 1,200–1,800Wh, not the 3,600Wh you’d get by multiplying 150W by 24.

Apply duty cycle and the 21-hour FridgePower result snaps into focus. Usable energy: ~1,700Wh after the inverter and floor deductions. Average fridge draw: ~80W (a 150W compressor at ~53% duty in a warm room). 1,700Wh ÷ 80W ≈ 21 hours. The unit performed exactly to spec — the owner’s estimate just skipped the duty-cycle step. The same logic governs AC compressors, well pumps, sump pumps and anything thermostatically controlled.

The formula you can reuse on any unit

Put the four deductions together into one reusable equation. Real runtime (hours) = (Nameplate Wh × 0.88) ÷ (Running watts × Duty cycle).The 0.88 bundles the inverter loss and discharge floor for AC loads — bump it to 0.95 for pure DC loads. Running watts comes from the appliance label or, better, a $15 plug-in meter. Duty cycle is 1.0 for constant loads (lights, a CPAP’s blower, a TV) and 0.3–0.5 for cycling thermostatic loads (fridges, AC, freezers).

Worked example on a DELTA Pro’s 3,600Wh nameplate running a 12,000 BTU window AC: usable ≈ 3,168Wh; running draw ≈ 1,100W at ~50% duty = 550W average; runtime ≈ 3,168 ÷ 550 ≈ 5.8 hours. That matches field reports far better than the naive 3,600 ÷ 1,100 = 3.3 hours (which ignores duty cycle) or 3,600 ÷ 550 = 6.5 hours (which ignores derating). Get both corrections in and your estimates land within 10% of reality. For sizing a unit and panel array from scratch using this same math, see our step-by-step worksheet, and when you’re weighing a station against a gas generator, the cost-and-runtime comparison uses these exact figures. The DELTA 3 Ultra Plus vs DELTA Pro breakdown applies the 0.90 multiplier to a head-to-head AC scenario.