How to Size a Solar Battery System for Your Electronics Project

Solar-powered electronics projects have never been more achievable. Small, cheap panels, efficient lithium batteries, and low-power microcontrollers mean you can build genuinely useful off-grid systems without a large budget or complex engineering.

The problem is that most hobbyist guides give you rules of thumb that work in theory but fail in practice. 'Just add 20% extra' isn't a design methodology — it's a guess. Proper solar sizing is a straightforward calculation once you know what to feed into it.

The three things you need to know

To size a solar battery system correctly, you need three pieces of information:

1. Your daily energy consumption (Wh/day)
2. Your location's peak sun hours (hours/day of full-intensity sunlight)
3. How many days of autonomy you need (days the battery must run without sun)

Every other number in the design flows from these three.

Step 1: Calculate your daily energy consumption

List every component in your system with its voltage, current draw, and daily operating hours.

Example system: weather station with ESP32, BME280 sensor, small OLED display.

ESP32 in normal operation: 160mA at 3.3V = 0.528W. Active for 1 minute per hour (takes reading, transmits, sleeps the other 59 minutes): 0.528W × (1/60) = 0.0088W average

ESP32 deep sleep: 10µA at 3.3V = 0.000033W × (59/60) = effectively zero

BME280 sensor: 3.6µA at 3.3V — negligible

OLED display (if used): 20mA at 3.3V = 0.066W, if on 10 min/day: 0.066 × (10/60/24) = 0.00046W

Total average power: roughly 0.009W

Daily energy: 0.009W × 24h = 0.22Wh/day

This is a very low-power system. A single small solar panel and a modest battery will run it indefinitely. Larger systems — Raspberry Pi projects, camera traps, irrigation controllers — need the same calculation but with bigger numbers.

Step 2: Find your peak sun hours

Peak sun hours (PSH) are not the number of daylight hours. They represent the equivalent hours of full-intensity (1000W/m²) sunlight your location receives per day, averaged over the year or over your worst month.

• Nairobi, Kenya: approximately 5.5–6.5 PSH (excellent solar resource)
• London, UK: approximately 2.5–3.5 PSH in winter, 4.5–5.5 in summer
• Mumbai, India: approximately 5.0–6.0 PSH
• New York, USA: approximately 3.5–4.5 PSH

For year-round systems, always design for the worst month (typically December in the northern hemisphere, June in the southern). For seasonal use, use the average for your operating period.

You can find precise PSH data for any location on the NASA POWER database or GlobalSolarAtlas.info.

Step 3: Size the solar panel

Panel wattage = Daily energy consumption (Wh) / Peak sun hours / System efficiency

System efficiency accounts for losses in the charge controller, wiring, and battery charging process. A typical value is 0.75–0.85.

For our weather station: 0.22Wh / 4.0 PSH / 0.80 = 0.069W panel minimum

In practice, you'd never buy a 0.07W panel — a 1W or 2W panel is the smallest sensible option and provides generous headroom. For real projects with more load, a 5W panel is a practical starting point.

For a more demanding system — say, a Raspberry Pi camera trap drawing 2W average:

2W × 24h = 48Wh/day

Panel = 48 / 4.0 / 0.80 = 15W minimum — a 20W panel gives comfortable headroom.

Step 4: Size the battery

Battery capacity (Wh) = Daily energy consumption × Days of autonomy / Depth of discharge

Days of autonomy: how many consecutive sunless days the battery must cover. For non-critical systems, 2–3 days is typical. For critical remote sensors, 5–7 days.

Depth of discharge (DoD): lithium batteries should not be regularly discharged below 20% capacity (80% DoD). Lead-acid batteries should not go below 50% (50% DoD).

For our weather station with 3 days autonomy and lithium battery:

0.22Wh × 3 / 0.80 = 0.825Wh capacity

For the Pi camera trap with 3 days:

48Wh × 3 / 0.80 = 180Wh capacity — roughly a 50,000mAh power bank at 3.7V

Common mistakes

Sizing for average sun, not worst-case sun: systems designed for summer averages fail in winter. Always use the worst month's PSH for the design.

Ignoring the charge controller: the controller must be rated above both the panel's short-circuit current and the battery's maximum charge current. A 10A controller is the practical minimum for most small systems.

Using lead-acid for portable systems: lead-acid is heavy, has lower DoD, and degrades faster in partial-state-of-charge cycling. Lithium (LiFePO4 for safety, LiPo for compactness) is almost always the right choice for electronics projects.

Forgetting voltage compatibility: a 12V panel, a 12V charge controller, and a 12V battery need to be matched. Mixing voltages without proper conversion is the most common wiring mistake.