More kWh means more runtime, but load size still controls the result. The same 5 kWh battery can last 45 hours or 3 hours depending on what you plug in.
Planning estimates: 2 kWh = 1,800 Wh and 5 kWh = 4,500 Wh at an assumed 90% usable/system efficiency.
Formula: runtime hours = usable Wh ÷ load watts. Example appliance wattages are representative planning values, not specifications; use your product label or a plug-in meter. Efficiency is a planning assumption, and inverter rating and startup surge are not checked here.
Constant load, 90% planning efficiency.
| Load | 2 kWh runtime | 5 kWh runtime |
|---|
A 5 kWh battery gives 2.5 times the energy of a 2 kWh battery, so at any constant load it runs 2.5 times longer. But moving from 100 W to 1,500 W cuts runtime 15 times on either battery. Load swings matter more than capacity.
With about 1,800 Wh of planning energy, a 2 kWh battery suits small, steady essentials. It is not a promise for any specific household.
Router, lights and a laptop.
2 kWh: 18 hRefrigerator running plus electronics.
2 kWh: 3.6 h (3 h 36 min)A portable space heater or similar.
2 kWh: 1.2 h (1 h 12 min)Refrigerators, freezers, furnace blowers, window ACs and pumps cycle on and off. DOE guidance for refrigerators is to divide plugged-in time by three to estimate hours at maximum wattage. Actual cycling varies with temperature, door openings, and settings.
Use a plug-in watt meter over 24 hours for a real energy number, then divide your battery's usable Wh by the average watts. See the refrigerator and window AC pages.
Energy (kWh) and power (W or kW) are separate. Motor loads draw a surge when starting; DOE guidance notes about three times the running current in the first second. If the inverter cannot supply the surge, the load can fail or the inverter can trip, even with a full battery.
Check continuous and surge ratings and the appliance's starting requirements before trusting runtime math.
Most U.S. appliances use 120 V; larger ones such as dryers and electric cooktops use 240 V (DOE). A battery's outlets and inverter must match the load's voltage. A dedicated voltage-compatibility guide is coming; start with the Full Backup Calculator.
Example: router 15 W + LED lights 40 W + laptop 50 W + CPAP 40 W = 145 W. 1,800 ÷ 145 = about 12.4 h, a full night plus the next morning.
Example: 3 hours of evening TV (100 W) plus router (15 W) = 345 Wh, leaving most of the battery for phones and lighting.
Example: those same 145 W plus a refrigerator averaging 50 W = 195 W. 4,500 ÷ 195 = about 23 h, close to a full day.
Example: a sump pump that runs briefly plus network and lights over a multi-day storm needs the margin. Verify startup power first.
Zaron planning estimates assume 90% usable/system efficiency. Actual usable capacity varies by system.
Limits: runtime ignores inverter rating, startup surge, temperature, battery age and low-state-of-charge cutoffs.
It depends on the average draw, not the running wattage. A unit that draws 150 W while running and runs about a third of the time averages about 50 W, so 1,800 Wh would last roughly 36 hours. This is an example; measure your own refrigerator.
Not at normal use. At 1,000 W, about 4.5 hours; at 500 W, about 9 hours. It suits essential circuits better than whole-house loads.
Possibly for essentials. 4,500 Wh over 24 hours means an average of about 187 W. Heating, cooling or electric cooking will exceed that.
Only a small unit, briefly, and only if the inverter handles the startup surge. At 600 W average, 1,800 Wh lasts about 3 hours.
It stores five times the energy, so it runs the same load about five times longer. It is not automatically five times more powerful; that depends on inverter output.
kW is power, the rate of use right now. kWh is energy, the amount over time. A 1 kW load running one hour uses 1 kWh.