A battery powered signage screen is sized from watts times hours: take the panel's rated watts, multiply by the hours per charge, add 20 percent for losses, and buy that many watt-hours. An 18 W panel needs about 86 Wh to cover a four-hour session.
A screen on an e-bike or a cart dies when its battery does, so the battery is the first thing to size, ahead of the screen. The arithmetic fits on one line: rated watts times hours, times 1.2 for converter and cable losses. This article works that line for a small signage panel on a bike, compares the four ways to power it, and covers the two things that end a mobile sign before a flat battery does, rain and daylight.
The answer: watts times hours, plus 20 percent
Battery packs are labeled in watt-hours (Wh). A screen's appetite is printed on its spec sheet in watts (W) as rated or typical power, and runtime in hours is watt-hours divided by watts.
Watt-hours needed = panel watts × hours per charge × 1.2
The 1.2 multiplier adds 20 percent for DC-DC converter loss, cable loss and capacity fade. Keep it even when the panel runs straight from a matching battery, because packs age and sheets quote best-case figures.
Small all-in-one signage panels, screen and player together, typically draw 10 to 30 W, and doubling the watts halves the runtime. A pack in the common small-bank size class therefore runs a 10 W panel about 8 hours and a 30 W panel about 3. The exact figure for any panel is on its sheet, and what a digital signage display spec sheet tells you shows where the rated power sits among the other numbers. If a sheet lists current instead, multiply amps by the rated input volts to get watts. Packs labeled in amp-hours convert the same way, amp-hours equals watt-hours divided by nominal volts.
What decides the size
Panel watts. The biggest lever. A 15-inch class all-in-one with its player sits in the high teens of watts, and brighter or larger panels climb toward 30 W.
Hours per charge. Your longest session, plus margin. We publish no duty-cycle rating for these units, so work per charge and recharge between outings.
The pack's real label. Watt-hours if printed, otherwise amp-hours times nominal volts.
Cold. Lithium packs deliver less usable capacity near 0 °C, so an early spring morning costs runtime.
A pack that is too small or worn shows the problem early: random restarts and black screens before total shutdown. The power-first checks for a screen that goes blank and freezes apply to a mobile rig just as much as to a wall-mounted one.
Power options for a mobile sign
A mobile sign has four practical power sources. The table assumes an 18 W panel, so scale the hours by 18 divided by your panel's rated watts.
| Power source | Suits | Watch for |
|---|---|---|
| Small USB power bank | Tablet signs and short sessions | Most signage panels need 12 V DC or more, so a plain USB port will not run one; the bank and lead must deliver the panel's input volts |
| 12 V lithium pack in the small-bank size class | An 18 W panel for about 4.6 hours per charge | Fuse the positive lead and charge with the pack's own charger |
| The bike's own battery through a DC-DC converter | Whole-day routes | Converter rated above the panel's watts, output set to the panel's input volts |
| Venue mains through a covered outlet | A cart parked in one spot | RCD-protected outlet and a cable route nobody steps on |
Worked example: an 18 W panel on an e-bike for a four-hour session
The rig: one 15-inch class signage panel on a cargo rack, playing from a card, ridden to a market and run through a four-hour session.
Inputs, with their sources:
- Panel draw 18 W, the static power figure listed for our panel-mount units. Use your own sheet's rated figure.
- Target runtime 4 hours per charge, chosen to cover the session with a recharge between outings.
- Overhead multiplier 1.2, the 20 percent adder from the formula.
- Candidate packs: a small USB power bank, and the bike's own battery, labeled 36 V and 10 Ah.
The rule in words: watt-hours needed equals watts times hours times 1.2, and a label in amp-hours converts to watt-hours by multiplying by nominal volts.
The arithmetic:
- 18 W × 4 h = 72 Wh
- 72 Wh × 1.2 = 86.4 Wh, the capacity the rig needs
- Effective draw 18 W × 1.2 = 21.6 W, so a bank labeled 100 Wh runs 100 Wh ÷ 21.6 W = 4.6 hours
- Margin 100 Wh − 86.4 Wh = 13.6 Wh, about 38 minutes extra
- Bike battery 36 V × 10 Ah = 360 Wh, which runs 360 Wh ÷ 21.6 W = 16.7 hours
Result: the small bank covers the four-hour session with about 38 minutes to spare, and the bike's own battery covers 16.7 hours.
Decision: up to about 4.5 hours per charge, a pack in that size class carries an 18 W panel. Past about 4.5 hours, take power from the bike battery through a fused DC-DC converter rated above 18 W.
Rain, daylight and mounting
Rain ends most mobile signs. Our panel-mount displays are not rated for outdoor use, and only the front face is rated IP65, so a mobile rig needs a housing around the whole screen, with sealed glands where cables enter. An ingress protection rating grades how well an enclosure keeps out dust and liquids, and the rating belongs to the assembled enclosure, so fit the housing's seals the way its instructions describe.
Daylight decides whether anyone reads the sign. An indoor-grade panel around 300 nits washes out in street light, while panels built to read in sunlight sit near 1000 nits. Pick the brightness option before the size, because a washed-out sign advertises nothing.
Mount to a rigid plate on the rack or box frame, metal to metal, with threadlocker on the bolts, and strain-relieve every cable so road vibration cannot back a connector out. Foam padding under the screen hides a loose bolt more often than it absorbs anything.
For content, keep it offline: a player that reads its playlist from a card needs no data plan on the road, the same playback behind small retail displays. Ours load images and video from microSD or USB with no subscription, and the player is designed to return to its playlist after a power cut, which matters when a pack runs flat. The article on power loss recovery for offline displays shows what that recovery looks like.
When to pick something else
If the sign lives outside through a season of weather, stop adapting an indoor panel: put it in a housing with a rated window and glanded cable entries, or use a display built for exposed installations. The watt-hour arithmetic stays the same, and the housing decides whether the electronics survive.
If you need to push new content while the screen is out, an offline card player is the wrong tool for that one job. A connected player with a data plan does it, and the modem's draw belongs in the watts line of the formula. Everything else here carries over.
What to ask for before you order
- The display's rated input voltage range and rated power in watts, or amps at nominal volts, so the watt-hour sum uses real figures.
- Brightness in nits, for the daylight check.
- The battery label in watt-hours, or amp-hours with nominal volts.
- A DC-DC converter rating in watts, above the panel's rated draw, if pack and panel voltages differ.
On a current-range model from us, the quote confirms the software, ports, power, orientation, rating, lead time and warranty; on the panel-mount series carried in the cart, the listed figures are the ones to use.
Recommendation: Size the pack before you shop for screens. Take the panel's rated watts, multiply by the hours you need per charge, multiply by 1.2, and buy a pack with at least that many watt-hours. Up to about 4.5 hours, a pack in the small-bank size class and an 18 W-class panel do the job with margin. Past that, run the screen from the bike's own battery through a fused DC-DC converter rated above the panel's watts. House the screen against rain, since indoor panels are not rated for outdoor use, and choose the brightness option for daylight before anything else.
Questions
Can I run the screen straight from the e-bike battery?
Only if the panel's DC input range covers the pack's nominal voltage. Some all-in-one signage panels accept anything from 12 to 36 V DC and can be wired to a matching battery through a fuse, with no converter; panels fixed at 12 V need a buck converter. Check the input label or the quote, fuse the positive lead close to the battery, and use a locking connector, because road vibration backs loose connectors out.
How long does a 100 Wh pack last with a 30 W panel?
About 2.8 hours. The effective draw is 30 W times 1.2, which is 36 W, and 100 Wh divided by 36 W gives 2.8 hours. Panels at the top of the 10 to 30 W range drain small packs quickly, and a high brightness option pushes the draw higher, so read the rated watts before you size the pack.
What happens when the pack runs flat mid-playlist?
The screen goes blank with the pack and returns when power does. A card-based player is designed to resume its playlist after a power cut, so the usual cost is lost selling time, not a corrupted loop. Size the pack so the flat point lands after your longest session, and recharge lithium packs at room temperature, since charging a cold pack shortens its life.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.