WattStack: Generator Sizer

FIRMAMBAS

Contains adsIn-app purchases

About this app

Storm on the way, or the power already out? WattStack tells you what size generator you need, what you can run at the same time, which motor to start first, and how long your gas or propane will last. Everything works offline, so it still works when the grid and the cell towers are down.

WHAT SIZE GENERATOR DO I NEED?
Build a list of what you want to power from about 50 common appliances: refrigerator, chest freezer, sump pump, well pump, furnace fan, window AC, central AC, space heater, microwave, CPAP, oxygen concentrator, TV, Wi‑Fi, lights, RV air conditioner, power tools and more. Each one shows its typical running watts and the extra watts its motor needs to start. Change any value to match the nameplate, set the quantity, switch loads on or off, or add your own appliance from its watts or from volts × amps.

WattStack adds up every running watt, adds the single largest starting surge (motors are started one at a time), and keeps 20% running headroom. You get the running and starting watts to look for and a common generator size class, or advice to talk to an electrician about a standby generator when the plan is bigger than a portable unit.

WHAT CAN MY GENERATOR RUN?
Enter your generator's running and starting watts from its spec label. The load meter shows the % of the generator in use and warns when a plan is over its running or starting watts. Dual-fuel generators also keep their propane rating for propane run times.

START MOTORS IN ORDER
The start-order list puts the largest motor first, while the generator is still lightly loaded, and shows the peak of every step. Steps that go over your generator's starting watts are marked in red.

HOW LONG WILL THE FUEL LAST?
See hours per full gasoline tank or per propane cylinder (15, 20, 30, 40 or 100 lb) at your plan's load, and slide to try another load. Add the run times from your generator's spec sheet for a closer estimate; without them WattStack uses a typical fuel curve checked against published spec sheets. The outage plan works out the fuel for N days at H hours a day, in gallons or liters, and how many cans or cylinders that is.

OUTLET AND RV CHECK
Check a plan against a 120 V household 5-20 outlet, an RV 30 A (TT-30), an RV 50 A (14-50) or a generator L14-30 outlet, using the 80% continuous-load rule. 240 V loads are split over both legs, and 120 V loads are balanced between them.

PLANS
Save plans such as Hurricane essentials, RV weekend or Job site, start from a template, and share a plan as text.

SAFETY FIRST
The safety card is one tap away from the Load, Generator size and Run time screens: run a generator outside only, at least 20 feet from the house, with carbon monoxide alarms inside; never plug it into a wall outlet; refuel only when it has cooled.

WATTSTACK PRO
• Unlimited plans and generator profiles
• Parallel kit: two identical generators combined
• My appliances: save your own appliances for every plan
• CSV export of a plan
• No ads
Sizing, start order, run time, the outlet check and the safety card are free for everyone. A one-time option to remove ads only is also available.

Your plans stay on your phone. No account needed. English and Indonesian.

Estimates only. Check your appliance nameplates and your generator's manual, and use a transfer switch installed by a licensed electrician to connect a generator to a house.