Most battery-sizing advice answers the wrong question. People ask "how many days of backup do I want?" — but on Dominion's Rate 7, the question that actually controls your savings is "can it carry my house through the on-peak window every day without leaning on the grid?" Get that right and the demand charge stays low. Get it wrong and the savings evaporate. Here's how I think about it, running this at my own house in Charleston.
Sizing is really about the demand charge
On Rate 7, your savings depend on keeping your on-peak grid usage low — both the energy you buy at the expensive rate and the demand charge set by your single worst 15-minute on-peak interval. A battery only delivers if it can shoulder your home's load for the entire window, every day it applies, so the grid never sees a big spike. So sizing isn't about backup days — it's about reliably covering the peak. There are two separate numbers that have to be right.
The two numbers that matter
1. Power (kW) — can it keep up with your draw? This is how much the battery can deliver at once. During the on-peak window your house might pull several kilowatts when the AC compressor, oven, and dryer all kick in. If the battery's output can't match that instantaneous draw, the grid makes up the difference — and that moment becomes your billed demand peak. A battery with plenty of stored energy but too little output power will still let demand charges through.
2. Energy (kWh) — can it last the whole window? This is how much total energy it holds. The summer on-peak window is several hours long, so the battery needs enough stored energy to run your house across that entire stretch without running dry. If it taps out an hour before the window ends, every remaining minute is back on the grid at peak — including, potentially, your worst interval.
You need both: enough power to cover your hardest moment, and enough energy to last the full window. A system that's strong on one and weak on the other won't deliver the savings.
My setup as a reference point
For context, I run an EcoFlow system at about 24 kWh of usable storage. That comfortably carries my house through the summer 4–8 p.m. on-peak window every weekday — the battery supplies the load, my grid draw stays low, and the demand charge stays small. That's the whole reason my June bill came in around $190 on Rate 7 versus about $311 it would've been on Rate 8 for the same 1,846 kWh. The full breakdown is in is a home battery worth it on Dominion Rate 7.
Your number will be different. A smaller, well-managed home might cover its peak with noticeably less. A big house with heavy late-afternoon AC and an EV charging at the wrong time needs more — both more power and more energy. There's no single right answer, which is exactly why "buy X kWh" advice on the internet is mostly useless.
A margin for the every-day change
One more reason not to size right at the edge: Dominion's proposed July 1, 2026 changes would treat weekends and holidays like weekdays, extending the on-peak window to every day. A battery that can just barely cover the weekday peak has no slack for that. Sizing with comfortable headroom protects you against both a hot day with unusually high load and rule changes like this one. More in Dominion's 2026 time-of-use rate changes.
How to find your actual number
The honest way to size a battery is from your usage, not a rule of thumb — specifically your on-peak load shape during the window, which is what determines both the power and energy you need. That's what the analyzer works out: it reads your real bill and usage, estimates your peak coverage, and translates it into a system size with honest pricing — including telling you if a battery isn't worth it for your situation.