SatBase

Power Budget

The solar array output and battery capacity an orbit’s loads require, with conversion losses on the eclipse and daylight paths, load contingency, and depth of discharge.

Inputs

Orbit

The Equatorial Orbit Designer gives the worst-case eclipse for an altitude.

min
min

Loads

Average power drawn in each phase, before contingency; the margin is applied to both.

W
W
%

Power system

W

Battery

Sized to carry the eclipse load without discharging past the depth-of-discharge limit.

%
%
batteries
V

Results

Array

Required array power 252W
Array margin +18.9%

Battery

Eclipse energy 57.8Wh
Capacity per battery 214Wh
Capacity per battery (Ah) 7.64Ah
Installed capacity 214Wh

Orbit budget

Sunlit time 56.5min
Orbit-average load 111W
Energy generated 282Wh
Energy to loads 171Wh
Conversion losses 66.8Wh
Energy surplus 45Wh

Method

The standard orbit-average power sizing: the array output needed to run the loads and recharge the battery through conversion losses, and the battery that carries the eclipse within its depth of discharge.

Psa = (PeTe/Xe + PdTd/Xd) / Td, C = PeTe / (DoD·N·n) Orbit-average sizing: the array carries the sunlit load and recharges what the eclipse took, through the losses on each path.

Derivation

1
Split the orbit and add contingency The array only works in sunlight, so everything it must supply comes out of the sunlit time. Early load estimates grow, so both loads carry the margin before any sizing. Td = T − Te, P′ = P·(1 + m)
2
Size the array through the losses Eclipse energy is stored and recovered, so it passes through the battery, its charger and its discharger. The eclipse path loses far more than the daylight path. Dividing each phase’s energy by its path efficiency and spreading the total over the sunlit time gives the array output required. Psa = (Pe′Te/Xe + Pd′Td/Xd) / Td
3
Check the array you have A positive margin means the array covers the loads, the losses and a full recharge every orbit. A negative one means the battery ends each orbit a little emptier than it began. margin = (Parray − Psa) / Psa
4
Size the battery to the eclipse The batteries supply the eclipse load, delivered through the battery-to-load efficiency, and may only discharge to the DoD limit, so the capacity is the eclipse energy divided by all three, shared across N batteries. Dividing by the battery voltage states it the way packs are rated. Ee = Pe′Te, C = Ee / (DoD·N·n), CAh = C / V
5
Account for every watt-hour Over one orbit the array’s output goes to the loads, to conversion losses, or to surplus. Counting the losses is what separates this from a simple generated-minus-consumed balance, which reports a margin the hardware does not have. Egenerated = Eloads + Elosses + Esurplus

Symbols

Input Description Unit
T Orbit period min
Te Eclipse duration per orbit min
Pd Sunlit (daylight) load, before margin W
Pe Eclipse load, before margin W
m Load margin (contingency) %
Xe Eclipse path efficiency: array to battery to load %
Xd Daylight path efficiency: array to load %
Parray End-of-life array output W
DoD Allowed depth of discharge %
n Battery-to-load efficiency %
N Number of batteries sharing the load
V Battery voltage V
Derived Description Unit
Td Sunlit time: T − Tₑ min
Psa Required array output W
Ee Energy the batteries supply in eclipse Wh
C Capacity per battery Wh
CAh Capacity per battery in amp-hours Ah

Reference values

First-order figures for sanity-checking inputs.

Regulation Xₑ % Xd %
Peak-power tracking 60 80
Direct energy transfer 65 85
Orbit Eclipse cycles Typical DoD %
LEO ~5,500 per year 20–30
GEO ~90 per year 60–80

This is orbit-average sizing after SMAD: one constant load per phase, no peak or pulsed loads, and an array power you supply at end of life. Degradation from radiation, temperature and pointing is not modeled. Battery capacity fade, cycle-life limits and thermal derating are not modeled either, and the depth-of-discharge bands above are starting points; a cell's own cycle-life data sets the real limit. Treat these results as first-pass sizing, not a power subsystem design.