100 vs 200 vs 400 Amp: Sizing Your Electrical Service

"What size electrical service do I need?" is one of the most common questions we get, usually right before someone adds an EV charger, a generator, or an addition. The honest answer is that 200-amp service covers most homes, 100 amps still works for smaller gas-appliance homes, and 400 amps is a specialty case — not the default upsell some outfits make it. Here's how to tell which one is actually yours.

Written by a licensed master electrician — TDLR EC #37785, 27+ years in the Galveston Bay / South Houston corridor.

What "amps" actually means for your house

Your electrical service size — 100, 200, or 400 amps — is the maximum amount of current your home can safely draw at one time. Think of it as the width of the pipe, not the water bill. A bigger service doesn't make you use more power or cost more to run; it just sets the ceiling on how much can flow at once before the main breaker cuts things off.

That's why the question isn't "how much do I use in a month" — it's "how much might everything I own try to pull at the same moment." Air conditioning, an electric range, a dryer, a water heater, a heat pump, an EV charger, a pool pump: add up the big simultaneous loads and you have the number that decides your service size. Electricians formalize that with a load calculation, and it's the honest way to answer the sizing question instead of eyeballing it.

100 vs 200 vs 400 amp: what each one supports

Between 100 and 200 you'll also see 150-amp services, which split the difference and are common in mid-size homes. The same rule applies: the calculation decides, not a round number that sounds impressive.

Signs your 100-amp service is maxed out

You usually don't need a meter to suspect you've outgrown your service. These are the everyday tells:

• The panel is physically full — every slot used, no room to add a circuit. (Full isn't always the same as out of capacity — more on that below.)
• Breakers trip when big loads run together — the AC kicks on while the dryer or oven is going, and something cuts out. (Our guide to why a breaker keeps tripping sorts the harmless causes from the real ones.)
• Lights dim when the AC or a big appliance starts. An occasional flicker can be normal; a consistent dip when heavy loads engage points at a service near its limit.
• You're relying on tandem breakers and power strips to stretch what's there, or an electrician has told you there's "no room" to add what you want.
• You're adding a major load — an EV charger, a heat pump, a second AC, a hot tub, or a generator — to a service that was sized for a much smaller electrical life.

Any one of these is a reason to get a load calculation. None of them is proof you need the biggest service on the shelf.

How a load calculation decides your number

Here's the part that keeps you from overbuying: the right service size is a math problem, not a judgment call. A load calculation (the method comes straight from the electrical code) adds up your home's square footage, its fixed appliances, the AC and heating loads, and any big additions like an EV charger — then applies standard demand factors, because not everything runs at full tilt at once.

The result is a real number that tells you whether your existing service has headroom, whether a 200-amp upgrade is the right call, or whether you genuinely fall into 400-amp territory. We do it before quoting an upgrade, not after — because the calculation is exactly what tells us whether you need one at all. If the math says your current service can take the new load, we'll show you that and save you the job.

When 400-amp service actually makes sense

Since 400 amps is where the cost climbs, it's worth being specific about when it's the right answer rather than an oversell. A 400-amp service earns its keep when several of these stack up:

• Large square footage with multiple AC systems and an all-electric kitchen and heat.
• An EV charger (or two) plus a whole-home generator — a combination that stacks heavy loads a 200-amp service can struggle to cover together.
• A detached shop, workshop, or ADU fed from the same service, adding its own panel and loads.
• A load calculation that actually exceeds 200 amps — the only reason that settles it.

On the Gulf Coast, the combination that most often pushes a home toward a bigger service is a whole-home standby generator alongside an EV charger and heavy AC — the hurricane-season backup and the electric-driveway future landing on the same panel. Even then, the calculation is what confirms it. (Planning a generator? See standby generator installation.)

"My panel is full" — do you even need an upgrade?

Before you budget for a service upgrade, the honest caveat: a full panel isn't automatically a panel that's out of capacity. Sometimes the fix is a sub-panel, a tandem-breaker review, or simply the load calculation proving your existing service can carry what you're adding. That's a far smaller job than a heavy up.

So we run the math first and let it decide. If it says a 200-amp upgrade is the right move, we'll show you why. If it says you don't need one, we'll tell you that just as plainly. Either way, you're not buying amperage on a hunch. When an upgrade genuinely is the answer, a panel upgrade in this area typically runs $1,183–$1,972 — see what a panel upgrade actually costs in Houston for the full breakdown of what moves that number.

Get the right-sized service — not the biggest one

The goal isn't the largest service on the shelf; it's the one your home actually needs, confirmed by a load calculation and installed to code. For most homes that's a 200-amp service; for some it's staying at 100; for a few it's a genuine 400-amp job. We'll tell you which — and show you the math.

Read the full service details on our panel upgrades & breaker replacement page, or if you're in Pearland, here's a 200-amp panel upgrade in Pearland.

Prefer to talk it through? Call or text (832) 315-5772 — same-day callback.