Planning off-grid solar around the way you live

This is a planning guide for discussing a standalone household system with a qualified designer. The examples below are deliberately simplified and are not installation instructions or an equipment specification.

Make an electricity inventory before shopping for panels

List everything the home must supply: refrigeration, lighting, communications, cooking, water pumping, wastewater equipment, ventilation, heating and cooling. Include equipment that runs when nobody is home. Then add intermittent loads such as laundry, tools and guest use. Separate essential operation from things you can reschedule or leave off.

Record the power while operating and the amount of time used. For cycling equipment, use measured energy over a representative period or suitable manufacturer data rather than multiplying the nameplate rating by 24 hours. Record how the estimate was obtained so the designer can identify weak assumptions.

Energy in watt-hours = operating watts × operating hours. Divide watt-hours by 1,000 for kilowatt-hours. If several identical devices operate together, include their quantity. For a measured daily energy figure, use that figure directly; do not multiply it by 24 again.

This example excludes electric space heating, water heating, air conditioning, cooking, vehicle charging and any wastewater equipment. It is not a complete household design. Adding just one 1,500 W load for four hours adds 6 kWh, bringing the example to 8.913 kWh. That is why lifestyle and heating decisions matter more than comparing panel counts first.

Download the blank electricity load list. Make separate columns or copies for the demanding winter day, summer day and essential-load operation. The CSV has no automatic formulas; use the equation above or add formulas in your spreadsheet.

Kilowatts and kilowatt-hours answer different questions

Energy consumption tells you how much electricity is used over time. Power tells you the rate at a particular moment. A battery with substantial energy capacity can still be unable to support the power demanded by several appliances starting or running together.

Ask the designer to identify continuous demand, startup or surge demand, and loads that may coincide. In the simplified table, the pump, washer, two laptops, six lights and router total 1,400 W while running at the assumed ratings. That arithmetic is not an inverter selection: it leaves out the refrigerator’s operation, motor startup and other design conditions.

DOE’s photovoltaic system overview explains the separate roles of modules, mounting, inverters and batteries. Ask for a complete system proposal that identifies how these components work together, including the intended standalone operating mode. A panel package alone is not the household’s electrical system.

Ask how much energy can actually reach the loads

Distinguish nominal battery capacity from the usable amount under the proposed settings and operating conditions. Ask whether a quoted usable capacity is measured on the battery’s DC side or as delivered AC energy, which losses have already been included, and what power limit applies. Otherwise, two battery proposals can look equivalent when they are not.

Worked example: two days of essential loads

Teaching assumptions: essential AC consumption is 3 kWh per day and the planning interval is two days without solar input. Required delivered energy is 6 kWh. If the assumed usable fraction of nominal capacity is 80% and discharge-path efficiency is 90%, simple nominal capacity is 6 ÷ (0.80 × 0.90) = 8.33 kWh.

Those percentages are invented for the calculation, not recommended settings or claims about a battery chemistry. The result excludes additional reserves, aging, temperature effects and power constraints. Do not apply another 80% reduction if a manufacturer’s stated usable capacity already accounts for it. Have the designer explain each allowance and avoid counting the same loss twice.

Then ask how the system recovers. Covering two poor-sun days does not explain how it will supply the next day’s loads and recharge the battery. The recovery plan may require reduced consumption, more generation or another accepted backup arrangement.

Use the difficult season, not just an annual total

A yearly generation estimate can conceal the period when supply is low and household demand is high. Ask for the site’s shading assessment and a seasonal model using the planned array, location and consumption pattern. Include conditions specific to the installation and explain how the backup plan is triggered.

The DOE homeowner guide describes PVWatts as a tool for estimating grid-connected PV production. It can help frame a generation discussion, but an annual output estimate is not proof that an isolated household will have power during every cloudy period. Standalone supply and storage need a separate design assessment.

Another arithmetic example: supplying 3 kWh per day with an assumed three equivalent full-sun hours and a 75% delivery factor gives 3 ÷ (3 × 0.75) = 1.33 kW. None of those assumptions has been established for a property. The calculation also leaves out recovery after depleted storage. Use it to understand a designer’s explanation, not to buy a 1.33 kW array.

Require the same operating assumptions in every quote

• Loads: the agreed household inventory, seasonal demand and essential-load plan.
• Production: array location, shading and weather assumptions, and the period used for the design.
• Storage: nominal and usable energy, power limits, reserves and conditions affecting operation.
• Backup: what happens during a long deficit, who acts, and how the system recovers.
• Site work: mounting, trenching, equipment location, access and any necessary structural work.
• Approvals: responsibility for design, permit submissions, inspections and final documentation.
• Ownership: warranties, service access, monitoring, consumables and replacement assumptions.

Ask for an explanation of exclusions and for the finished system to be demonstrated against the agreed operating brief. Keep manuals, settings, commissioning records and service contacts with the property file. Electrical and battery design should be performed and installed by appropriately qualified people under the applicable requirements.

Finally, confirm that the dwelling can be approved with the proposed arrangements. A sound technical proposal does not decide a utility connection requirement. Keep that investigation separate using the utility obligations guide, and include water and wastewater loads in the same plan.

Agree what the installer will demonstrate

Before installation, ask the designer and installer to define the commissioning checks for the agreed loads, controls, storage and backup. Record who will perform each check and the result you will receive. Include the assumptions and conditions that a short site visit cannot establish, such as performance through a prolonged winter deficit.

At handover, request the installed circuit and equipment documentation, final settings, test records, required inspection documents, warranties and service arrangements. Ask the installer to explain the monitoring display, normal operating limits and the owner response to an alarm. Have qualified personnel demonstrate approved shutdown and restart procedures using the equipment instructions.

Confirm which devices remain essential during a shortage and how the owner will know when to reduce demand or seek service. Save this with the system-handover worksheet and use the build sequence to coordinate pump, wastewater and other electrical work across contractors.