Sizing for a Window AC or Space Heater — Cooling and Heating in an Outage
Last verified: September 2026
Cooling and heating are two loads folks get wrong in opposite ways: a window AC surges with a big starting jolt, while a space heater never surges but draws heavily and steadily. Here's how to size each. Rather talk it through? My number is (330) 200-8042.
Sizing for a Window AC or Space Heater, at a Glance
Cooling and heating behave in almost opposite ways, and that's the key to sizing them. A window or portable air conditioner is a compressor motor — it surges with a big starting jolt, then runs modestly. A space heater has no surge at all, but it's a heavy, steady draw the whole time it's on, because electric heat is watt-hungry. So you size an AC for its surge and a heater for its steady appetite. This guide walks through both, plus central air, soft starters, and cycling — with a printable page-by-page version up top.
Common Questions
1 What's the difference between sizing for an AC and a heater?
An air conditioner is a compressor that surges — a big brief jolt to start — so you size for that surge. A space heater has no surge but a heavy, steady draw, so you size for its constant appetite in your running total.
2 Why won't my window AC start on my backup?
Almost always its compressor's starting surge came up short. A window AC surges several times its running watts to start; your unit needs the surge headroom, not just enough for the steady running draw.
3 Why does one small space heater use so much power?
Because making heat is watt-hungry. A heater turns electricity straight into heat with no motor and no efficiency trick, so it draws heavily and steadily the whole time — and two heaters stack straight up.
4 Can a portable generator run my central air?
Usually not comfortably — central air has a much bigger surge than a window unit. That points toward a standby generator or a soft starter to shrink the surge; see our well-pump and standby guides.
5 Would a soft starter help my air conditioner?
Often, yes — especially for central air, sometimes a window unit. A licensed pro installs it to ease the compressor up to speed, shrinking the surge so a smaller unit can start it. It doesn't change the running watts.
6 Should I cool the whole house or just one room?
For most portable backups, one room. Conditioning the whole house is a far bigger load; keeping one room comfortable is kinder to your backup and your budget, and cycling helps a modest unit stretch.
Would You Rather I Just Help?
Cooling and heating trip people up in opposite directions — the AC's surge and the heater's steady appetite. I'll help you read your nameplates, size each one right, and decide what fits your backup season by season, with nothing to sell you. $99 flat for a visit, 30-day come-back-free.
Would you rather I just do this for you?
No shame in it — plenty of folks would rather hand it off, and that's what I'm here for. I'll come to your home, set it up right, and make sure it's working before I leave. Flat $99 per visit — no jargon, no upsell, and every visit is 30-day come-back-free.
Not a hardware job? I can often fix it remotely for $49 — a secure screen-share, up to 2 hours, that you watch the whole time and can end whenever (included free for $39/mo Support Plan members, and it counts toward the $99 if it turns out I need to come out).