The lunar night is a battery problem, too

Five watts sounds like the kind of load you can lose in the rounding. Leave it on for a lunar night and it becomes the reason you are discussing a different lander.

For this exercise I am using 14 Earth days of darkness. That is a convenient rounded assumption, not a lighting model for every latitude and landing site. The average electrical load is 5 W, including whatever survival heating we have put on the bus. No science observations, no transmission, and no heroic midnight drilling.

Suppose the installed battery pack supplies 120 usable Wh per kilogram after the allowances we choose to include. Then 1,680 / 120 gives 14 kg. At 10 W the mass becomes 28 kg. These are illustrative pack assumptions, not claimed performance figures for a particular flight battery.

Write “usable” in ink

A cell’s catalogue energy density is not the installed pack’s delivered energy density. Housings, interconnects and management electronics weigh something. Temperature changes available capacity. Limiting depth of discharge consumes part of the nameplate capacity. Ageing takes another share. You must decide whether those effects are already included in your 120 Wh/kg before adding them again.

The recharge deserves a line on the same page. If you can actually charge for 200 hours of the following day and the relevant charging path is 80% efficient, replacing 1,680 Wh requires an average 10.5 W at its input during those hours. Daytime science and communications are additional loads. So are any conversion losses that your chosen boundary leaves outside that 80%.

A larger panel solves only the illuminated part. Better insulation may reduce the night load and therefore the battery as well. That is why I would put the electrical and thermal engineers at the same table before ordering either component.

There is a real design behind the arithmetic

The LuSEE-Night team describes a proposed lunar far-side radio instrument with a 7 kWh battery. Its published power study considers night-time observations, daytime recharge and changing thermal conditions together. The paper is a useful antidote to the claim that a solar-powered spacecraft simply cannot work through lunar night. It describes a design and simulations, rather than an operational result. ^[1]

None of this makes a radioisotope heater unnecessary. A small continuous heat source can save a large amount of stored electrical energy if it reaches the right components. But a watt of heat delivered to an enclosure and a watt available on an electrical bus are different entries. I have seen prettier spreadsheets than this ruined by treating them as the same one.

My envelope has room for one more line: can the thing be colder while asleep? That answer may save more mass than the next improvement in battery chemistry.