RV Power Calculator: Shore Power Watts and Battery Runtime
The calculator multiplies hookup volts by amps to get the watts available, subtracts what is already running and tells you whether your heater or AC fits. It also divides your usable battery watt-hours by the appliance’s average draw. A 30 A RV hookup at 120 V supplies 3,600 W.
This calculator answers two questions: will an appliance fit on your campground hookup, and how long will your battery run it? For the hookup, it multiplies volts by amps, subtracts the loads already running and checks whether your heater or air conditioner fits. For the battery, it divides usable watt-hours by the appliance’s average draw.
The voltage defaults to 120 V (US and Canada). Set the amps to match your pedestal: 15 or 20 A for a household-style outlet, 30 A or 50 A for RV outlets.
1. Campsite hookup
2. On battery
Hookup supplies: 0 W · headroom 0 W →
Battery runtime: 0 h
Formulas: available W = V × A. Hours = Wh × usable depth × inverter efficiency ÷ (W × duty). Compressors start with surges of 2-3 times their power: leave headroom on the hookup.
How does the calculation work?
Two formulas, one for each part.
- Hookup: available W = volts × amps. Headroom = available W − other loads. If the heater or AC wattage is at or below the headroom, it fits; if not, the calculator warns that the breaker will trip.
- Battery: hours = battery Wh × usable depth % × inverter efficiency ÷ (appliance W × average duty %). The inverter factor is 0.88 if you choose Yes and 1 for direct 12/24 V appliances.
Example, the battery defaults: a 1,280 Wh battery, 90 % usable, through an inverter, running an 800 W heater at 60 % duty. That is 1,280 × 0.9 × 0.88 ÷ (800 × 0.6) ≈ 2.1 hours. The same load on direct 12 V would last about 2.4 hours.
For batteries rated in amp-hours, Wh = V × Ah: a 100 Ah battery at 12.8 V holds 1,280 Wh.
How do I read the result?
Treat “it fits” as fitting at running power, and leave margin for starts. The calculator compares running watts only. Air conditioner compressors start with surges of 2 to 3 times their running power, so a unit that barely fits can still trip the breaker.
| Hookup | Watts available | Left after 150 W of other loads | Where to look |
|---|---|---|---|
| 15 A at 120 V | 1,800 W | 1,650 W | Low-wattage heaters in the best electric space heaters |
| 20 A at 120 V | 2,400 W | 2,250 W | Heaters on high, or a small AC with its start-up surge in mind |
| 30 A at 120 V | 3,600 W | 3,450 W | The best RV rooftop air conditioners |
| 50 A (two 120 V legs) | 12,000 W in total, 6,000 W per leg | 5,850 W on one leg | Loads split across legs by your RV’s wiring |
For battery results, a few hours is normal for heaters and air conditioners. Roof vent fans and small dehumidifiers run far longer; compare their draw in the best RV roof vent fans and the best RV dehumidifiers. For heating off-grid, a diesel heater uses little battery; check it with the diesel heater fuel calculator.
Cooling capacity is often given in BTU/h: multiply by 0.293 for watts of cooling. An 8,500 BTU/h unit gives about 2,490 W of cooling, which is not the same as the electrical power it draws. Use the electrical figure from the listing in the calculator. More background is in camper air conditioners.
Frequently asked questions
How many watts can I use on a 30 amp RV hookup?
Available watts = volts × amps, so 120 V × 30 A = 3,600 W in total, shared by everything that is on. Leave headroom for air conditioner compressors, which start with surges of 2 to 3 times their running power.
How many watts does a 50 amp RV service give?
A 50 A RV outlet carries two 120 V legs, 12,000 W in total. The calculator multiplies one voltage by the amps, so selecting 50 shows 6,000 W, the capacity of one leg. Loads are split between the legs by your RV’s wiring.
Can I run a space heater on a 15 amp outlet?
120 V × 15 A = 1,800 W. A heater on its low setting usually fits alongside a few small loads, but a heater on high plus a kettle or microwave can trip the breaker. Enter your loads to check.
How long will my battery run an appliance?
Hours = battery Wh × usable depth × inverter efficiency ÷ (appliance W × average duty). With the defaults, a 1,280 Wh battery running an 800 W heater at 60 % duty through an inverter lasts about 2.1 hours.
What inverter efficiency does the calculator use?
It uses 88 % when you choose Yes (an inverter typically loses 10-15 %) and 100 % for appliances that run directly on 12/24 V.