The short answer
A first approximation is energy available to the loads divided by average load. It is only meaningful after backup compatibility, power limits, starting charge and reserve have been established.
State where the energy is measured
Keep nameplate capacity, usable capacity and energy actually available to the loads separate. If a reserve or loss has already been deducted, do not deduct it again.
For a real installation, ask the designer which part of the displayed state of charge can be used in backup operation and which operating restrictions apply.
Compare three illustrative loads
Assume 6 kWh is available at the loads after reserve and losses. At a constant 0.3 kW average load, the arithmetic is 6 ÷ 0.3 = 20 hours. At 1 kW it is 6 hours; at 2 kW it is 3 hours.
These are arithmetic examples, not measured results or product promises. Cycling appliances, standby use, temperature, charge level and changing demand can change the outcome.
Check whether the loads can start
An average-load calculation says nothing about whether the inverter can start or continuously run a particular appliance. The power requirement must fit the system’s backup capability.
Ask for high-demand loads to be reviewed individually. Decide which loads can be postponed so that essential services last longer.
Keep solar recharge as another scenario
Start with a no-recharge case, then consider what compatible solar could add under stated conditions. Do not assume the full clear-day forecast will be available during an outage.
SolarIQ’s savings forecast does not calculate a guaranteed backup duration. Use these questions to prepare for a dedicated assessment.
Before your assessment
Bring these questions with you.
- Starting charge and backup reserve.
- Available energy measured at a clear boundary.
- Average load plus peak and starting requirements.
- No-recharge and possible-recharge cases separated.