Some assumptions here:
- You currently have 200A service feeding a 200A panel
- You want to add a 100A panel for the garage, presumably for EV charging or use as a workshop (welder, big table saw, kiln, whatever) or conversion to an apartment (Accessory Dwelling Unit)
Assuming that's the case, you very likely don't need a service upgrade at all. The first step is to figure out (a) how much electricity you currently use and/or are expected to use based on your existing stuff and (b) how much additional electricity you need.
The proper way to do this is a Load Calculation. One for the existing feed and another for the new stuff. A Load Calculation takes into account:
- Size of your house
- Standard amounts for kitchen, bathroom and laundry circuits
- Oven, cooktop, etc. if applicable (you might have gas)
- Water heater if applicable (you might have gas)
- Clothes dryer if applicable (you might have gas)
- Any other known fixed and/or large loads
It is not simple "add up the loads", because there are adjustments built in based on "not everything runs at the same time". It is not exact because everyone's usage is different, but it is a standard guide and it generally works quite well.
There is a rough alternative if your utility provides either a peak demand value on your bill (typical on commercial bills, but varies by utility; if you have peak demand in kW on 240V service then divide it by 240 to get Amps) or downloadable detailed usage (available both commercial and residential, typically in either 15-minute or hourly increments; divide hourly kWh by 240 or 15-minute kWh by 60 to get Amps).
The "clamp meter" method is reliable in the long term - that's essentially what the utility usage detail does. But in the short term it is useless because it is too easy to go high (run the HVAC in the maximum mode, turn on the dryer, turn on the oven and 4 burners, etc.) or low (harder to force the electric water heater to turn on when you need it, or "emergency heat", etc.).
Once you have good load calculations (old and new) or at a minimum historical peak usage + load calculation for the new panel, you see what you've got.
If the old is already over 200A then you are living on borrowed time and need to look at cutting usage or a service upgrade. But that's not likely.
If the total can be handled by 200A then you're done, no upgrade needed.
If the total is close to 200A then take a serious look at what you are adding. Two key things to seriously think about (if they are on your plan) are:
- Tankless electric water heating. If you are turning your garage into a small apartment (ADU) then you might think that will save a few square feet. But it is usually not worth it because of the HUGE demand on the electricity supply. You can get a small tank and hide it in a cabinet in the bathroom or kitchen.
- EV charging. 30A is enough for almost everyone. 20A is even enough for most. So if your load calculation is a little bit over 200A but includes 50A or 60A for EV charging, lower the charging rate (it has to be installed that way, not "I'll remember to run it at a low rate") and you're done.
Beyond that, there are a number of new ways to manage power to significantly lower the need for > 200A to a typical house, but we need more details to help.