THE SUNSHINE BANK

What a Month of Solar Data Has Taught Me About Living Off-Grid

There is something wonderfully satisfying about parking a campervan in a field, switching everything on and knowing that, somewhere above your head, daylight is quietly paying the electricity bill.

No plug. No campsite hookup. No generator chuntering away in the corner.

Just the sky.

That, essentially, is what I have been experimenting with in Vanilla since fitting a new 200 watt PV Logic solar panel to the roof and replacing the original controller with a Victron SmartSolar MPPT 75/15.

And over the first full month of proper data, something rather interesting has emerged.

The solar panel is working very well.

The Victron controller is working very well.

But the most fascinating discovery is that the battery currently underneath my passenger seat has sometimes been the thing holding the whole system back.

And that is precisely why my forthcoming move from an AGM battery to a Fogstar lithium battery could change Vanilla’s off-grid abilities far more than I first thought.

First, forget the electrical jargon

Solar power can become unnecessarily intimidating because people start throwing around watts, watt-hours, volts, amps, absorption voltages and charge curves until everyone except the electrician has quietly left the room.

So here is Vanilla’s solar system explained using a garden hose.

☀️ The solar panel is the water supply.

It collects energy from daylight.

🧠 The Victron controller is the clever tap.

It constantly works out how to extract the best available energy from the panel and safely send it towards the battery.

🔋 The battery is the water tank.

It stores that energy so I can use it later.

And Vanilla’s fridge, lighting, Webasto heater, USB chargers and other equipment are effectively taps taking water back out of that tank.

That is basically the entire system.

The clever bit is what happens when the tank gets full.

Thirty days of sunshine

Between 31 August and 29 September I collected thirty complete days of solar history from the Victron.

In that period Vanilla’s roof produced:

⚡ 8.41 kilowatt-hours of usable solar energy

That is 8,410 watt-hours harvested from daylight.

The average was:

☀️ 280 Wh every day

The best day produced:

🔥 570 Wh

And the highest instantaneous output I recorded from the nominally 200 W panel was:

⚡ 147 W

There were no controller errors recorded during the entire period.

For a flat panel mounted permanently on the roof of a campervan during September in Britain, I am very happy with that.

But there is an important distinction here.

The panel didn’t necessarily produce everything it could have produced.

It produced everything the system was able to use.

And those are not always the same thing.

What does 200 watts actually mean?

This catches people out.

A 200 W solar panel does not produce 200 W continuously from sunrise until sunset.

If it did, campervan electricity would be hilariously easy.

The 200 W figure is essentially the panel’s maximum rated output under very specific test conditions.

Real life introduces Britain.

Clouds. 🌥️

Sun angle.

Temperature.

Trees.

Buildings.

Shade.

September.

And the inconvenient fact that a campervan roof spends very little time perfectly pointing at the sun.

So seeing 147 W from my 200 W panel means it achieved around 74% of its official rated power at its best moment.

That doesn’t worry me at all.

Quite the opposite.

During the month it exceeded a 100 W peak on 21 separate days.

It reached 142 W, 146 W and 147 W on its strongest days.

The panel therefore isn’t sitting on Vanilla’s roof looking decorative.

It is working.

But watts aren’t the whole story

This is where solar starts becoming interesting.

A huge instantaneous number isn’t necessarily what you want.

Imagine two solar days.

On Day A the panel briefly touches 150 W for ten minutes before clouds arrive.

On Day B it sits at 80 to 120 W for several hours.

Day B can produce vastly more electricity overall.

That is why I pay so much attention to Wh, or watt-hours.

Watts tell you how hard the system is working right now.

Watt-hours tell you how much energy it has actually collected over time.

Think of it as speed versus distance.

147 W is the speedometer.

570 Wh is the odometer.

And one particular day demonstrated that beautifully.

26 September: the monster day

On 26 September the panel’s maximum output was only:

124 W

So it wasn’t even Vanilla’s highest-power day.

Yet by the end of that day the Victron had collected:

🔥 570 Wh

That remains the best day in the dataset.

Why?

Because rather than producing one spectacular burst, the system appears to have been producing useful power for a long time.

The battery remained hungry enough to take it.

And this is where my AGM battery enters the story.

Meet AGM

Vanilla currently has a conventional AGM leisure battery.

AGM batteries have done sterling service in campervans, caravans and boats for years. They are proven technology and there is nothing inherently wrong with them.

But they have limitations.

One of those limitations concerns how they charge.

When an AGM battery is fairly empty, it will happily accept plenty of charging current.

As it gets fuller, things change.

The charging process gradually slows.

Eventually the Victron moves through stages called:

Bulk

Absorption

Float

Forget the technical names for a moment.

Think of filling a pint glass from a tap. 🍺

When the glass is empty you can open the tap properly.

As the beer approaches the rim, you naturally slow the flow because otherwise you’ve got foam all over the bar and an unhappy landlord.

AGM charging behaves a little like that.

The battery can accept lots of energy when it is emptier, but as it approaches full charge the controller has to become increasingly gentle.

Eventually it reaches float.

At that point the battery is essentially saying:

“I’m fine, thanks. Just keep me topped up.”

Which creates an interesting problem.

What if the sun is still shining?

The solar panel might still be capable of producing plenty of power.

But the battery doesn’t need it.

The Victron therefore backs the panel off.

The available sunlight hasn’t disappeared.

There simply isn’t anywhere useful for all that electricity to go.

And my solar records appear to show this happening again and again.

The number that changed the whole story

Across the 30 complete days, Vanilla reached float on 22 days.

On those days average solar harvest was approximately:

246 Wh per day

But on the eight days when the battery didn’t reach float, average harvest jumped to approximately:

🔥 374 Wh per day

That is about 52% higher.

Now, this absolutely does not mean changing batteries will magically give me 52% more solar every day.

Weather changed.

My electrical use changed.

Cloud cover changed.

There are too many variables to make that claim.

But the pattern is extremely interesting.

When the battery remained hungry, the solar system generally harvested much more energy.

When the battery filled, solar harvesting frequently dropped.

That makes complete sense.

And 8 September is perhaps the best example.

The strange case of the terrible solar day that wasn’t terrible

On 8 September Vanilla harvested only:

140 Wh

Looking at that figure alone, I might have concluded:

“Rubbish solar day.”

Except the charging history tells another story.

The system spent nearly five hours in float.

The battery was full.

The Victron therefore had little reason to extract maximum power from the solar panel.

In other words, that 140 Wh isn’t necessarily telling me:

“The sun only provided 140 Wh.”

It is telling me:

“Vanilla only needed to collect 140 Wh.”

That is a completely different thing.

On 23 September exactly the same pattern appeared.

Solar harvest:

160 Wh

Time in float:

Almost five hours.

Again, the battery had effectively shut the restaurant because it had eaten enough.

Meanwhile the chef was still standing in the kitchen.

☀️👨‍🍳

And this is where lithium changes things

My next upgrade is a Fogstar Drift 105 Ah LiFePO₄ battery.

LiFePO₄ stands for lithium iron phosphate.

That sounds suspiciously like something involving a laboratory coat and goggles, but for our purposes it simply means a modern lithium leisure battery chemistry.

The important thing isn’t the chemistry lesson.

It is how differently lithium behaves.

Lithium batteries can generally accept substantial charging current through much more of their charging cycle.

They don’t spend ages slowly squeezing the last part of the charge into the battery in the same way an AGM does.

So return to our pint glass.

AGM is the barman carefully slowing down as the pint approaches the top.

Lithium is closer to filling a much bigger container that can happily accept a strong flow for considerably longer.

And Vanilla’s Victron solar controller can deliver a maximum of around 15 amps.

For the Fogstar, that is not a particularly aggressive charging rate.

So if the lithium battery is sitting at 40%, 50% or 60% charge and sunshine appears, it should be perfectly happy taking pretty much everything my 200 W solar installation can provide.

Lithium does NOT create more sunshine

This bit matters.

Installing a lithium battery will not suddenly turn a grey Staffordshire Tuesday into California.

☁️➡️☀️

The solar panel remains 200 W.

The weather remains British.

The sun remains roughly 93 million miles away and stubbornly outside my control.

What changes is my ability to capture and store more of the energy that is available.

Suppose the panel could produce 120 W.

With the AGM already nearly full, perhaps the battery only needs a small proportion of it.

The Victron backs off.

Potential solar is effectively left on the roof.

Now imagine the lithium battery is only half full.

The panel offers 120 W.

Lithium says:

“Yes please.”

The Victron keeps working.

That’s the difference.

It also gives me a much bigger usable fuel tank

Battery capacity is another area where the headline figures can be misleading.

A conventional lead-acid battery does not particularly enjoy being repeatedly discharged very deeply.

Lithium is far happier operating across a much larger proportion of its nominal capacity.

So although I’m moving from an AGM of roughly 100 Ah to a Fogstar of about 105 Ah, the headline numbers make them sound almost identical.

They aren’t.

The amount of energy I can comfortably use should increase substantially.

And the voltage should remain much more stable while I am using it.

That matters enormously in a campervan.

The fridge doesn’t care about impressive battery specifications.

Neither does the Webasto.

They just want reliable electricity.

Those low AGM voltages caught my attention

My month of Victron data also contains another interesting clue.

The minimum recorded battery voltage dropped below 12 V on numerous days.

On 28 September the lowest recorded value was:

10.63 V

On 29 September:

10.58 V

Those aren’t resting battery voltages, so they shouldn’t be treated as a direct measurement of state of charge.

A heavy load can temporarily drag an AGM’s voltage downward.

But that is exactly the point.

AGM voltage sags.

Lithium’s voltage curve is considerably flatter.

So when the Fogstar goes in, one of the things I will be watching closely is how stable Vanilla’s 12 V supply becomes when the fridge, Webasto and other equipment are operating.

Then there’s the Jackery experiment

This becomes even more interesting because Vanilla isn’t just powering herself.

I’ve also been using the leisure electrical system to charge my Jackery power stations through the Anderson connection at roughly 90 W.

So imagine a sunny campsite next year.

Vanilla is parked.

The fridge is cold.

The Fogstar is perhaps 60% full.

The panel is producing 120 W.

The Jackery is taking around 90 W through the Anderson connection.

Suddenly the solar roof isn’t merely charging a battery.

It is feeding an entire little ecosystem.

Sunlight lands on Vanilla.

The Victron extracts it.

The Fogstar acts as the buffer.

The fridge runs.

The Jackery charges.

The Webasto has energy waiting for that evening.

That is exactly the type of off-grid system I’ve been trying to build.

Not an enormous complicated installation.

Just one where all the components work together intelligently.

So how good has the first month actually been?

Strip away every technical term and the answer is:

Really encouraging.

☀️ 8.41 kWh collected

⚡ 280 Wh average per day

🔥 570 Wh best day

🚀 147 W maximum solar power

🧠 Zero Victron controller errors

🔋 Battery reached float on 22 out of 30 complete days

And perhaps most importantly:

When the battery had room for electricity, the system demonstrated that it could produce substantially more than the headline average suggests.

That is the bit I didn’t fully appreciate when I began this experiment.

My 280 Wh daily average isn’t necessarily the average amount of solar energy that was available.

It is the average amount Vanilla actually harvested.

Those are subtly different things.

Phase Two starts with lithium

Once the Fogstar goes in, this entire experiment effectively resets.

But this time I have a proper baseline.

Thirty days of AGM data.

I know the average.

I know the peak.

I know the best day.

I know how often the existing battery reaches float.

I know how far its voltage can sag.

So over the following weeks I can compare the Fogstar properly.

I’ll be watching:

☀️ Daily solar harvest 🔋 Morning battery percentage 🌙 Overnight consumption ⚡ Maximum solar output 🔥 Webasto use 🧊 Fridge consumption 🔌 Jackery charging 📉 Lowest battery voltage 📈 Solar recovery during the following day

And eventually I’ll be able to answer the question that actually matters:

How long can Vanilla genuinely live away from a plug?

That, for me, is far more interesting than arguing about whether one battery chemistry is theoretically better than another.

I want to know what happens in the real world.

Parked in a field.

Fridge running.

Heating on.

Lights glowing.

Jackery charging.

Rain arriving.

Sun returning.

Day after day.

Because that’s where an off-grid system either works or it doesn’t.

The sunshine bank

I’ve started thinking about the roof of Vanilla as my little energy bank.

Every hour of useful daylight makes a deposit.

The fridge makes withdrawals.

The Webasto makes withdrawals.

Lights make tiny withdrawals.

Charging Jackery makes a larger one.

The battery is the bank account sitting between them.

My original solar upgrade gave me a better income.

The lithium upgrade gives me a better account.

And the really exciting part is that those two things together could change how I use Vanilla altogether.

Less thinking about hookup.

Less worrying about battery voltage.

More staying where I actually want to stay.

For me, that is what off-grid power is really about.

Not watts.

Not amps.

Not battery chemistry.

Freedom measured in nights rather than numbers. ☀️🔋🚐