7-Day Off-Grid Simulator

Why a week, and not a day

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Almost every van power calculator on the internet, including ours, answers the same question: how many watt-hours do you burn in a day? That is a useful number. It is also the wrong number to buy a battery with.

Nobody runs out of power on a good day. You run out on day four of a bad stretch, at two in the morning, because the last three days were overcast and you never fully recovered from any of them. A daily total cannot show you that. It has no memory.

This tool has memory. It walks an hour at a time through a full week, tracks what goes in and what comes out, and carries the deficit forward the way a real battery does.

The week this is built on

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Santa Rosa Beach, Florida, summer of 2019. Seven straight days of overcast. Brutal heat, worse humidity, a fridge working overtime, fans we could not turn off, and almost nothing coming off the roof. I hauled the Goal Zero Yeti 1400 into a coffee shop nearly every day that week. On a 25-hour charger, two hours in a booth bought back very little.

What I learned that week is the thing the whole site is built on. When you run out, comfort goes first. You sweat. You stop running fans. You sit still and you get quiet. The one thing you cannot let fail is the fridge, because losing the food costs more than being uncomfortable.

Load the “Our actual 2018 setup” preset above and you are looking at that week. It is not a hypothetical.

What the curve is telling you

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The line is your battery percentage. The shaded columns are night, when nothing comes in and the fridge keeps cycling anyway. What you want to look for is not the low point on any single day. It is the shape across the week.

A setup that works has a sawtooth that returns to the top most days. A setup that is about to fail has a staircase going down, where each morning starts a little lower than the last one did. The staircase is the tell. By the time you notice it in real life you are usually two days from a problem.

Where the numbers come from

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Fridge draw, charge speeds, and the cold cutoff are measured on gear we owned and ran, not pulled from a press kit. Solar harvest, alternator output, and overcast yield are estimates built from known inputs, and they are labeled that way in the assumptions panel above. Open it. If a tool will not show you its constants, it is asking you to trust a number it made up.

Two honest limits worth stating. Panel angle, shading, and local weather move solar harvest more than any constant can capture, so treat the solar figures as a planning range rather than a forecast. And alternator output varies with your charger, your station’s input limit, and how long your engine actually runs.

The cold thing nobody puts on a box

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Pick either winter climate with the Yeti 1400 selected and watch what happens. The panels produce and the battery refuses to take it.

That is not a bug in the model. Our Yeti would not accept a charge once its internal temperature dropped near freezing, which in a Colorado winter meant most of the night and a good chunk of the morning. Full sun outside, charge controller sitting there doing nothing. The only fix we had was carrying a 45-pound battery into a heated house and waiting six to twenty-four hours.

Newer LiFePO4 stations handle this much better. It is one of the least discussed and most consequential differences between an older station and a current one, and it will not show up in any watt-hour comparison.

Using this to actually buy something

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Set it up honestly. Real climate, real loads, a realistic number of overcast days for where you are going. If the week fails, the tool tells you the smallest station in our lineup that survives it, and whether more solar would have solved it instead. Sometimes the answer is neither, and the real fix is running less.

Size for your typical week, not your worst one, and make sure you have more than one way to put power back in. That is the whole argument.

Related reading

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