A home inverter may keep one household comfortable through an evening outage and run out early in another. The difference often starts with the load: an extra fan, a television and several lights all draw from the same battery. Calculate your essential appliances first, then allow for battery condition and conversion losses.
This guide concerns a home inverter connected to a suitable battery bank and professionally installed backup circuits. EchoRepel sells a 200W car inverter, discussed later for portable electronics. Its output rating is not a home-battery capacity or a promise of backup hours.
Estimate hours from energy and load
Use this planning formula: backup hours = battery capacity in Ah × battery voltage in V × usable fraction × inverter efficiency ÷ load in watts. Ah multiplied by volts gives nominal watt-hours. The usable fraction allows for the portion you plan to draw; efficiency allows for energy lost while converting DC battery power to AC output.
Enter fractions as decimals: 50% becomes 0.50 and 85% becomes 0.85. These are assumptions, not settings to copy into a charger. Use the battery maker's permitted discharge and inverter data when available. Battery chemistry, temperature, age, starting charge and discharge rate change the result. Low-voltage cutoff may end backup before the estimate.
A worked example for essential appliances
Assume a fully charged 150Ah, 12V battery, a usable fraction of 0.50 and efficiency of 0.85. The calculation gives 150 × 12 × 0.50 × 0.85 = 765Wh available for AC loads. Divide 765 by each total below. Appliance figures are illustrative planning inputs; replace them with measured consumption or appropriate label values for your equipment.
| Illustrative connected load | Total watts | Approximate backup |
|---|---|---|
| One fan, assumed 70W | 70W | 10.9 hours |
| Fan + two 9W LED lights | 88W | 8.7 hours |
| Above + router, assumed 12W | 100W | 7.7 hours |
| Above + TV, assumed 70W | 170W | 4.5 hours |
| Above + second 70W fan | 240W | 3.2 hours |
Each row is an alternative combination, not extra runtime to add together. The estimate assumes steady consumption and does not separately model inverter idle draw, which matters more at light loads. Fan speed, TV brightness and equipment design affect consumption. Keep a reserve rather than planning an important call around the final few minutes.
Check the battery bank and output rating
Read the installed bank specification. Two identical 12V, 150Ah batteries in series form a 24V, 150Ah bank, not 24V at 300Ah. Do not multiply both figures when calculating energy. Ask the installer to confirm the configuration; changing connections or adding batteries is electrical work, not a runtime shortcut.
The inverter's VA or watt rating limits what it can supply at once. Battery energy determines how long it can supply an allowed load. Motors can also have starting demands. For choosing loads and leaving output headroom, read what size inverter you need, while keeping your home system's own manual as the controlling guide.
Stretch backup by choosing priorities
Switch off unnecessary lights and the TV when nobody is watching. Use one occupied room where practical, and avoid heavy loads such as an iron, kettle or heater on a small backup system. Do not defeat overload protection or add extension boards to bypass a limit. Know which sockets remain powered before the next outage.
Keep charging and ventilation arrangements as the installer specifies. For serviceable flooded tubular batteries, check water-level indicators at the manual's interval and arrange topping up with battery-grade distilled water as directed. Never add acid or tap water, overfill cells or open sealed batteries. Keep sparks and flames away; use trained service help if you cannot safely follow the maintenance procedure.
The manufacturer's battery maintenance guidance explains water checks and maintenance precautions. Record backup duration under a familiar load. A marked drop after a full charge needs diagnosis of the battery, charging and connections rather than repeated deep discharge. Frequent outages may also leave insufficient recharge time between cuts.
Use the car for a few portable devices
A compatible car inverter can help charge a phone or laptop or power a router temporarily. Keep total demand below both the inverter rating and vehicle socket limit, whichever is lower. A separately powered fibre terminal may also be needed. Our guide to using a car during a power cut covers this limited backup role.
Connect devices directly; never feed inverter output into a wall socket or household wiring. Extended engine-off use can leave the starting battery depleted. If longer use requires the engine running, the car must be outdoors in open air, away from doors and windows, never inside a garage or enclosed parking area. Keep connected equipment attended.