Living Off-Grid in a Container Home

Most off-grid systems fail in the second week of January. Not because the equipment breaks, but because it was sized against an annual average instead of against the worst nine days of the year. If you take one thing from this piece, take that.

Size against the dark week, not the year

A solar quote will usually tell you how many kilowatt-hours your array produces annually and compare it to your annual consumption. Those two numbers can match perfectly and you can still be sitting in the dark. What matters off-grid is the longest stretch your site goes without meaningful generation, and in the Pacific Northwest that is routinely four to nine days of heavy overcast in midwinter.

We size every off-grid unit for four consecutive days of no useful generation at winter loads, with the battery never planned to drop below twenty per cent state of charge. On our Ridgeline 40 that works out to a 6.4 kW array against a 20 kWh lithium bank. A lot of quotes you will see pair a similar array with 10 kWh, because the battery is the expensive half and shrinking it is the easiest way to win a bid.

The battery needs to be warm

Lithium iron phosphate loses usable capacity below about 40°F and should not be charged at all below freezing. An uninsulated steel box in a mountain winter will happily sit at 20°F for a week. This is the single most common failure we are called out to look at on systems somebody else installed: a perfectly good battery bank bolted to the inside of an uninsulated container wall, quietly refusing to accept a charge every morning until the sun has been on it for two hours.

Our battery banks sit in a conditioned service bay inside the insulated envelope, with its own small ventilation path and a temperature sensor tied into the system monitor. It costs floor area. It is not optional.

Loads you forget to count

People model the fridge, the lights and the laptop. Then they move in with:

• A well pump or pressure pump, which is a large intermittent load with a nasty starting surge
• An induction hob, which will pull 3 kW the moment somebody boils a pan at dusk
• A kettle, a toaster and a hair dryer, all of which arrive in the same box as the person who uses them
• Standby draw from an inverter that never sleeps, which is small but runs 8,760 hours a year

We model consumption hour by hour rather than as a daily total, because the daily total hides the evening peak, and the evening peak is when the array is producing nothing.

Water is the half nobody plans

A 40 ft container roof catches roughly 900 gallons from an inch of rain. That is a genuinely useful number in a wet climate and a useless one in a dry summer, so the cistern has to be sized for the dry gap rather than the annual rainfall. We generally specify 1,500 gallons buried below the frost line, fed through a first-flush diverter and a four-stage filter to potable standard.

Greywater from the shower and basin is separated at the trap. Where local code allows it, that goes to a reed bed or a subsurface irrigation loop. Where it does not, it goes to the same tank as the blackwater and you should know that before you design the bathroom, not after.

What it is actually like

Quieter than people expect, and more attentive. You develop a relationship with the weather forecast that grid-connected people do not have. You run the washing machine at one in the afternoon rather than nine at night, because that is when the array is making more than the house is using, and after a month you stop noticing that you are doing it.

The Okonkwo family have been on our Ridgeline 40 for two winters. Their bank has not dropped below forty per cent state of charge, including a nine-day January overcast. That is not because the site is unusually sunny. It is because the system was sized for that week specifically.