I run my portable AC overnight at 8p/kWh. Here's what it actually costs — and whether the obsession is worth it.

It's 6am. The sun's already up. I'm moving through the house opening every window and door, trying to capture the last of the cool overnight air before the day heats up. At 9am I'll close everything. Curtains too. My wife thinks I'm losing the plot. She might be right. But I've done the maths — and the numbers are more interesting than I expected.

We have a 9,000 BTU portable air conditioning unit — about 900 watts at full power. And for the past few summers, I've been quietly obsessive about running it as cheaply as possible. Not because I can't afford to run it normally. More because I can't help myself. I'm the kind of person who gets more of a buzz from optimising something than from spending the money I saved. My wife, meanwhile, would cheerfully have it running all day. She's probably more rational about this than I am.

With electricity prices rising again from July 1st — Ofgem's new price cap pushes the standard unit rate up to 26.11p/kWh, a 13% increase — I thought it was worth actually writing down what the numbers look like. Because a lot of people are going to be staring at their portable AC this summer wondering the same thing I do: is this worth turning on?

The overnight trick — and why a smart tariff changes everything

I'm on the EON Next Smart EV tariff, which gives me electricity at 8p/kWh between midnight and 6am, rising to 24p during the day. The tariff was originally designed around charging electric vehicles overnight, but there's nothing stopping you from running any appliance on it — including an air conditioner.

The routine I've settled into: run the AC in the bedroom from around midnight until 6am, get a genuinely comfortable night's sleep, then open everything up at 6am to let the cool air circulate before the day heats up. By 9am, windows and curtains are closed to keep the heat out.

Here's what that actually costs:

That overnight run — six hours of cool, comfortable sleep — costs 43 pence. It's genuinely one of the better deals available in the UK energy market right now. Running the same unit for eight hours during the day on the standard rate costs over four times as much.

What the new July price cap means

From 1 July 2026, Ofgem's energy price cap pushes the standard electricity unit rate to 26.11p/kWh — up 13% from the previous quarter. If you're on a standard variable tariff, that makes every hour of daytime AC use about 23.5p, and a full day of working from home with it on around £1.88.

Over a two-week summer heatwave, a habit of running the AC during working hours adds up to roughly £26 at the new rate. That's not catastrophic — but it's not nothing either, particularly if you're already stretched. For context, a fan costs almost nothing: a typical desk fan running 8 hours a day costs around 10p.

The fan comparison is instructive. A fan doesn't cool the air — it just moves it, which makes you feel cooler through evaporation. On a genuinely hot night, a fan in a 28°C bedroom isn't going to cut it. That's the honest case for the AC.

Does the window-and-curtains routine actually help?

The short answer: yes, meaningfully. Keeping direct sunlight off your windows during the day can reduce heat gain through glass by 40-70%, depending on your curtains. Closing windows once outside air becomes warmer than inside air stops that warmth from flowing in. These aren't marginal gains.

The longer answer: whether it's worth the mental overhead depends entirely on your situation. I'm aware that I'm spending a non-trivial amount of cognitive energy — and apparently some marital goodwill — saving roughly £1 a day. That's a choice I make consciously, and I'm not sure it's entirely rational.

The honest answer: can you put a price on it?

Here's where I land, having thought about this probably more than is healthy: you can't really put a price on health and comfort. If heat is making you stressed, anxious, sleep-deprived or unproductive — the £1.73 to run the AC for a day is trivially worth it. Bad sleep compounds. Overheating while trying to work from home is a genuine productivity cost.

What's changed for me as I've got older is the relationship between money and suffering. When I was younger, I'd have sweated through a hot week without a second thought. Now I have the means to be more comfortable, I use them — even if I still quietly calculate every penny while I'm doing it. Old habits.

My wife's position is simpler and probably healthier: be comfortable, don't overthink it. She's right that the AC should be on if it's needed. I just reserve the right to run it at 8p instead of 24p when I can.

Tips if you're genuinely watching the bills

If you're on a tight budget and weighing up whether to use the AC at all, here's how I'd think about it:

Check if a smart tariff is available to you. You don't need an EV to benefit from off-peak electricity rates. Several providers offer time-of-use tariffs with cheap overnight rates — Octopus Go, EON Next Smart, OVO Beyond. If you can shift appliance use to overnight, the AC maths changes completely.

Fan plus cold water is surprisingly effective. A fan blowing over a bowl of ice or cold water, or pointed at damp skin, provides real cooling at almost zero cost. Not the same as AC, but meaningful on a budget.

The bedroom is where it matters most. If you can only afford to cool one room, make it where you sleep. The cost of running a portable AC for 6 hours overnight at standard rates (around £1.30) is much more defensible than running it all day.

Check your unit's wattage. 9,000 BTU portable ACs typically draw around 800–1,000W. Larger 12,000 BTU units draw 1,100–1,400W and cost proportionally more. Your unit should have a label on the back with its wattage; multiply by your unit rate to get the cost per hour.

The bottom line: running a portable AC overnight on a cheap tariff costs less than 50p for a full night. Running it all day on the standard rate from July will cost around £1.88. That's not a reason to suffer — but it is a reason to think about when you run it, not just whether you run it.

Now if you'll excuse me, I need to go close the curtains.

Sources: Ofgem price cap rates July–September 2026 (26.11p/kWh electricity, announced June 2026). Fan wattage estimates from manufacturer specifications. AC running cost calculations based on 900W unit at stated tariff rates.