- Audit loads first: record measured energy use, running power, duty cycles and manufacturer-stated surge requirements.
- Model the worst month you intend to rely on, using site-specific solar data and shading rather than an annual average.
- Define critical loads and the number of low-production days you want to cover; those are design inputs, not universal settings.
- Battery chemistry, voltage, inverter, protection, cable, isolation and placement are professional design decisions.
- Use a licensed and appropriately accredited designer and installer, and comply with current local electrical, building and fire rules.
This article has not been reviewed by an electrical engineer or electrician and is not a system design. Solar arrays and batteries can remain hazardous when the grid is off, and faults can cause shock, arc flash or fire. In Australia, the Clean Energy Regulator requires accredited, licensed designers and installers for eligible systems and points to state and territory requirements. Elsewhere, follow the current licensing, code, permit and inspection rules for the site; the US Department of Energy homeowner guide likewise recommends a qualified professional.
A solar system is an energy budget: generation is variable income, batteries hold a limited reserve and loads draw from both. That analogy is useful for planning, but the final equipment and protections must come from a site-specific professional design.
Part oneThe load audit: one evening, one notebook
Walk through the home with a notepad and, where compatible and safe, a correctly rated plug-in energy meter. For each device, record measured energy use, running power, duty cycle and the manufacturer's start-up information. Do not open equipment or work on fixed wiring to collect readings.
| Load | Watts | Hours/day | Wh/day |
|---|---|---|---|
| Fridge (modern, 10–15 cu ft) | ~90 avg | cycles | 1,000 |
| Chest freezer (7 cu ft) | ~60 avg | cycles | 700 |
| LED lighting, whole house | 60 | 5 | 300 |
| Laptop + phone charging | 70 | 6 | 420 |
| Internet (router / satellite) | 40 | 16 | 640 |
| Well or transfer pump | 750 | 0.5 | 375 |
| Washing machine (cold, 4 loads/wk) | 500 | ~0.4 | 300 |
| Kitchen small appliances | — | — | 400 |
| Total | ~4,100 Wh |
Heating, cooling, hot water and cooking can dominate an off-grid load. Do not replace one energy source with wood, gas or another fuel from a solar spreadsheet alone; each option has ventilation, fire, carbon-monoxide, fuel-storage, cost and local-code implications. First reduce avoidable demand and measure actual appliances, then ask the designer to compare whole-system options.
Part twoThe worst-month rule
Panel ratings are laboratory values. Site output changes with season, latitude, orientation, shade, temperature, soiling and system losses. Model the worst month you intend to rely on with site-specific data, and have the designer explain the low-production and backup assumptions.
The designer should apply equipment-specific efficiency, temperature, ageing, depth-of-discharge, reserve, shading and conversion losses. Autonomy and backup are household risk decisions; neither a fixed number of battery days nor a generator is universally safer or cheaper.
Turning the example into a professional brief
If the illustrative table measured 4.1 kWh a day, the next step would be to give the designer the underlying appliance readings, seasonal variation, critical-load list, site address, shading and desired autonomy. The total alone is not enough to specify equipment.
- Array question: what output does the proposed layout model in the lowest-production month, including shade and stated losses?
- Battery question: how much certified usable energy remains at the site's temperature, reserve setting and expected end of warranty?
- Inverter question: which measured loads may overlap, and what start-up behaviour do their manufacturers document?
System voltage changes current, conductor, protection, isolation and equipment-compatibility requirements. Do not choose or reconfigure it from a rule of thumb; it belongs in the compliant system design.
Part threeWhat a comparable quote should show
| Area | What the quote should identify | Check |
|---|---|---|
| Design basis | Load data, solar model, shade, autonomy, backup and growth assumptions | Documented |
| Equipment | Exact approved models, ratings, compatibility, warranties and operating limits | Current |
| Safety and compliance | Protection, isolation, labels, battery location, ventilation, permits and applicable standards | Signed off |
| Installation | Licensed and accredited people responsible for design, work, testing and commissioning | Verified |
| Handover | Certificates, as-built drawing, shutdown procedure, monitoring, maintenance and emergency contacts | Included |
| Whole-of-life cost | Installed price, maintenance, replacement assumptions, finance and disposal | Compared |
Battery chemistry affects fire behaviour, usable capacity, temperature limits, maintenance, enclosure, recycling and warranty. Compare certified products within the professional design and read the warranty's test conditions; cycle-life and usable-energy figures are not interchangeable guarantees.
Backup generation can reduce required storage, but it adds exhaust, carbon-monoxide, fire, fuel-storage, noise, maintenance and connection risks. Compare it with load shedding and extra storage, and have any electrical connection designed and installed compliantly.
Part fourSafety belongs in the signed design
- Fault protection and isolation: the designer specifies devices, ratings, locations, coordination and shutdown behaviour for the exact equipment.
- Conductors and connections: a licensed installer sizes, routes, terminates, torques, tests and documents them under the applicable standard.
- Earthing, bonding and surge protection: these are site- and jurisdiction-specific; a one-line rule is not safe.
- Battery and array hazards: location, access, clearances, weather, fire response, labels and emergency isolation must be reviewed and signed off before operation.
Part fiveStart with one circuit
You do not have to move the whole house off-grid at once. Ask a licensed designer to scope a compliant critical-load system around a short, measured list such as refrigeration, communications and lighting. Confirm which existing circuits can be supplied, how transfer and isolation work, what runtime is modelled and whether the proposed components can safely support a later expansion.
AppendixThe commissioning checklist
- Load audit, critical-load list and worst-month model documented.
- Designer and installer licence and relevant accreditation verified.
- Exact equipment approvals, compatibility and warranty conditions checked.
- Permits, network approvals and required inspections completed.
- Electrical safety or compliance certificate and as-built drawing received.
- Shutdown, isolation, monitoring, maintenance and emergency procedures demonstrated.
- Low-production and equipment-failure scenarios tested on paper before handover.