This page is a worked reference design. It is deliberately specific so every number can be traced from a load assumption to a component constraint.
It is not a site-specific plan and does not prescribe grounding, conductor, fuse, disconnect, or transfer-switch sizes for a real building.
The five flagship guides share the same module, controller, battery family, and calculations so that the system remains internally consistent.
1. Freeze the load model first
A component-first design often becomes a collection of hardware searching for a load. Start with daily energy and simultaneous power instead.
| Load | Daily energy |
|---|---|
| Efficient refrigerator | 1.00kWh |
| Lighting | 0.35kWh |
| Well pump | 0.45kWh |
| Laptops / electronics | 0.55kWh |
| Kitchen / small appliance allowance | 0.90kWh |
| Internet / networking | 0.75kWh |
| Miscellaneous / controls | 0.80kWh |
| Total | 4.80kWh/day |
For this example, also assume:
- 3.2kW plausible simultaneous continuous load
- motor/appliance events that justify a 6kW continuous inverter class
- inverter/load surge requirement of at least 9kW for several seconds
- 2 days modeled battery autonomy
These are design assumptions, not claims about every cabin.
2. Choose the battery-bus architecture
At multi-kilowatt power, a 48V-class bus keeps battery current far lower than 12V or 24V.
At a 6kW AC load, 92% inverter efficiency, and 47.5V battery voltage:
That current is still substantial, but it is much more manageable than the same load on a 12V-class bus.
For the reasoning behind that choice, see 12V vs 24V vs 48V.
3. Size the battery from energy
Daily AC energy:
With an 80% planned usable fraction and 92% inverter efficiency:
Each reference Pytes module is 5.12kWh nominal:
Three modules provide:
- 15.36kWh nominal
- 300Ah nominal at 51.2V
- simple summed maximum continuous current of 300A, subject to proper parallel installation/current sharing
- simple summed peak rating of 306A for 15s, based on the published per-pack value
Under the planning assumptions:
That is:
Read LiFePO4 Battery Bank Sizing for the energy-versus-current method.
4. Verify continuous and surge battery current
Continuous 6kW example:
Three reference batteries have a simple summed 300A maximum continuous rating, so the bank passes the continuous-current screening check.
A 9kW short surge:
Three packs have a simple summed published peak value of 306A for 15 seconds. That also passes the screening check.
This does not prove the inverter will start every motor. Actual motor starting depends on inverter surge envelope, DC voltage sag, cabling, BMS behavior, load torque, and the appliance itself.
5. Build the PV array from one real module
Reference module:
- Canadian Solar CS6.1-54TM-450H
- 450W Pmax
- 33.0V Vmp
- 13.66A Imp
- 38.9V Voc
- 14.55A Isc
- −0.25%/°C Voc coefficient
Use 8 modules:
Wire them 4S2P:
| Array value | Calculation | Result |
|---|---|---|
| Vmp | 4 × 33.0V | 132.0V |
| Imp | 2 × 13.66A | 27.32A |
| Voc at STC | 4 × 38.9V | 155.6V |
| Isc | 2 × 14.55A | 29.10A |
| Power | 8 × 450W | 3,600W |
See Series vs Parallel Solar Panels for the topology comparison.
6. Verify cold Voc and MPPT limits
At −25°C:
Four modules in series:
Reference controller: Victron SmartSolar MPPT 250/70.
Published values include:
- 250V absolute maximum PV Voc
- 245V startup / operating maximum
- 70A maximum battery current
- 70A maximum PV short-circuit current in the current manual
- 4,000W nominal PV power at 48V battery
The 4S2P array checks out:
| Constraint | Array | Published controller value | Result |
|---|---|---|---|
| Cold Voc | 175.05V | 250V absolute | Pass |
| Operating max | 175.05V cold Voc | 245V start/run max | Pass |
| PV Isc | 29.10A | 70A | Pass |
| PV power | 3,600W | 4,000W @ 48V | Pass |
See MPPT Charge Controller Sizing for the four-check method.
7. Model winter energy without pretending it is guaranteed
For illustration only, assume:
- 3.6kW array
- 2.5 peak-sun-hours/day
- 75% whole-system yield factor after the planning losses represented in this simplified model
That exceeds the 4.8kWh/day modeled load on an average day under these assumptions.
It does not guarantee a full recharge every day.
NREL’s PVWatts exists specifically because real PV output depends on weather, location, tilt, azimuth, losses, and other inputs. A real design should replace this simplified energy line with site-specific modeling.
8. Worked stress case — 48 hours of poor winter solar
Suppose two days produce only 15% of the modeled normal winter harvest.
Poor-day harvest:
Daily deficit:
Two-day deficit:
Starting from the modeled 11.30kWh usable AC battery energy, remaining modeled energy after those two days is:
That is less than one full normal day of the modeled load.
This is where an operational plan matters. Load shedding might prioritize:
- refrigeration
- water
- lighting
- communications
while deferring high-energy convenience loads. A remote or four-season cabin may also justify generator backup, more storage, more array, or a different autonomy target.
9. What is intentionally not specified here
This reference design does not prescribe:
- DC conductor gauges
- fuse or breaker ampere ratings
- grounding-electrode configuration
- neutral-ground bonding location
- transfer-switch topology
- PV rapid shutdown approach
- service/panel configuration
- local permits
Those depend on the exact equipment, wiring method, jurisdiction, and building.
10. Acceptance checklist for a real design
Before installation, replace every example assumption with site/equipment data:
- measured or defensible daily load
- actual pump/motor startup requirement
- exact inverter model and surge envelope
- exact battery manual and BMS limits
- actual module SKU
- actual local minimum design temperature
- site-specific solar production model
- actual cable lengths
- current locally adopted electrical code
- Authority Having Jurisdiction requirements
When comparing off-grid capital investments against grid extension costs, see our energy price reference for official national electricity-price statistics across selected countries. These figures are broad statistical baselines, not local utility tariffs or site-specific payback estimates.
A blueprint is useful when it makes the reasoning visible—not when it hides uncertainty behind a bill of materials.
Explore this decision in an appliance-based plan
Use the Build My Off-Grid System calculator to adjust your own appliances and compare daily solar balance, battery reserve and optional reserve recovery. The preset values are illustrative, not tested equipment specifications or local weather data. The tool is a planning screen, not an electrical compliance check.
To stress-test the example against synthetic low-solar winter days, explore the Winter Solar Survival Simulator. This scenario model is not a location-specific weather forecast, PV output guarantee or equipment design approval.