Battery Backup for Power Cuts in Thailand: How Long Will It Last?
Figures reviewed 14 October 2026
Power cuts and what you actually need
Short power cuts happen in many parts of Thailand, particularly during storms in the rainy season and in rural areas. Before you look at batteries, list what must keep running: usually the fridge, fans, lights, the internet router and phone charging. That list drives the size of the battery and its price.
Energy and power are different
kWh (energy) is how much a battery stores.
W (power) is how fast your appliances use it.
A 5 kWh battery doesn't automatically mean it can supply 5 kW at once. The inverter has its own maximum output, so check both figures.
How usable capacity is calculated
You can't use all of a battery's rated capacity:
Depth of discharge (DoD): the calculator’s 80% is an editable example, not a specification for your battery. Use the limit on your model’s datasheet.
Inverter efficiency: the calculator’s editable 90% is an example of conversion losses, not a measured figure for your system.
Example from the calculator's defaults: a 500 W load for 8 hours needs 4 kWh delivered. 4 ÷ (0.8 × 0.9) = 5.56 kWh rated capacity. In the other direction, a 5 kWh battery runs that 500 W load for about 7.2 hours.
Two scenarios in a Thai home
The wattages below are energy and runtime examples. A plug-in meter can measure compatible plug-in appliances; fixed-wired loads need suitable professional measurement. Check actual running and start power separately for output sizing.
| Scenario | Load | Hours | Energy delivered | Rated battery needed |
|---|---|---|---|---|
| Essentials: fridge (avg 150 W), 1 fan (50 W), LED lights (40 W), router (15 W) | 255 W | 6 h | 1.53 kWh | ~2.1 kWh |
| Essentials plus one bedroom aircon (~1,000 W average in this scenario) | 1,255 W | 6 h | 7.53 kWh | ~10.5 kWh |
Adding one aircon multiplies the battery size about fivefold, which is why most people back up only the essentials.
Check the power limits too
Motor start-up surge: fridges and pumps may draw a short peak above their running watts. Check simultaneous running watts and motor start demand against inverter continuous and surge ratings; this calculator does not verify those ratings.
Batteries age: usable capacity falls over the years, so leave a margin.
Professional installation: a qualified installer must check the electrical protection, compatibility and how the system connects to your home. If you also have rooftop solar, the battery must meet your utility's standards (see the solar guide).
FAQ
Can I use 100% of the battery? Only if the manufacturer says so. Use the depth-of-discharge figure from the datasheet.
Is the runtime guaranteed? No. Real loads vary, and battery condition and output limits change the result.
Size backup for a 255 W load over six hours