Watts to Microwatts

Writing an Implant Power Budget in Microwatts

Implanted and body-worn electronics are designed backwards from the cell. A device that must run ten years inside a person cannot be tuned by trimming watts — it is tuned by arguing over microwatts, one subsystem at a time. Datasheets, however, quote power in watts and milliwatts, so the first move in any implant or wearable budget is to drag every figure down onto a single μW scale where the sensing front end, the stimulation output and the telemetry can be compared honestly.

Conversion factor: 1 W = 1 000 000 μW, so multiply by a million. A hearing aid averaging 0.0015 W is 1 500 μW — about 150 times the roughly 10 μW housekeeping budget of a pacemaker, which is exactly why one runs for a week on a zinc-air cell and the other for a decade on a sealed lithium cell.

What the Microwatts Are Spent On

The sensing front end never sleeps

An amplifier watching a cardiac or neural signal has to stay biased around the clock. A few microamps of bias at 2 to 3 volts is single-digit μW, and it accrues every second of every year.

Therapy output dominates when it fires

Driving charge into tissue through a lead is the one part of the device that touches milliwatts. Pulse amplitude, width and rate turn directly into microwatts of average draw.

Telemetry is priced per session

A link that costs tens of milliwatts while it is up costs almost nothing averaged over a month — unless someone specifies continuous streaming, at which point it dwarfs everything else.

Diagnostics quietly add up

Episode storage, self-checks and lead-impedance measurements each look negligible in isolation, yet together they routinely account for a meaningful slice of a long-life device's μW ledger.

Taking a Component Datasheet Down to Implant Scale

Component vendors publish watts; implant reviewers ask for microwatts and years. This is the loop between the two.

Enter the watt figure from the datasheet

Type the value on the left — 0.0015, 0.005, 0.02 — and the μW column fills in as you type. A comma is accepted as the decimal separator and stray spaces in a pasted value are ignored.

Weight it by how long the block is actually on

Convert the block's active power first, then scale by duty cycle. A 30 mW link is 30 000 μW while it is up; run for five milliseconds a minute it contributes only about 2.5 μW to the average.

Flip to μW → W to talk to the component vendor

Once the budget line is agreed in microwatts, press ↔ so the figure comes back as watts in the form a supplier or a simulator expects — or divide by 1,000,000 by hand.

Copy the clean value into the budget table

The copy button puts the bare number on the clipboard without a unit or spaces, which is what a design-history spreadsheet cell wants. Ctrl + C in a field does the same.

Average power is not the whole story: a cell also has to deliver the peak current a therapy pulse demands without its voltage sagging, and it loses capacity to self-discharge whether the device is busy or idle. Both belong in the budget alongside the μW figures.

Power Budgets Across Implanted and Body-Worn Devices

Representative average draws, from the deepest long-life implant to a device that is recharged or replaced routinely. The span is four orders of magnitude, and it maps almost exactly onto how each device is powered.

Device	What it is doing	Average power (W)	Average power (μW)
Cardiac pacemaker	Sensing continuously, pacing on demand	0.00001 W	10 μW
Implanted glucose sensor	Dormant between readings, queried every few minutes	0.0002 W	200 μW
Hearing aid	Continuous signal processing and receiver drive	0.0015 W	1 500 μW
Neurostimulator	Delivering stimulation on a therapy schedule	0.005 W	5 000 μW
Cochlear implant system	External processor plus implanted receiver	0.02 W	20 000 μW
Ingestible capsule sensor	Imaging and reporting for a single passage	0.025 W	25 000 μW
Wrist-worn body sensor	Display, optical sensing and a wireless link	0.03 W	30 000 μW

Only the top two rows belong to devices expected to be sealed and forgotten. Everything from about a thousand microwatts upward carries a battery the patient recharges or a cell someone changes, because no primary cell that fits in the body holds enough energy to run at milliwatts for years.

What the Converter Adds to a Medical Design Review

Keep watt and microwatt columns in step

Both fields are editable and update together, so a budget table that mixes vendor watts with reviewer microwatts can be reconciled line by line without retyping.

Turn an agreed budget back into a supplier figure

One press of ↔ runs μW → W, the direction you need when a microwatt allocation has to become a purchasing specification.

Milliwatt blocks on the same page

The searchable dropdowns hold every power unit, so a stimulation stage quoted in milliwatts sits beside a μW sensing stage in one comparison.

Nothing leaves the browser

The conversion runs entirely on your machine after the page loads, which matters when the numbers come from an unreleased device file.

Implant and Wearable Power Questions

What average draw does a device need to hold if it must last ten years on one cell?

Divide capacity by hours. Ten years is 87 660 hours, so a 1 Ah cell allows an average of about 11.4 μA; at 3 V that is roughly 34 μW, or 0.000034 W for the entire device. Take off self-discharge and the replacement-indicator margin and the working allowance is smaller still — which is why long-life implants live at ten microwatts rather than a hundred.

What does a single telemetry session really cost the battery?

Convert the active power, then divide by the interval. A 5 ms exchange at 0.03 W is 150 μJ of energy. Once a minute that adds 2.5 μW to the average; once a second it adds 150 μW — fifteen times a pacemaker's whole housekeeping budget. Session length matters far less than how often someone decides the device should talk.

Why can a pacemaker cell be tiny while a neurostimulator needs recharging?

Because the therapy is five hundred times heavier. Pacing pushes a few volts through a lead for under a millisecond, roughly a hundred times a minute, which averages to single microwatts on top of the sensing floor. A stimulator holding 0.005 W — 5 000 μW — would drain the same cell in weeks, so those devices carry a rechargeable cell and an inductive charging coil instead.

Can wireless power replace the battery in an implant altogether?

For shallow, coil-sized devices, yes — an inductive link across a few centimetres of tissue can deliver milliwatts, which is how cochlear systems and rechargeable stimulators work. For millimetre-scale devices deep in the body the delivered power falls to tens or hundreds of microwatts, so those designs are built to run at 0.0001 W or less and to wake only while the external transmitter is present.

Is there an upper limit on power set by the tissue rather than the battery?

Yes. Every watt a device dissipates becomes heat in surrounding tissue, and implant standards are written around keeping the outer-surface temperature rise small — conventionally on the order of two degrees. For a small implant with limited surface area that caps dissipation at tens of milliwatts regardless of what the cell could supply, so thermal headroom, not capacity, sets the ceiling for continuously powered devices.