Understanding Electrical Loads and Designing a Solar System

Off-grid electrical design is one of the few domains where doing the math correctly on paper prevents years of frustration in practice. The math is not complicated. But it requires patience and honesty about how you actually live — not how you imagine you will live once you go off-grid.

The Load Audit

Start with a spreadsheet. Column one: device name. Column two: rated wattage (found on the label or in the manual). Column three: hours used per day. Column four: watt-hours per day (columns two times three). Sum column four. That sum is your daily load in watt-hours.

A few nuances. Motors have a startup surge current that can be two to seven times their running wattage. A well pump rated at 1,000 watts may surge to 3,500 watts for two seconds when it starts. Your inverter must tolerate that surge. Label ratings are often optimistic — measure actual consumption with a Kill-A-Watt meter or clamp meter if you can. Refrigerators and freezers cycle on and off; their average consumption is 30–50% of their rated wattage.

Separate your loads into two categories: essential (refrigerator, lights, water pump, phone charging, medical equipment) and comfort (entertainment systems, high-powered tools, washing machine). Design your battery bank to cover essential loads for your autonomy period. Design your panel capacity to cover total loads on an average day. This hierarchy lets you shed load gracefully during extended cloudy periods without losing critical function.

Panel Sizing in Detail

Panel output is rated at Standard Test Conditions (STC): 1,000 watts per square meter of irradiance, 25°C cell temperature. Real-world conditions are almost always worse. On a hot summer day, cell temperature can reach 65°C, reducing output by 15–20%. Panels facing slightly off-south lose additional production. A derating factor of 0.75 (75% of nameplate capacity) is a conservative and realistic planning assumption.

Peak sun hours (PSH) are not the same as daylight hours. They represent the equivalent hours of full-strength irradiance. Phoenix, Arizona averages 6.5 PSH in summer, 4.5 in winter. Boston averages 5.5 in summer, 2.5 in winter. Design for your worst month if you need year-round reliability. For a seasonal cabin, design for the months you occupy it.

Formula: Required panel wattage = (Daily load in Wh) / (PSH × derating factor)

For 3,000 Wh/day, 4.5 PSH, 0.75 derating: 3,000 / (4.5 × 0.75) = 889 watts. Round up to the next standard panel count — at 400 watts per panel, that is three panels for 1,200 watts installed, giving you a reasonable buffer.

Battery Chemistry and Sizing

Lead-acid (flooded, AGM, gel) batteries are the historical standard. They are cheap per kilowatt-hour upfront but require careful management. Discharge them below 50% regularly and their lifespan collapses from 500 cycles to 200. They require temperature compensation — cold temperatures reduce capacity significantly. Flooded batteries off-gas hydrogen during charging and require ventilation. They work, but they demand attention.

Lithium iron phosphate (LiFePO4) is the modern choice for those who can afford the higher upfront cost. Usable depth of discharge is 80–90%. Cycle life is 2,000–5,000 cycles versus 300–500 for lead-acid. No off-gassing. Built-in battery management systems (BMS) handle protection. Over a 10-year period the cost difference largely evens out when you factor in the lead-acid replacements you avoid.

Battery bank sizing formula for lead-acid: (Daily load × autonomy days) / (depth of discharge × system voltage × battery amp-hour rating) = number of batteries needed. More practically: nominal bank capacity in Wh = (daily load × autonomy days) / depth of discharge.

For 3,000 Wh/day, 3 days autonomy, 50% DoD: 3,000 × 3 / 0.5 = 18,000 Wh nominal. At 12V, that is 1,500 amp-hours. A bank of eight 6V 200Ah golf cart batteries wired in series-parallel gives you 400Ah at 24V = 9,600 Wh. Clearly you need to either increase the bank, reduce the autonomy target, or shift to a higher voltage system with larger batteries.

Running a 48V system is worth considering for any installation above 3 kWh/day. Higher voltage means lower current for the same power, which means thinner wire, less heat loss, and smaller charge controller ratings.

Charge Controllers

Two types: PWM (pulse-width modulation) and MPPT (maximum power point tracking). PWM is simple and cheap. MPPT extracts 10–30% more power from the same panels by continuously finding the optimal voltage operating point. For any installation worth building, use MPPT. The efficiency gain pays for the cost difference within the first year.

Size the charge controller by dividing total panel wattage by system voltage to get maximum input current. Add 25% safety margin. A 1,200-watt array at 48V produces 25 amps. An MPPT controller rated at 40A provides adequate headroom.

Inverter Selection

Pure sine wave inverters output power identical to grid power and run all appliances without issues. Modified sine wave inverters are cheaper but can damage sensitive electronics, cause motors to run hot, and create audible hum in audio equipment. Always use pure sine wave.

Inverter efficiency matters at partial load. Many inverters are 90–95% efficient at full load but drop to 80% or lower at 10% load. If you run many small loads for long hours, an inverter with good part-load efficiency (look for 90%+ at 20% load) saves meaningful energy over time.

Inverter-chargers combine the inverter with a battery charger and automatic transfer switch. For any system with a backup generator, this integration is valuable: when the generator runs, the unit charges the batteries; when it stops, it switches back to battery/solar seamlessly.

System Monitoring

A battery monitor (shunt-based, not voltage-based) is not optional — it is how you understand your system. A shunt measures actual current flow in and out of the battery bank and tracks state of charge accurately. Voltage-based estimation is unreliable under load. Brands like Victron, Renogy, and Bogart Engineering make quality monitors. Pair it with a data logger and you will know, within two weeks, exactly which appliances are costing you the most power.

Designing for Resilience

The most robust off-grid systems have multiple input sources. Solar covers sunny-day loads. A small backup generator handles extended cloudy periods and high-draw tasks like welding or running a wood splitter. Some systems add a small wind turbine or micro-hydro input for geographic diversity. The battery bank acts as the buffer that smooths all of these inputs into a stable supply.

Finally: design for growth. Install conduit. Run wire with capacity to spare. Put your charge controller and inverter in a location where you can add a second unit. The system you design today will be expanded in year three when you add a workshop or an outbuilding. Planning for that now costs almost nothing. Retrofitting it later costs significantly.