How to size a backup battery
Battery capacity starts with an energy budget. A large Ah number is not enough to tell you what will run, for how long, or whether the battery can start the load.
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.
Build the energy budget
For each essential appliance, multiply measured running watts by operating hours during the outage. Add the results in Wh. A router at 15 W for 8 hours needs 120 Wh; a 60 W laptop for 4 hours needs 240 Wh. A refrigerator requires measured daily consumption or a justified duty cycle, not its starting power multiplied by every hour. Keep the startup event in a separate power column.
Convert delivered energy into storage
Nominal storage Wh = required AC Wh / usable discharge fraction / inverter efficiency. For 1000 Wh, an illustrative 0.80 usable fraction and 0.92 conversion efficiency give 1359 Wh. These factors are inputs, not universal promises. Add standby consumption and a justified reserve for temperature and ageing. If you add future loads, show them explicitly so reserve is not counted twice.
Check the second constraint: current
At 1000 W AC and 12.8 V nominal DC, 92% efficiency implies about 85 A. Current rises as battery voltage falls. A bank that stores enough energy may still trip its BMS on a compressor start. Compare continuous and time-limited discharge ratings, then coordinate the cable, fuse and terminals. Only use series or parallel grouping explicitly permitted for the model.
Worked case and decision
Consider 1000 Wh of essential AC consumption over an evening. At 80% usable discharge and 92% conversion efficiency, the nominal requirement is approximately 1359 Wh. A documented 12.8 V, 100 Ah battery stores 1280 Wh nominally, so one such battery falls short even before ageing or standby losses. Two batteries are not automatically the answer: the manufacturer must permit the grouping, the batteries must be suitable for joint operation and protection must be designed for the resulting fault current. Alternatively, reduce the load budget, choose a larger approved battery or shorten the autonomy target. Record which change made the system fit. Before comparing price, check whether the proposed inverter can draw its required current without exceeding the battery's continuous discharge limit. Passing the energy calculation only resolves one part of the selection.
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.