Halloween tombstone LED nights
Estimate battery-pack nights for lighted Halloween tombstones from stone count, LED draw, and nightly hours for Prince William County yard displays.
nights = (pack_Wh × usable%) ÷ (stones × W_each × hours_night)
When to use
Use this when battery-powered LED tombstones sit away from outdoor outlets and you need pack nights, not just runtime hours.
Typical foam LED stones draw 0.5–2 W each; check the label. Usable capacity of 70–80% leaves margin for cold October evenings.
Lighted stones on one pack or circuit group.
Label watts or measured draw per tombstone.
Timer window from dusk.
Power-bank or battery pack capacity.
USB packs are often 5 V; 12 V packs use 12.
Leave headroom for cold nights and aging packs.
Worked example
Example defaults — 6 stones × 1 W × 5 h = 30 Wh/night.
Pack: 10000 mAh × 5 V = 50 Wh nameplate; 75% usable → 37.5 Wh.
Nights: 37.5 ÷ 30 ≈ 1.3 — plan on 1 full night before a mid-evening swap, or upsizing the pack.
Second example: one input changed
From the defaults (Tombstone count = 6; LED watts per stone = 1; Hours per night = 5; Pack mAh = 10000; Pack volts = 5; Usable capacity % = 75), the calculator shows:
Nightly load: 30.0 Wh
Usable pack energy: 37.5 Wh (nameplate 50.0 Wh)
Display nights: 1.25
Continuous hours if left on: 6.3 h
Now raise Tombstone count from 6 to 9 and keep the other entries. The result becomes:
Nightly load: 45.0 Wh
Display nights: 0.83
Continuous hours if left on: 4.2 h
Comparing the two runs tells you how sensitive the answer is to that one measurement. If a small change in it swings the result a lot, measure it twice.
Limits / assumptions
Units: watts, hours, mAh, volts.
Assumptions: steady LED draw; no solar recharge; one pack feeding the listed stones.
Not a substitute: manufacturer runtime charts or lithium pack charge limits in freezing weather.
Keep battery packs dry and off wet mulch. Do not enclose lithium packs in sealed foam without ventilation.
FAQ
Corded tombstones instead?
Skip this tool — use outlet load / GFI headroom for plug-in stones.
Different watts per stone?
Average the draws, or run separate groups with different packs.
Solar stake packs?
Treat daytime harvest as bonus; size for nights without sun.
How is this different from generic LED runtime?
Yes — this is stones × W × hours → nights, not a single string wattage.