What Size Solar Panel Do I Need? An SMA Installer's Honest Take (and the EverStart 100W Trap)

I've been speccing residential and small commercial PV systems for seven years. In that time, I've personally made and documented 14 significant design mistakes—most of them avoidable. Total wasted budget? Roughly $32,000. That's how I learned that the question 'what size solar panel do I need?' is usually the wrong first question.

If you're here because your search history contains NSLIJ ESS login, let's clear that up first. In healthcare, ESS can mean 'employee self-service.' In solar, ESS means 'energy storage system.' SMA's ESS login is part of the Sunny Portal monitoring environment, not a hospital HR page. I only mention it because I've seen the search traffic collide in my analytics more than once.

The real issue is that people try to size a grid-tied solar array the same way they'd pick a portable power inverter. An EverStart 100 watt power inverter is a fine device for running a small appliance from a car. It is a terrible mental model for sizing a roof.

Two ways to answer 'what size solar panel do I need'

I'll compare them side by side. Approach A is the portable-inverter method: watts in, watts out. Approach B is the system-design method: model the building, the roof, the inverter, and the battery as one system.

Dimension 1: What the question actually asks

With the EverStart 100 watt power inverter, the math is simple. If the load is 100 watts, you buy a 100-watt inverter. If the load grows, you buy a bigger one. That's appliance-first sizing.

With grid-tied solar, the question isn't 'how many watts do I need?' It's 'how many kilowatt-hours does this building use in a year?' The panel count is the last answer, not the first. What I mean is: start with the utility bill, not the product page. Too many first-time buyers jump straight to '15 panels' before looking at a year of usage data.

Dimension 2: The method: rule of thumb vs. modeled design

The rough method goes like this: take daily load in kWh, divide by peak sun hours, add 20% for losses. That's not wrong, but it's incomplete. Put another way: it gets you in the ballpark, and then the ballpark moves.

On the SMA Solar Technology AG official website, you'll find Sunny Design. Enter your location, roof orientation, module type, and inverter. It models AC output, DC/AC ratio, and clipping. In my experience, this catches what the rough method misses. I don't have hard data on how many installers skip this step, but based on the 47 drawings I've reviewed from other contractors, my sense is about one-third never open a design tool.

Dimension 3: The cost of getting it wrong

My worst mistake happened in 2018. I sized a 6 kW array for a 6 kW load and ignored inverter losses and winter production. The customer's true-up bill came in ugly. I paid for the extra panels, racking, and labor out of pocket: $4,300, plus a bruised relationship.

The upside is that I now verify every startup with SMA's monitoring portal. That's where the SMA ESS login comes in. It shows per-string watts, battery state, and daily yield. If you don't log in and compare the model to actual output, you're flying blind.

Dimension 4: Inverter selection: appliance vs. grid-connected component

The EverStart 100 watt power inverter converts 12V DC to 110V AC for small loads. It has no grid connection, no anti-islanding, and no monitoring. That's fine for an emergency kit.

An SMA Sunny Boy or Sunny Tripower is chosen based on PV string voltage, module power, and grid code. If you pick it before the array is designed, you'll get clipping or underutilization. This isn't a knock on EverStart; it's a knock on conflating the two categories.

It's tempting to think you can just compare watts. But two arrays with the same total panel wattage can produce very different annual kWh because of MPPT voltage windows, shade behavior, and battery charge curves.

The 'bigger inverter is always safer' thinking comes from an era when panels were expensive and clipping was the main enemy. That's changed. With today's panel prices, a higher DC/AC ratio often makes sense—but only up to the inverter's rated limit. If you don't model it, you'll leave generation on the table or pay for an inverter you don't need.

Which one should you use?

If you only need to keep a phone charged in a car, an EverStart 100 watt power inverter is probably enough. Don't buy an SMA inverter for that.

If you're installing panels on a house or business, you need the SMA Solar Technology AG official website planning tools. Sunny Design is free as of January 2025, and it will keep you from making the mistakes I made.

Also subscribe to SMA solar news through the official website. Firmware updates, grid-code changes, and battery compatibility lists show up there first. In 2024, those updates changed how we configured a storage system in New York. It wasn't in the marketing emails; it was in the technical release notes.

My experience is based on roughly 180 residential and light-commercial projects in the northeastern US. If you're designing for Arizona, Texas, or Europe, your sun hours and utility rules are different. Don't copy my numbers. Use the tools. At least, that's been my experience. And if your bookmarks still say NSLIJ ESS login, update them—because SMA ESS login and a hospital employee portal are not the same thing.