What Size Solar System for Off-Grid? Honest Guide (2026)
Short answer: there is no universal one-number system size. Start with measured loads, local weather, the circuits you need to keep running, and your backup policy. Model those choices for your site before buying equipment.
Off-grid size is a reliability choice
An off-grid design must say which loads stay available during low solar production, how long storage should support them, and when another source or load shedding is acceptable. Daily or annual energy totals alone cannot describe that reliability choice.
Choose a backup policy first
- Minimize generator use: model difficult local weather, preserve an energy reserve, and identify loads that can be shifted or shed. This requires careful design and still cannot guarantee independence in every event.
- Allow occasional backup: design for expected seasonal operation and document which loads or periods may use a safe backup source.
- Use backup routinely: prioritize essential circuits, fuel availability, maintenance, transfer equipment, and clear operating procedures alongside PV and storage.
These are planning choices, not guaranteed annual coverage tiers. Results depend on loads, the weather record, shading, equipment, and control settings.
Use local solar and weather data
Latitude is only a rough clue. Check seasonal daylight, clouds, roof orientation, shading, temperature, snow or dust, and consecutive low-production days at the actual site. Test the difficult season across more than one plausible weather period.
NREL PVWatts estimates grid-connected PV production at locations worldwide. It is a useful first estimate, but does not prove off-grid load coverage or battery autonomy.
Build the estimate from measured loads
| Evidence | Input to model | What it tells you |
|---|---|---|
| Load profile | Measured timing and appliance demand | Energy use, peaks, and flexible loads |
| Solar resource | Coordinates, roof layout, shading, seasonal weather | PV production when loads occur |
| Storage and backup | Usable battery limits and backup policy | Periods that need reserve, backup, or load shedding |
Keep the weather period, loads, battery limits, and backup rules the same when comparing designs. The calculator is an estimate, not a site survey or electrical design; have the final configuration checked for equipment compatibility and local code.
Why difficult seasons govern reliability
Annual-average production can hide long periods when generation is below demand. A system designed around the hardest season may need more PV, storage, load flexibility, or planned backup than an annual total suggests. The balance depends on local weather and the cost of an interruption for the household.
Model low-sun sequences
Storage bridges times when production is below demand; it does not create energy. Estimate the essential loads during consecutive low solar periods, include conversion losses and the battery maker's permitted operating range, then test when recharge is available. Avoid a fixed autonomy multiplier that ignores the site and battery.
NREL's System Advisor Model battery documentation describes storage dispatch and degradation modeling.
Compare designs with the same assumptions
- Essential-load design: serves named circuits and allows noncritical loads to wait.
- Whole-home design: includes heating, cooling, cooking, and motor loads and their peak power.
- Backup-supported design: documents when another source starts and what it can serve.
Compare the full equipment scope
| Equipment | Compare | Include in scope |
|---|---|---|
| PV array | Models, ratings, layout, and loss assumptions | Racking, wiring, and roof work |
| Battery | Nominal and usable energy, power, limits, warranty | Controls, enclosure, and replacement path |
| Inverter | Continuous and surge power, operating modes | Protection, transfer equipment, commissioning |
| Installation | Permits, labor, inspection, maintenance | Local code and site conditions |
Prices vary by place, date, design, labor, freight, taxes, and what is included. Compare itemized local quotes on the same scope; a global price-per-watt or battery-price range is not a project estimate. See the DOE Homeowner's Guide to Solar for site and local-policy questions to review with an installer.
Batteries and mains wiring can cause fire, injury, and death. Use qualified professionals and follow local electrical and fire codes.
Your site and backup policy determine the design. Run the free simulation with your coordinates and measured loads, then review the assumptions with a qualified installer.
Run free simulationRelated guides
- Off-grid battery bank sizing
- Off-grid vs grid-tied payback
- Nairobi sizing · Berlin sizing · all 66 cities
Frequently asked
How do I choose an off-grid solar array size?
Build an hourly load profile, estimate PV output for the actual site and difficult season, then test the chosen backup policy in a system model. No latitude band or daily average guarantees a particular array size.
How do I estimate battery capacity?
Start with essential loads and the autonomy you want. Account for conversion losses, usable-energy limits, temperature, power limits, and realistic recharge opportunities. Test low-production sequences instead of a universal multiplier.
Can I design an off-grid system without a generator?
Possibly, if modeled low-sun periods, essential loads, storage, and load shedding meet your reliability needs. Use local time-series weather and professional review; annual average production does not prove autonomy.
Does latitude determine solar system size?
No. Seasonal clouds, shading, roof orientation, temperature, snow or dust, load timing, and backup expectations also matter. PVWatts estimates grid-connected production, not off-grid coverage.