Solar Backup for Load Shedding: Size Critical Loads (2026)
A backup system is sized from the circuits you choose to keep running and the outages you want to cover. There is no universal "essential home" package: load profile, outage timing, weather, and other uses for storage change the design. The U.S. Department of Energy lists load profile, outage duration and timing, critical loads, and other battery uses among the inputs for a resilience design (DOE sizing factors).
Check the outage schedule for your location
Outage length and timing depend on the local utility and service area. Eskom directs customers to look up the schedule for their suburb and notes that municipal schedules can differ; customers elsewhere should use their own utility's current notices (Eskom schedule guidance). Record the longest outage you actually plan to cover and the time between outages when the battery can recharge.
Build a critical-load list
- Write down which circuits and appliances must stay on during an outage. Keep optional loads in a separate list.
- For each load, note its measured or manufacturer-stated power and the hours you expect to use it during the outage. For a steady 100-watt load used for 5 hours, the arithmetic is 100 × 5 ÷ 1,000 = 0.5 kWh. A refrigerator cycles, so a full-day energy measurement is more useful than multiplying its running watts by every hour.
- Add the power of loads that may run at the same time, then check the appliance and inverter specifications for starting power and other limits.
The DOE's resilience guidance also calls out the building load profile and the timing and duration of outages; measured use at the site is a better starting point than a generic appliance bundle (DOE report).
Translate the list into system requirements
| Input | How to use it |
|---|---|
| Energy during the outage | Sum the energy for the selected loads over the backup window. |
| Continuous and starting power | Check simultaneous demand, appliance startup requirements, and inverter limits. |
| Outage timing and duration | Plan for the outage pattern at the service address and the next opportunity to recharge. |
| Usable battery energy | Account for the battery's stated usable range, reserve settings, conversion losses, and discharge limits. |
| Solar and recharge | Use local weather, shading, orientation, and the time available before the next outage. |
Use weather data as a planning input
This calculator requests hourly sunlight and temperature data for the last five complete years from NASA POWER when the service is available; if it cannot retrieve that data, it falls back to a nearest-city typical-year profile. NASA describes POWER as an analysis-ready solar and meteorological data service with hourly records available from 2001 to near real time (NASA POWER hourly API documentation). Historical modeled weather is a planning input, not a forecast or a guarantee of output at a particular roof.
Confirm the system design and local requirements
Solar panels alone are not automatically backup during a grid outage. A backup design needs equipment that can isolate from the grid and serve the selected loads; the U.S. Department of Energy explains this islanded solar-plus-storage mode in its solar resilience guide. Confirm battery and inverter compatibility, transfer equipment, protections, and the local utility's connection rules with a qualified installer before purchasing or changing wiring.
Costs need local written quotes
This guide does not publish a universal equipment price or savings promise. Ask for a written quote that identifies the exact battery, inverter, panels, installation work, permits, warranty, and any utility charges for your address. Estimates are not a wiring plan or a substitute for site-specific professional review.
Start with the loads you need to protect. Run your address through the calculator, then have a qualified designer verify equipment compatibility and site conditions.
Size my backupRelated guides
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- How to cut your bill with solar
- Johannesburg sizing · Lagos sizing · all cities
Frequently asked
What size solar system do I need for load shedding?
There is no universal panel or battery size. Start from the critical circuits, measured energy during your outage, outage duration, peak demand, and local solar conditions.
Can I start small and expand later?
Only if the inverter, battery, controls, wiring, and protection are compatible with the planned expansion. Ask the installer to document expansion limits before buying.
Will my grid-tied solar work during load shedding?
A grid-only system is not automatically backup. Backup requires equipment designed to isolate from the grid and supply selected circuits. See DOE's solar-resilience guide.
How long will a given battery last through a blackout?
A rough upper bound is usable battery energy in kWh divided by average load in kW. For example, 4 kWh divided by 0.5 kW is 8 hours before conversion losses, reserve settings, cutoff limits, and load changes.