CubeSat Power Budget: Sizing Solar Panels & Batteries

A power budget answers one question: over a full orbit, do your solar panels generate more energy than your satellite consumes? If the answer is no, your battery slowly drains and the mission ends. This guide shows how to build one, with a worked 3U example.

The key idea: orbit-average power

A CubeSat in LEO orbits the Earth roughly every 90 minutes. For part of each orbit it is in sunlight (generating power) and for part it is in the Earth’s shadow, or eclipse (generating nothing). The fraction of the orbit spent in eclipse is typically 30–40% for low orbits.

So the real test is energy balance over the whole orbit: energy generated in sunlight must cover consumption across the entire orbit, including the eclipse when you run purely off the battery.

Step 1: Add up your load

List every subsystem and its average power draw, then separate average from peak loads (the radio transmitting, for example, is a big peak but only for a few minutes per pass). A simple 3U example:

Step 2: Work out the energy needed per orbit

Over a 90-minute (1.5-hour) orbit, an average load of 3.1 W needs about 3.1 W × 1.5 h = 4.65 Wh of energy. But you only generate during the sunlit ~60% of the orbit, so your panels have to produce that entire 4.65 Wh in roughly 54 minutes of sunlight.

Step 3: Size the solar panels

Required sunlit generation = 4.65 Wh ÷ 0.9 h ≈ 5.2 W (plus margin). A 3U face covered in ~30% efficient triple-junction cells generates roughly 2–2.5 W when pointed at the Sun; body-mounted panels rarely face the Sun perfectly, so real orbit-average generation per face is lower. This is why many 3U designs use deployable panelsor accept a tight power budget with careful duty-cycling.

Rule of thumb: always design to generate at least 20% more than you consume. Panels degrade, pointing is imperfect, and missions always grow.

Step 4: Size the battery for eclipse

During the ~36-minute eclipse your load of 3.1 W draws 3.1 W × 0.6 h ≈ 1.9 Wh from the battery. To protect battery life you should only use a fraction of its capacity each cycle — the depth of discharge (DoD). At a conservative 20% DoD you need a battery of at least 1.9 Wh ÷ 0.20 ≈ 9.5 Wh. Two common 18650 Li-ion cells (~2.6 Ah at 3.6 V ≈ 9.4 Wh each) give comfortable margin.

Common mistakes

• Using peak load instead of average, and massively oversizing.
• Forgetting eclipse — designing as if the Sun always shines.
• Ignoring panel pointing: body-mounted cells rarely see full Sun.
• Running the battery at high depth of discharge and killing its cycle life.

Once you understand the method, doing it by hand for every design change gets tedious fast — the power & mass budget tool recomputes generation, consumption, eclipse fraction and battery need live as you add components.