Designing Small Standalone Systems: Panel, Battery, Load Math

By Solar Panel Kits · September 12, 2026

Designing a small standalone solar system is sequential arithmetic: calculate the load, size the battery, size the panel, select the controller, and specify the wiring. Each step feeds the next, and skipping a step or guessing means the system either costs more than necessary or fails in the field. This guide walks through each step with the math, the decisions, and the common errors that trip up first-time builders.

Step 1: Calculate Your Daily Load

List every device the solar system will power. For each device, record its wattage and the number of hours per day it will run. Multiply watts by hours to get daily watt-hours (Wh) per device. Sum all devices for total daily consumption.

If any device cycles on and off (like a motion-triggered camera or a thermostat-controlled fan), estimate the average duty cycle. A camera in motion-detect mode may average 2 watts instead of its 5-watt active power, reducing daily consumption from 120 Wh to 48 Wh. Be conservative with duty cycle estimates: underestimating load is the most common design error.

If any device requires AC power, add the inverter efficiency loss. Inverters typically waste 5–15 percent of the energy as heat during DC-to-AC conversion. Multiply the AC load's watt-hours by 1.15 to account for this loss. Better yet, choose DC-native devices where possible and eliminate the inverter entirely.

Step 2: Size the Battery

The battery must store enough energy to power the load through overnight hours, cloudy days, and seasonal output lows. The sizing formula is:

Battery capacity (Wh) = Daily load (Wh) × (1 + Days of autonomy) ÷ Maximum depth of discharge

Days of autonomy is how many consecutive cloudy days the system must survive without any solar input. One day is typical for non-critical applications. Two to three days for critical systems in cloudy climates.

Maximum depth of discharge (DOD) protects the battery from excessive drain. For AGM lead-acid, use 0.5 (50 percent DOD). For LiFePO4 lithium, use 0.8 (80 percent DOD).

Using the example above: 432 Wh daily load, two days of autonomy, LiFePO4 battery.

Battery capacity = 432 × (1 + 2) ÷ 0.8 = 432 × 3 ÷ 0.8 = 1,620 Wh

For a 12V system: 1,620 Wh ÷ 12V = 135 Ah. A 150Ah 12V LiFePO4 battery provides a comfortable margin.

For AGM at 50 percent DOD: 432 × 3 ÷ 0.5 = 2,592 Wh = 216 Ah at 12V. This illustrates the capacity disadvantage of lead-acid: you need a physically larger and heavier battery for the same effective storage.

Step 3: Size the Solar Panel

The panel must generate enough energy each day to power the load and recharge the battery. The sizing formula is:

Panel wattage = Daily load (Wh) ÷ Peak sun hours × Derating factor

Peak sun hours vary by location and season. Use the worst-month value for year-round systems. At 40° North latitude, winter peak sun hours are approximately 3.0 hours. In summer, approximately 5.5 hours.

Derating factor accounts for real-world losses: panel soiling, wiring losses, controller conversion losses, and temperature effects. Use 0.75 as a conservative derating factor (25 percent total system losses).

Panel wattage = 432 ÷ 3.0 ÷ 0.75 = 192 watts. Round up to a 200-watt panel or two 100-watt panels.

If you size to summer sun hours instead: 432 ÷ 5.5 ÷ 0.75 = 105 watts. This system works in summer but fails in winter, which is why the worst-month sizing is critical for year-round systems.

Step 4: Select the Charge Controller

The charge controller regulates charging and protects the battery. Choose between PWM and MPPT based on system size and budget. For systems under 100 watts, PWM is adequate and cost-effective. For systems over 100 watts, MPPT recovers 15–30 percent more energy and usually pays for itself through reduced panel sizing requirements.

Controller amp rating must exceed the maximum current from the panels. For a 200-watt panel array on a 12V system, maximum current is approximately 200W ÷ 12V = 16.7A. With a 25 percent safety margin: 20.8A. Choose a 20A or 30A controller.

Verify the controller's maximum input voltage exceeds the panel array's open-circuit voltage (Voc). For two 100-watt panels in series, Voc is roughly 44V. Add a 20 percent cold-temperature margin: 53V. Your controller needs a maximum input voltage above 53V. Most 20A MPPT controllers accept 75–100V input, which provides adequate headroom.

Ensure the controller has the correct battery profile for your battery chemistry. LiFePO4 batteries need a controller with a lithium charging profile. Using a lead-acid profile on a lithium battery risks overcharging if the battery's BMS does not intervene.

Step 5: Specify Wiring

Wire gauge depends on current and distance. Use the 3 percent voltage drop rule: total voltage drop from panel to controller should not exceed 3 percent. From controller to battery, keep drop under 1 percent since this is a shorter, higher-current run.

For the panel-to-controller run: calculate the one-way distance in feet. Apply the voltage drop formula and select the wire gauge that keeps the drop at or below 3 percent. For a 200-watt 12V system (about 12A) over a 20-foot run, 10 AWG wire keeps drop at about 2.4 percent.

For the controller-to-battery run: this is usually short (under 5 feet), but the current can be high, especially with MPPT controllers that boost current. Use heavy gauge wire (8–6 AWG) for this short, high-current connection.

Fuse both the positive panel lead (near the controller) and the positive battery lead (near the battery) at 125 percent of the maximum expected current. Use automotive-style inline fuse holders or ANL fuse blocks rated for the voltage and current.

Step 6: System Configuration Decisions

12V vs 24V

12V is the standard for small systems. Most small solar devices (cameras, pumps, fans, LED lights) run natively on 12V. Single 12V batteries are widely available in all chemistries and capacities. The limitation of 12V is current: at 12V, a 500-watt load draws over 40 amps, which requires heavy wire and large fuses. If your total system capacity exceeds about 500 watts of panels, consider stepping up to 24V to halve the current.

Series vs Parallel Panel Wiring

Panels in series add their voltages while current stays the same. This is useful for long wire runs (higher voltage reduces current and voltage drop) and for MPPT controllers that convert excess voltage to current. Panels in parallel add their currents while voltage stays the same. This is simpler and more forgiving of partial shading since a shaded panel in parallel only reduces its own output, not the entire string.

Inverter: When You Need One

If any load requires 120V AC, you need a modified sine wave or pure sine wave inverter. Pure sine wave inverters are compatible with all devices and produce cleaner power, but cost more. Modified sine wave inverters work for resistive loads (heaters, incandescent lights) but can cause humming, overheating, or malfunction in sensitive electronics, motor-driven devices, and battery chargers. Size the inverter's continuous wattage rating above your total AC load, and the surge rating above any motor starting loads.

Common Design Errors

Undersizing the battery for overnight and cloudy-day autonomy. Sizing the panel to summer output and discovering the system fails in winter. Ignoring voltage drop on long wire runs. Using a charge controller without the correct battery chemistry profile. Omitting fuses, which turns a minor fault into a fire hazard. Placing the panel in a location that receives shade during critical midday hours. Each of these errors is preventable with the step-by-step sizing process above.

Real-World Design Example: Multi-Device Ranch System

This example walks through a complete design for a ranch gate monitoring system at 40° North latitude with the following loads: one 2K security camera (5W average, 24 hours), one WiFi repeater (8W, 24 hours), and one LED dusk-to-dawn light (15W, 10 hours). These are spread across a 50-foot area near the gate, all powered from one centralized system.

Daily load calculation: Camera = 5W × 24h = 120 Wh. WiFi repeater = 8W × 24h = 192 Wh. LED light = 15W × 10h = 150 Wh. Total daily load = 462 Wh.

Battery sizing: Using LiFePO4 with 80 percent DOD and two days of autonomy. Battery = 462 × 3 ÷ 0.8 = 1,733 Wh = 144 Ah at 12V. Select a 150 Ah LiFePO4 battery.

Panel sizing: Using worst-month peak sun hours of 3.0 and a derating factor of 0.75. Panel = 462 ÷ 3.0 ÷ 0.75 = 205 watts. Select two 100W panels (210W total) wired in parallel for shade tolerance.

Charge controller: 210W ÷ 12V = 17.5A, with 25 percent margin = 21.9A. Select a 30A MPPT controller with LiFePO4 profile. Maximum panel Voc in cold = 22V × 1.2 = 26.4V per panel (panels in parallel, not series, so Voc stays at 26.4V). Confirm the controller's maximum input exceeds this value.

Wiring: Panel to controller is 15 feet. Current is about 12A. Using the quick sizing table, 10 AWG handles 12A at 20 feet with under 3 percent drop. Controller to battery is 3 feet: use 8 AWG for the short high-current run. Battery to each device: run 12 AWG for the camera (5A max), 12 AWG for the WiFi repeater (8A max with startup spike), and 14 AWG for the LED light (under 2A).

System Monitoring

Once the system is installed, knowing its state of health prevents surprises. Battery voltage alone is a crude indicator because voltage does not map linearly to state of charge, especially for LiFePO4 where the voltage stays nearly flat from 20 to 80 percent charge. A Bluetooth-enabled BMS reports actual state of charge, charge and discharge current, individual cell voltages, temperature, and cycle count. This data is invaluable for diagnosing problems before they cause failures.

Many MPPT charge controllers also include monitoring features: daily and historical energy harvest, peak and average current, battery voltage trends, and fault codes. Some connect to phone apps via Bluetooth. Reviewing these metrics monthly tells you whether the system is producing as expected or whether something has changed: a panel getting dirty, a battery losing capacity, or a load drawing more power than planned.

For remote installations where visiting the site for Bluetooth readings is impractical, cellular-connected monitoring systems report data over the mobile network. These add cost and complexity but provide continuous visibility into system health from anywhere. For critical systems like livestock water or security cameras, remote monitoring is worth the investment.

Seasonal Adjustment Strategies

A well-designed system runs unattended most of the time, but seasonal adjustments improve performance and longevity. In spring, clean panels after pollen season, adjust tilt angle for summer (latitude minus 15 degrees), and inspect all connections and mounting hardware after winter weather. In fall, clean panels again, adjust tilt angle for winter (latitude plus 15 degrees), reduce loads where possible (switch cameras from continuous to motion-detection, lower lighting brightness), and verify battery health before the low-production season.

Winter is the tightest period for energy budgets. If your system barely keeps up in winter, consider these reductions: lower camera resolution, reduce WiFi repeater transmit power, shorten light-on duration with earlier-off timers, and reduce aerator CFM to match reduced biological oxygen demand in cold water. Each small reduction extends battery autonomy through the critical low-sun months.

Troubleshooting Common Problems

Battery not charging: Check panel voltage at the controller input terminals. If voltage is present but no charging current flows, the controller may not have detected the battery (reconnect battery first, then panel). If panel voltage is low, check for shading, dirty glass, or a disconnected MC4 connector. If panel voltage is zero, check the fuse.

Battery draining overnight: Compare actual overnight consumption to designed consumption. A camera switching to high-resolution continuous recording due to constant motion triggers (wind, wildlife, shadows) can draw three to five times its expected average. Narrow the camera's detection zone and reduce sensitivity. Check all loads for parasitic draw when they should be in standby.

System fails in winter: The system was likely sized for summer output. Options are to add panel capacity, add battery capacity, reduce loads, or combine all three. Sizing to winter from the start prevents this problem, but retrofitting a second panel is usually the most cost-effective fix.

Charge controller displays fault codes: Consult the manufacturer's manual for the specific code. Common faults include overvoltage (panel Voc exceeded controller maximum, usually from panels wired in series getting cold), overtemperature (controller in direct sun or enclosed without ventilation), and battery overcharge (incorrect battery profile selected). Each fault has a specific fix that the code identifies.

Scaling Up

Small standalone systems can grow incrementally. Adding a second panel to an existing controller is the simplest expansion, provided the controller has headroom in its amp rating and input voltage. Adding a second battery in parallel doubles capacity. Adding new loads requires recalculating the energy budget and verifying that the panel and battery can handle the increased demand. The modular nature of 12V DC systems makes incremental expansion straightforward compared to AC electrical work, which often requires new circuits, breakers, and potentially panel upgrades.

The beauty of small standalone solar systems is their modularity. Every component can be upgraded independently as needs change or technology improves. A system designed thoughtfully today, with headroom in the charge controller rating and properly sized wiring, can grow with your property's needs for years without requiring a complete redesign. Start with the math, build for today's load with tomorrow's expansion in mind, and the system pays for itself through reliable independent power that owes nothing to the grid.