How we work out sizes and runtimes

By the Ferncharge team · Figures last checked 18 October 2026

Advertising disclosure: This page contains no affiliate links.

In one line: we use manufacturers' published specs and official sources, apply a simple formula that we show in full, and label every assumption. We have not tested any product. Nothing on Ferncharge is based on hands-on use or our own measurements.

1. Where the numbers come from

What	Where it comes from
Power station capacity (Wh), AC and DC outputs, surge, weight, battery chemistry, cycle life, charging inputs	The manufacturer's UK product page, spec sheet, manual or help centre, linked from each table, with a "Checked" date for each row
Typical appliance wattages	The Centre for Sustainable Energy's Energy Savers list, or the appliance maker's spec sheet. We use your appliance's own rating label first if you enter it
Medical devices (CPAP)	The device manufacturer's own published specifications only
Safety, travel and legal points	Regulators and official bodies: GOV.UK, Ofgem, Ofcom, the Civil Aviation Authority, HSENI, UKHSA, the FSA, fire and rescue services, and Electrical Safety First
Solar estimates	The European Commission JRC's PVGIS monthly radiation data

If a manufacturer doesn't state a figure, we write "not stated by the manufacturer". We don't fill gaps from forums, retailer listings or review sites.

2. The runtime formula

Runtime (hours) = capacity (Wh) × usable share × efficiency ÷ appliance watts

Assumption	Default	Status	Why
Usable share, LiFePO4	90%	Planning estimate	EcoFlow's depth-of-discharge article gives 70–90% for lithium-ion and LiFePO4 batteries, with 80% recommended for its LiFePO4 DELTA Pro (EcoFlow). Its 100Ah worked example uses 100% for LiFePO4 (EcoFlow), and Dometic says lithium iron phosphate batteries are "nearly 100%" usable (Dometic). We use 90%, a planning estimate within that spread. It's at the top of EcoFlow's 70–90% range, so it isn't a cautious figure: lower it in the calculator if you want more margin
Usable share, NMC lithium-ion	80%	Sourced	EcoFlow says "lithium-ion batteries have a recommended DoD of up to 80%" (EcoFlow). Dometic says lithium NMC batteries are "roughly 80%" usable (Dometic). We use 80% when the chemistry is unknown
Inverter efficiency (AC sockets)	90%	Planning estimate	EcoFlow's worked examples use 90% (EcoFlow)
DC conversion (12 V and USB)	95%	Our estimate, not a published figure	DC outputs skip the inverter but still pass through a converter
Cold or age derate	Off (0%)	Planning estimate	EcoFlow treats cold as a planning factor rather than a fixed penalty: "There is no universal percentage loss for every model" (EcoFlow). Add a derate if your battery is cold or old (battery age is our own addition, not from that article)
Solar output	70% of panel rating at peak-sun equivalent	Our estimate	Jackery lists light, angle, temperature, cables and conversion as causes of loss but gives no single figure (Jackery)
Headroom warning	80% of continuous AC rating	Rule of thumb	We warn (it isn't a fail) when the running load is above 80% of the continuous rating
Start-up (surge) check	All running watts + the largest single extra start-up	Method	Appliances rarely all start at the same moment. If several motors may start together, add them yourself

For LiFePO4 on the AC sockets, 90% × 90% gives an overall factor of about 0.81. That's close to the single factors manufacturers publish: Jackery's blog uses 0.85, "calculated considering 85% efficiency" (Jackery), and its FAQ uses 0.8 (Jackery). On the 12 V and USB outputs we use 90% × 95%, about 0.855.

Worked example: a 1,000 Wh LiFePO4 power station running a 200 W appliance from the AC socket: 1,000 × 0.9 × 0.9 ÷ 200 = about 4.0 hours.

These are estimates for planning. Real runtimes vary with temperature, battery age, how hard the appliance works (thermostats switch on and off), and the product itself. Every calculator lets you change each assumption.

3. "Boost" modes

Some makers offer modes that let resistive heating devices (heaters, kettles, hair dryers) run above the rated AC output: EcoFlow X-Boost, Anker SurgePad and BLUETTI Power Lifting. How each one works is described on the maker's UK product page or in its manual: EcoFlow X-Boost, Anker SurgePad and BLUETTI Power Lifting. We report the figure the maker states and say which kind of device it applies to. We don't treat a boost figure as the continuous rating.

4. Comparison tables

Order: alphabetical by brand, then model. We don't rank.
Like with like: we compare the same specs, from the same type of source (the maker's UK page), checked on a stated date.
Prices: we don't quote prices. Where we list example models, we link to the maker's own UK product page so you can check the current price there. No affiliate links are live yet.
Freshness: specs are rechecked at least every 3 months, and after new launches. Spec tables carry a "Checked" date for each row, and each page shows its last-checked date. Our build system won't publish a spec table with a row older than 45 days, so most specs are rechecked sooner than the 3-month limit.
Spec-only statement: every comparison carries the line "Compared on manufacturers' published specs. Ferncharge has not tested these products."

5. Checking and AI assistance

New and updated pages go through a separate fact-check against their cited sources before publishing. Specs are rechecked at least every 3 months, and each page shows its last-checked date. We use AI tools to help research, draft and check pages.

6. What we don't do

No testing, measuring, star ratings, scores, "best" picks, awards or customer reviews. If we ever do test anything, we'll publish exactly how, and we'll say clearly which pages use it.

7. Corrections

See corrections for how we handle errors.