Many estimate conversations focus on panels and battery capacity first, but inverter sizing is just as important. The inverter is the device that converts DC power to AC power and controls how your loads are supplied in a hybrid or backup system. If it is undersized, the system may trip or fail to carry the loads you expected, even when the battery has enough energy.
Inverter sizing is not only about the rated power number on the box. Real loads behave differently at startup, and some appliances can demand much more power for a short moment than they do while running normally.
Why inverter size matters
Your inverter must handle the instantaneous power demand of the loads on the backup or protected circuits. This is different from battery sizing, which mostly answers the question "How long can the system run?"
- Battery capacity (kWh): affects runtime.
- Inverter output (kW): affects what can run at the same time.
- Surge capability: affects whether motor/compressor loads can start reliably.
Example: a site may have enough battery energy to run essentials for eight hours, but if the inverter cannot support the fridge compressor plus a pump starting at the same time, users will still see interruptions.
Surge loads explained
Some appliances draw a short burst of high current when they start. This is often called startup surge, inrush, or motor starting current. The surge may last only a moment, but the inverter must be able to tolerate it without shutting down.
Common surge-producing loads include:
- Refrigerators and freezers (compressor start)
- Water pumps and borehole pumps
- Air conditioners and some fans
- Workshop tools with motors
Not all surge events happen together, which is why engineering design looks at load coincidence and circuit planning. Good design can reduce nuisance trips without forcing the client to pay for a much larger inverter than they need.
Why nameplate ratings are not enough
Appliance labels are useful, but they do not always reflect real startup behavior, especially for older equipment or motors under load. This is one reason Iselle treats sizing as an engineering exercise rather than a fixed package assumption.
How Iselle sizes inverter capacity
We size inverter capacity from a combination of load mapping, usage patterns, and backup goals. The design question is not simply "what is your biggest appliance?" It is "what loads need to operate together, under what conditions, and with what reliability expectation?"
Typical design inputs
- Essential-load list: Which circuits must stay on during outages?
- Simultaneous usage: What loads are likely to overlap in real use?
- Motor/compressor loads: Which appliances introduce startup surge demand?
- Future growth: Are you planning added loads (e.g., extra fridge, office equipment, pump)?
- Electrical integration: How the DB split and circuit separation will be done.
Where appropriate, we reduce risk through circuit planning and load separation rather than oversizing the entire system. For example, high-demand loads may be excluded from the backup board or handled on a separate strategy.
Inverter size, battery size, and user expectations must match
A common mismatch is buying a large battery but expecting a small inverter to behave like a whole-house supply. Another is selecting a bigger inverter without defining which circuits will actually be backed up. Reliable systems come from matching these decisions to a clear load plan.
Common mistakes to avoid
- Choosing inverter size from "house size" instead of load behavior
- Ignoring startup surge loads for pumps, fridges, and AC units
- Assuming all circuits can be backed up without a DB split plan
- Comparing inverters on kW rating only, without checking system design context
- Planning today’s load only, then adding equipment immediately after installation
For a budgetary starting point, use the pricing calculator. For the broader load-analysis framework, read System Sizing 101 and the Engineering-led Sizing glossary note.