Calculate battery runtime
Runtime is an estimate based on usable energy and real load. Explain the assumptions before treating the result as a purchasing requirement.
Sources and methodology
Updated: 2026-10-17
Source-based desk research and explicit calculations; no hands-on testing claimed. Missing values remain unknown. Installation compatibility needs a separate assessment.
A transparent equation
Runtime hours = nominal Wh × usable fraction × conversion efficiency / average AC watts. A 1280 Wh battery at 80% usable depth and 92% conversion supplies about 942 Wh AC. At 200 W, the simple result is 4.71 hours. This excludes additional inverter idle draw, ageing and temperature effects. State whether those losses are already included in the efficiency you selected.
Variable loads need energy measurement
A refrigerator label is not a constant load. Measure Wh over a representative interval and divide by hours to estimate average watts. A laptop may charge at high power then settle to a lower demand. If devices operate only part of the outage, use a timed load schedule instead of a single average. Keep essential start events in the separate inverter check.
Validate without over-discharging
After installation, test the intended load with monitoring and a conservative stop threshold. Compare measured delivered energy with the plan and investigate differences such as standby draw or unexpected loads. Respect the battery's discharge limits; a BMS emergency cutoff is not a target operating point. Revisit autonomy as batteries age or the essential-load list changes.
Worked case and decision
Take a documented 1280 Wh nominal battery, an assumed 80% usable fraction and 92% inverter efficiency. Ignoring standby draw initially, the available AC budget is about 942 Wh. A steady 100 W load therefore has an illustrative runtime of 9.4 hours; at 500 W it is about 1.9 hours. Now include a 15 W inverter standby demand on the DC side. For the 100 W load, DC demand becomes approximately 100 / 0.92 + 15 = 124 W, reducing the estimate to about 8.3 hours. The same standby draw matters less proportionally at larger loads. These are planning calculations, not a discharge test or warranty claim. Replace the usable fraction and losses with model-specific data, check current limits independently and leave a suitable reserve rather than planning to operate until an emergency BMS cutoff.
Worked example: internet and home working
The values below are assumptions for a reproducible example, not measurements. Replace power and time with your appliance data. Actual running time can differ from the period an appliance remains connected.
On small screens, scroll the table sideways to see all values.
| Load | AC/DC | W | Hours | Wh |
|---|---|---|---|---|
| Router + optical terminal | AC | 18 | 4 | 72 |
| Laptop | AC | 45 | 4 | 180 |
| LED lighting | AC | 12 | 3 | 36 |
| Total delivered to loads | 288 | |||
Calculation assumptions: 80% usable battery fraction; 92% inverter efficiency for AC loads only. Nominal storage is (DC Wh + AC Wh / 0.92) / 0.80. 391.3 Wh.
This threshold excludes inverter idle consumption, DC distribution losses, ageing and weather reserve. For example, an additional 10 W for 4 hours adds 40 Wh to the DC budget and 50 Wh to nominal storage at an 80% usable fraction. Enough energy does not establish startup capability, BMS limits or UPS operation.
What changes the result?
A 90 W laptop replacing the 45 W example for four hours adds 180 Wh of AC demand: about 245 Wh more nominal storage. A 1500 W kettle running for ten minutes needs 250 Wh but also 1500 W instantly; the power check can rule out a system even when its energy would be sufficient.
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.