# What are reasonable units of energy measurement for my sci-fi RPG?

I am currently creating my own custom sci-fi tabletop RPG system. While I'm willing to make concessions where necessary, my goal is to use plausible physics whenever possible, at least when it comes to units of measurement.

At the moment I am creating a system to track energy usage for various equipment, such as hand-held tools, suits, and laser weapons. Most equipment is charged by power cells, which will function similarly to ammo in a video game sense. For example, a laser rifle might be able to fire 10 blasts before its power cell needs to be reloaded. A small handheld scanner, on the other hand, could run for 5 hours before requiring a new power cell.

Power cells will come in 4 sizes.

1. Small (hand-held scanners, flashlights, computer tablets, laser pistols, etc.)
2. Medium (power tools, laser rifles, remote operated drones, etc.)
3. Large (laser cutters, spacesuits, floodlights, etc.)
4. Huge (power armor, mounted laser turrets, etc.)

Obviously, I'm aware that some elements of my setting aren't completely realistic (e.g., laser rifles) but I would like to track the capacity of each power cell size with a real world unit such as watt-hours.

What units of measurement would make the most sense for each power cell size?

• Are you sure you want to ask about units of measure? Regardless of the capacity every energy storage device will be measured using the same units (probably Watt Hours, like every battery on earth). I edited your question to remove unrelated follow up questions. Please remember that we have a strict 1 question per post rule. Commented Dec 31, 2021 at 2:44
• ${fir}\frac{{fur}^2}{{ftn}^2}$ FTW Commented Dec 31, 2021 at 3:01
• From a writing, rather than a worldbuilding perspective, I would advise that you not give units. Describe them in terms of how long or how many times they'll charge a given device, and then scale everything else based on those charges. Otherwise you'll end up with whoppers like the "three megs of hot RAM" in the opening chapter of Neuromancer. Commented Dec 31, 2021 at 3:04
• @sphennings Thanks for the edit, my bad Commented Dec 31, 2021 at 3:30
• Why don't you want to use watt-hours? That's a standard that most readers will be familiar with. Commented Dec 31, 2021 at 8:37

Depending on how far in the future you intend to be, or if standard Earth based units of measure are to be used at all, you may want to create your own unit base. Using a real unit of measure for something very deeply imbedded in the realm of real physics invites over analysis and may break the suspension of disbelief for more science minded players, while becoming an annoyance for players that care less about the true details. In a tabletop RPG it also invites over zealous players to challenge the realm of accepted actions by simply being better at math than the DM (assuming a DnD type setup).

Personally I would recommend making the power cells themselves the base units and then compare them to each other. If we assuming that everything in this universe is powered by the same types of power cells we can then simply indicate that a device requires a certain number of such power cells, and have a handy chart to compare there values to each other.

If we use real life batteries as model, maybe the smallest cells are 'A cells' power cells, the second level are 'B cells', then 'C cells' and 'D cells'. Each could then be considered an order of magnitude greater than the last. Maybe 100 As = 1 B, 100 Bs = 1C, etc.

I use mass energy. 1 kilogram of mass energy is approximately equal to 90 petajoules. Or generators could generate some number of gigawatts, with a total maximum output between refueling expressed in mass energy.

I feel like mass energy is a pretty handy term, because it gives you some sense of how much fuel is involved.

1. Metric.
This is sensible. Metric prefixes are all divided up already. A millibat can power your scanner. A decibat would be good for the energy rifle. Maybe a gigabat for the spacecraft?

2. American money. A nice thing about American money is that it predictably features the same old dudes and it has nicknames. In this scenario the sum would correspond to how much it costs (or used to cost) to charge the battery. A dimebat would charge for the scanner. An Abebat would power the gun. You would need a BenBat for your turret and maybe more than one.

3. Predecimalization British money. Same as #2 but with implications of an alternate timeline and with excellent names. A farthing for the scanner. Thruppence for the rifle. 10 Crowns for the laser turret.

Tergs.

What? Well that's tera ergs (equivalent to 100,000 joules stored energy).

1. The Computer/tablet category would by normal standards use say 25 Watts or joules per second, so a 10 Terg pack would last a shade under 4 hours.

2. A Perg pack (peta erg), would give 100 times more than that 10 terg one of the torch/tablet/pistol - possibly just right for the power-tools laser rifles and drones.

3. 100 Pergs gives you perhaps enough for a spacesuit/laser-cutter etc..

4. Because of the awkwardness of the next multiplier (eta -> eerg), you might chose to call a thousand Pergs a kilo-Perg. 10 K-Pergs would give you 100 times the energy of #3), so perhaps enough for the heavy equipt..

.... or scale it as you will for the energy-usage needs that seem realistic to you.

• Providing wrong advice to a person who honestly wants to learn is not a good thing. (1) It would be Terg not **TErg. The symbol of the unit erg is erg, with a lower case e. Upper and lower case letters are distinct in the symbols of units of measurement. For example, lower case s is the symbol for a second (a unit of time), while upper case S is the symbol for siemens (a unit of electric conductance). (2) The symbols of units of measurement cannot be used in the plural; only their names can: so that it would be "teraergs" not **Tergs. Commented Dec 31, 2021 at 8:35
• ... And anyway, why would you use SI prefixes with CGS units? Back when various CGS systems were much more commonly used than now, ten trillion ergs would have been written $10^{16}$ erg, not 10 Perg. (I don't even remeber ever seeing CGS units with SI prefixes.) Commented Dec 31, 2021 at 8:40