How many kWh do I need?
Power tells you what can run together; energy tells you how long it can run. Keep both columns in your planning sheet.
Sources and methodology
Source-based desk research and explicit calculations; no hands-on testing claimed. Missing values remain unknown. Installation compatibility needs a separate assessment.
W, Wh and kWh
Watts measure instantaneous power. Watt-hours measure power integrated over time. A constant 200 W load for five hours uses 1000 Wh, or 1 kWh. A 2 kW kettle running for six minutes uses 0.2 kWh, yet still requires an inverter able to deliver its 2 kW demand. Do not confuse a high-energy low-power load with a short high-power event.
Measure the right interval
A plug-in energy meter over a full day captures refrigerator cycling more accurately than one instantaneous reading. Record ambient temperature, appliance mode and whether the day is representative. For fixed wiring, ask a qualified person for appropriate measurement. A monthly utility bill includes loads you may not back up and does not reveal their simultaneous peaks.
Turn the budget into a decision
Mark each load essential, deferrable or excluded. Multiply each essential load by its outage operating time, then add conversion and standby losses. If the resulting battery is too large, shorten runtime or defer loads before shopping. Save the assumptions so you can compare products against the same requirement and explain to an installer why a particular capacity was selected.
Worked case and decision
A 10 W router running for 24 hours uses 240 Wh. A 100 W workstation used for six hours uses 600 Wh. Four 8 W lamps used for five hours add 160 Wh. Together they require 1000 Wh, or 1 kWh, at the loads. A 2000 W kettle used for ten minutes adds about 333 Wh, even though it runs briefly; it also creates a much larger instantaneous power requirement. Decide whether that kettle belongs in the backup budget or can be excluded during an outage. This single choice changes both equipment power and energy needs. A monthly electricity bill cannot reveal that simultaneous peak or identify essential loads. Build a daily table, then a separate outage table using realistic operating times. Measure cycling appliances over a representative interval and repeat the budget for winter if heating controls, lighting or occupancy changes.
Turn the explanation into a procurement brief
Record included and excluded loads, system voltage, simultaneous power, starting demand and required duration. Keep efficiency and seasonal assumptions beside the result. Compare only documented values for the exact model; a missing catalog field means it needs verification, not that a feature exists. Ask the supplier for the manual, Romania delivery terms and warranty procedure before ordering.
After selecting components, a qualified installer should verify cables, protection, source isolation, ventilation and site conditions. Do not use a worked energy example as wiring instructions. When changing a load or battery, revisit both the energy budget and power check. A supervised operating test should confirm assumptions without exceeding manufacturer limits.
Sources and technical reading
Independent editorial explanation. Numerical examples are illustrative calculations, not specifications attributed to a product.