Every field below is editable, including the wattage. The presets are the standard published figures for each bulb type, but the number that actually governs is the one on the tag of your own string — so if it differs, type yours in.
| What | Watts each | How many | Watts |
|---|---|---|---|
| 50 W | |||
| 100 W | |||
| 0 W |
- Total load
- 150 W
- Current at 120 V
- 1.25 A
- Safe budget used
- 10 %
Within budget. A 15 A circuit gives you 1440 W of continuous load once you leave the 80 percent headroom, and this display asks for 150 W — about 1290 W spare.
The formula, in plain text
There is nothing hidden here. The whole calculation is four lines:
- Total watts = sum of (watts each x how many), plus anything else already on the circuit.
- Amps = total watts / 120 V.
- Safe budget = breaker amps x 120 V x 0.8.
- Budget used = total watts / safe budget, as a percentage.
The assumptions, stated
- 120 volts. Standard US residential supply. Actual line voltage varies slightly; it does not move the answer meaningfully.
- The 80 percent factor. The standard continuous-load convention for anything running three hours or more, which a holiday display always is.
- The preset wattages. Standard published ratings per bulb type. Yours may differ; the field is editable for exactly that reason.
- Everything is on one circuit. If your display is split across two circuits, run the calculator twice.
| Breaker | Full capacity | Continuous budget |
|---|---|---|
| 15 A | 1,800 W | 1,440 W |
| 20 A | 2,400 W | 1,920 W |
The continuous budget is the number that matters for a display running every evening for six weeks.
Source: Watts = volts x amps at 120 V; the 80 percent factor is the standard continuous-load convention.
Using it well
- Find out what is already on the circuit. An outdoor outlet often shares with a garage, a bathroom or exterior lighting. Whatever those draw comes out of the same budget, and that is what the “other watts” field is for.
- Read the tags. Every string prints its wattage. So does every inflatable blower — on the housing or the power supply — and that is the figure no manufacturer publishes in a listing.
- Add a row per item type. Not per string. Ten identical strings is one row with a quantity of ten.
- Aim well under 100 percent. The 80 percent factor is already built into the budget, so 100 percent here means you are exactly at the continuous limit. Leave real headroom.
The second limit this does not check
Your circuit budget is one constraint. The other is the maximum number of sets you may connect end to end, which is published on each set and is about the first plug in the chain rather than the breaker. You can be comfortably inside the circuit budget and badly over the series limit by running one very long chain. Both apply at once — the explanation is in how many amps do Christmas lights use.
If it comes out over
Three options, in order of how much they cost:
- Convert to LED. Roughly an eight-fold reduction for the same length of lit strand — the arithmetic is in LED vs incandescent.
- Split across a second circuit. A different exterior outlet, on a different breaker. Check which breaker each one is on before assuming.
- Reduce the display. Which nobody wants to hear, but it is cheaper than the alternative.
And make sure the cord carrying it is rated for it — see extension cords by gauge.