Good physical "solar sinks" for excess PV power?

I'm sure some of you here have encountered this: due to seasonality & buffer, you probably oversize your PV system a little. Even though you have battery storage, there's a lot of time throughout the year that your batteries are fully charged by 10am and all that juicy solar electricity is going down the drain (or wherever it goes lol).

Someone phrased it in another thread about pre-cooling your house with AC during that time as a "solar sink" and I like that concept: are there physical things we can "sink" that excess energy into, given that we already have the solar incoming? It's sort of like adding a rain catchment system under a gutter, I suppose. Just to stop the waste of an existing resource flow.

Ideas:
- Pre-cool the thermal mass of your house (even if that's just a stickframe and your furniture) with AC
- Chill down the freezer
- Pre-heat something in the winter
- Dry firewood with an electrically powered kiln
- Dry/harden something more intensive with a kiln or oven (isn't cement made this way?)
- Power a flywheel for later energy use
- Pump water uphill, could let it run downhill later (potential energy battery)
- Melt iron ore or something very energy intensive lol
- Mine crypto

Anybody else have any great solar sinks?

Heat some water for taking a bath

Pre-Heat the thermal mass of your house (in winter)

Run a dehumidifier (if you have moisture problems)

Run an electric food dehydrator

Or my favorite:
Get grid access and give it to your neighbors, be that for free or even selling it to the grid!

Possibly add a grid tied inverter that will dump the excess to the grid.

Oh, it would be nice to share it with the neighbors, but selling to the power company is a losing proposition.
They pay pennies on the dollar for it and require YOU to purchase transfer equipment to protect linemen working on the transmission lines.
So, use A/C if it's summer and electric heaters in the winter, OR run your wood splitter and your welders.

thomas rubino wrote:Oh, it would be nice to share it with the neighbors, but selling to the power company is a losing proposition.

It often is, but depends on where you are. Here (in Spain) there's contracts in which you use the grid as a virtual battery. You only pay them for what you take more than what you've given them. Which to me is a pretty sweet deal.
Unfortunately I'm 100 meters from the next connection point and a neighbor got a quote for around 10k euros to get connected...

Run a compressor and store extra compressed gas.
Pump water uphill to use for microhydro later

M Pearson wrote:Anybody else have any great solar sinks?

One useful comparison is how much energy the proposed store actually holds, and whether you already need the output. A hypothetical water example: lifting 1,000 litres through 10 metres stores only about 0.027 kWh of gravitational energy (mass x gravity x height), before pump and turbine losses. That can still be worthwhile if you need the elevated water for irrigation; it is a very small electricity battery at that scale.

By contrast, warming 100 litres of water by 20 degrees C stores about 2.3 kWh of heat, before tank losses. That is heat for later use, not 2.3 kWh of recoverable electricity. The distinction makes a big difference when comparing sinks.

I would first list jobs that are already needed and can move into the sunny window: water pumping, laundry, or drying a food harvest. Then compare their useful output and storage losses rather than adding a load solely to consume surplus. For any automatic diversion, the battery reserve and existing equipment limits still need to be respected; these examples are energy arithmetic, not control or wiring instructions.

What is your usual surplus window, and do you already have a hot-water or irrigation need?

A cheap and very automatable sink is a resistive water-heating element wired through the inverter or charge controller's auxiliary output, so excess PV gets dumped into the hot water tank only once the batteries reach float. Even without fancy controls, a simple timer on the water heater, freezer, or dishwasher set to run during the usual surplus window does most of the work. Around here that window is roughly 11:00-15:00, so anything heat-based that can run on a schedule lands perfectly.

For this purpose I built a wifi voltage meter for my batteries.      When the batteries are full,  a relay clicks and the power from the solar panels go directly to my heating element in my hot water heater...     I love solar powered hot showers which are automated.


But I have given this much thought with time, and some of the ideas I have had:

1)   Sand battery store heat in sand for later use or even to cook with,  in my testing I had heating elements up to over 700 deg F.      

2)  Store heat chemically.       Using limestone if one heats it to a certain temp it changes chemically.     This can be stored and added to water to give off heat, hand warmers work with this principle.      I know of a gent in Germany that uses the excess solar to create hydrogen which he burns right away to give off heat, I have not gone this route but I found it most interesting.

3)   Distill ....    I bought a still for making essential oils.      At some point I would like to investigate this even thought about making alcohol to burn for heating / run engines.

4)    Use extra solar to dry wood,  I have seen solar dehydrators but consider combining a j tube rocket heater with a standard electric heater to dry wood....      I have seen the Permies rocket heater design and had considered why not put a heating element in it with solar to give yet another option for dehydrating....  

5)    Use excess to power more devices in the home   like doing dishes,   running AC ...      I have been off grid for well over 3 years,  when it is a sunny day I pull more energy from the batteries in the morning to do tasks  that the solar can fill the now empty batteries.....      

6)     grow plants...    charge another set of batteries that can run grow lights, fans, heaters for year round gardening...     The thing is winter can have many cloudy days and one has to have a backup plan for when there is little sun.

7)    I guess one has to get an idea of  how many days a year they have extra power, and to know how much it is so they can know what they can do  and not do....        I have been looking at making syngas as a concept and storing it....      I adore propane because it can store for many years before it is needed...    some fuels can do that....

8)    Run a 3d printer or laser cutter  or CNC to make useful items.

The major problem with batteries is that they do not store energy well for many months at a time, I have been looking for methods of long term energy storage ( years )       I thought you know the extra energy I had from last years summer is all gone, I can't use any of that now....

Oh,  something I am working on testing, I bought one of these that outputs 65 watts of power via usb  hooks directly to solar panel,   my plan is to have it as a backup to charge my car jump starter and or run my laptop.        I tested last week and I was able to play a movie off the charge stored there,   for me it gives me another option when everything else does not work...

https://www.walmart.com/ip/Outdoor-Solar-65W-PD3-0-USB-C-Plate-for-Laptop-and-Tablet-Paper-Box-Packaging-No-Battery-Required-ABS/20687016061


I was thinking it also could run a fan in the van to blow out hot air....

Put a batch of bone broth to cook in the crockpot or instant pot!

On a slight tangent we had a similar thread a while ago looking at ways of storing the excess energy in embedded energy of goods - like making bricks or other manufactured items see https://permies.com/t/209079/Manufacturing-energy-storage-brainstorm for some more ideas.

Distill water, and bottle it.
Sell it as solar distilled or just use it later.

Make and sell dry ice, the smallest machines seem to be a couple of grand each.

Oxygen concentrators start at a similar price point.

i've only had really small systems, but even then occasionally this does happen. especially since most of the stuff i use when i am off grid is non electrical...

but a few simple things that come to mind when i get to that 100%

- break out the blender and make a smoothie
- grind my coffee, although i cannot go too over the top with that as i love it fresh...usually pre grind at least a few days to a weeks worth
- sew on my machine
- vacuum
- a few other things that are on the fly as needed  - perhaps i need power tools for a project and get inspired to do it now when i can, device batteries to charge up, and similar things

The easiest solar sinks may be the loads already sitting around waiting to run. Water heating, laundry, dishwasher, well or irrigation pumping, tool batteries, an EV if there is one, and food dehydrating can all be pushed into that late-morning surplus window without building another storage system. A simple priority list could handle the boring recurring loads first, then use whatever is still left for experiments like extra thermal storage or pumping water uphill. Turning normal chores into dispatchable loads seems a lot simpler than converting electricity into another form and then converting it back later.

I wonder, instead of dump loads is there an advantage to disconnecting some panels? Could this prolong their lifespan? Don't know -- just a notion.

I have a tracking solar heat concentrator, the sun scoop since April 2025.   Most people use these types of things as a solar cooker. But I did a water boil test and it delivers over 600 watts for about 8 hours a day in Summer.  Perhaps enough heat to cook a months worth of food.   BUT it is unreliable.  If it is cloudy for a few hours,  scary bacteria might grow.  So,  I have been thinking of and trying dump loads all this time. My go to dump load is about 10 liters or soil and chopped up weeds in a steaming pot. 2 inches of water in the bottom, and after 2 hours it is nicely steamed, and there are no weed seeds left alive.  In theory it should be much quicker but heat travels really slow through soil, and most of it gets radiated from the sides of the pot as water bubbles up,  and the middle takes a long time to get hot.  I use it as a 3.5 inch mulch when I transplant or grow seeds. (3.5 inches because I can level things off with a 2 by 4.   A lot of permies don't like this because "It kills the life" but as I see it,  there is plenty of life 3.5 inches below and it colonizes it really quick. (a lot of bacteria, protozoa and fungi are necrophiliacs and they breed faster than rabbits and they love my "dead" soil.  Plus, the plant roots have a great opportunity to encourage the life that they prefer as co-contributors.  I solar steam 10 liter batches,  in pots but if you are using pv,  you can do the same thing with an insulated metal barrel as a giant steaming pot, and get it up to about 80 or 90 C and have a thermostat to shut it off.  200 liters of soil at a time and you might get it done in a day.     That's almost commercial scale.     I take about a week to fill a 200 liter barrel with the 10 liters per batch thing.   AND I steam up lawnmowed weeds, and clippings,  that can go in the barrel too, and it steams much quicker than soil because the heat zips through it. (Soil is a good insulator).   Steamed weeds rot really quickly, and I just use them straight away as mulch. I skip the composting stage totally. And I also skip the weeding stage. It is pretty cool.  I also use the heat to dehydrate fruit herbs, vegetables, herbs and tobacco (for insecticide).  And I adapted a vevor 3 gallon distiller for the heat concentrator,  (just barbecue black painted the bottom black) and it delivers about 6 ml per minute of distilled water,  or hydrosol if you are distilling essential oil.  Of course you can distill alcohol too if you are allowed. It would be good for cleaning, and if you know how to distill of the meths first,  might be ok for drinking.   I found fennel to be the best but I only started very late in the year when everything, including the mint was woody.  When doing essential oils,  you make it like a steamer with maybe 4 inches of water and then wire mesh and then your herbs.  I read up.   When they steam rose petals for its essential oil as  perfume, the water left in the bottom of the still is "rose water" and that is valuable stuff!  But what is in the water after distilling fennel, or mint or weeds or grass from your lawn?    Probably sugars and proteins.   And I bet that if you added some yeast to it,  or pour it slowly on mushroom straw,  stuff will grow!  And if it is yeast, you might be able to get very useful cleaning alcohol from your waste grass, etc.   And maybe some yeast to feed to animals or fish.      So yeah,  what I do with solar heat,  anyone else could do with solar PV as a dump load and they could actually control it better.   My reflector is a bit under 1.5 sq meters in area, and I plugged the heating numbers into google.  It told me that on a sunny day,  it gives me the same heat amount as you could get from three to five grid connected 400 watt  roof mounted solar panels.    (of course on a cloudy day,  mine gives you nothing).     I hope to work on the soil steaming some more,  and either get the big  insulated barrel thing going,  or get a smaller system going, that is continuous flow. (As the soil gets to a good temperature, a thermostat turns on an auger, and soil gets moved) because that would be really neat. So yeah, distilling, dehydrating and soil steaming  can use up some of that spare energy to do very useful stuff.      sun scoop playlist

Douglas Alpenstock wrote:I wonder, instead of dump loads is there an advantage to disconnecting some panels? Could this prolong their lifespan? Don't know -- just a notion.

The heat of the sun is what I believe destroys the panels, but it is a trade off between long life and the amount of energy you get out of the panel.

One could place the panel to where it would miss the direct heat of mid day,  but that is often where the most energy is,  as one could have it shaded at that time to avoid the temp extremes..

Another option to be considered is to have a couple of panels that would run a water cooling system for the panels,   but this adds complexity and another system to maintain.

Nancy Reading wrote:Run a compressor and store extra compressed gas.
Pump water uphill to use for microhydro later

There was an Amish man that bought a railroad car tank.       He then used that to fill with compressed air, and converted all of his tools to air..

I highly recommend browsing around Low Tech Magazine's website - he has written a lot about these sorts of questions, particularly from a societal level. Probably the most directly to this question is his article about compressed air, which he dives very deeply into, even talking a bit about using it for refrigeration.
https://solar.lowtechmagazine.com/2018/05/ditch-the-batteries-off-grid-compressed-air-energy-storage/
And if the more societal / historical parts interest you, he writes about the history of compressed air here, including Paris's public utility that lasted from 1881 to 1994: https://solar.lowtechmagazine.com/2018/05/history-and-future-of-the-compressed-air-economy/

This one’s fun
I’d run my excessive summer time solar

Easiest would be into a hot water tank

If one had a work shop and needed compressed air
Run a compressor into a huge propane tank modified for compressed air storage

Wire placed deep into a thermal mass bed in and around either a greenhouse, Wofati, earthship.   Deep enough that by the time winter rolls around the heat reaches the outer wall.  Just right so that it doesn’t overheat in the summer

Running a compressor to freeze down a mass of water using the cool for refrigeration once stored

Lots of thought to go further with this but ….  See a need fill a need.

I have an inverter relay that flips a specific exterior outlet on when the batteries are full, and that outlet is used to charge my EV.

We bought an electric side by side utility vehicle as a prep for when gas becomes unavailable or just too dang expensive (which looms precariously closer....)  It plugs into a 110 outlet to charge, so I'd be recharging it during peak solar hours.  Or doing laundry, running a canner on an electric burner, vacuuming, using power tools, recharging the electric mower and chain saw, running the freeze dryer or dehydrator, baking stuff in the electric oven, using the infrared sauna, and so on.  

Although an actual solar sink to hold power would be something more along the lines of pumping water into a high tank or uphill pond to release later as needed to run a hydroelectric unit.  Or the compressed air others mentioned.  We bought undamaged Tesla car batteries from wrecked cars to store power, but they only hold so much, just like any other batteries.  So they can cover us for a cloudy day or two, but that's about it.  

Donna Lynn wrote:We bought an electric side by side utility vehicle as a prep for when gas becomes unavailable or just too dang expensive (which looms precariously closer....)  It plugs into a 110 outlet to charge, so I'd be recharging it during peak solar hours.  Or doing laundry, running a canner on an electric burner, vacuuming, using power tools, recharging the electric mower and chain saw, running the freeze dryer or dehydrator, baking stuff in the electric oven, using the infrared sauna, and so on.  

Although an actual solar sink to hold power would be something more along the lines of pumping water into a high tank or uphill pond to release later as needed to run a hydroelectric unit.  Or the compressed air others mentioned.  We bought undamaged Tesla car batteries from wrecked cars to store power, but they only hold so much, just like any other batteries.  So they can cover us for a cloudy day or two, but that's about it.  

hi donna, what is the model of the electric side by side?

Byron Gagne wrote:If one had a work shop and needed compressed air
Run a compressor into a huge propane tank modified for compressed air storage

Good idea in theory, running maybe 100 psi as a maximum working pressure. That's a lot of useful energy.

Unfortunately propane tanks are not typically lined for corrosion protection. Propane has been heavily dewatered. Introducing atmospheric air also admits moisture, which rots out the pressure vessel pretty quickly from the inside.

Your grid electricity power generator works best at full and steady load
BUT electricity is used up immediately it is produced which is a waste
since Independant Power Producers are paid to balance things out.

It is especially dire in Australia since there is so much solar piped
into the grid. Investigate the duck curve to know more.



I have a solution. What if we have a load that can be switched on and off
to balance the fluctuations and doesn't get harmed at all?

Introducing . . .



Banks of these can stabilize the grid and produce biochar and sequester
carbon.

What's not to like?

In your smaller scale, pack a biochar retort in a sandbox and place it
on an induction cooker.

It sounds feasible although the nearest I've gotten is to try to dry a milk
tin and the tin rapidly  glowed red and the paint started to smoke.

Naturally some experiment is required to get the sweet spot. Done but
not overdone.

David Baillie wrote:

Donna Lynn wrote:We bought an electric side by side utility vehicle as a prep for when gas becomes unavailable or just too dang expensive (which looms precariously closer....)  It plugs into a 110 outlet to charge, so I'd be recharging it during peak solar hours.  Or doing laundry, running a canner on an electric burner, vacuuming, using power tools, recharging the electric mower and chain saw, running the freeze dryer or dehydrator, baking stuff in the electric oven, using the infrared sauna, and so on.  

Although an actual solar sink to hold power would be something more along the lines of pumping water into a high tank or uphill pond to release later as needed to run a hydroelectric unit.  Or the compressed air others mentioned.  We bought undamaged Tesla car batteries from wrecked cars to store power, but they only hold so much, just like any other batteries.  So they can cover us for a cloudy day or two, but that's about it.  

hi donna, what is the model of the electric side by side?

It's a Hisun Motors Sector E1.  We had to drive several hours to pick it up because not many places were selling them.  Ours is a 2022.  My guy is partially disabled, and the Hisun made it possible for him to help with some homestead chores he just couldn't walk enough to do before.  He learned all about the batteries and how best to make them last, and that's how we use and charge it.  We also got the optional snow plow, and now that we've moved it's saved us a lot of money plowing our long hilly driveway.  I can't say it's paid for itself yet, but for our situation it's made a world of difference as far as "aging in place,"

Edward Lye wrote:
It is especially dire in Australia since there is so much solar piped
into the grid. Investigate the duck curve to know more.

It's a good problem to have, because now a whole bunch of Aussies get three hours a day of free electricity. Flattening the duck curve by doing loads of laundry, charging EVs, and heating pools.

Edward Lye wrote:Banks of these can stabilize the grid and produce biochar and sequester
carbon.

That's super interesting, especially the use of molten salt as the carrier. The video was a little murky on what is happening to the syngas, but maybe they're burning it to spin the turbines.

I'd love to be able to make a batch of biochar in ten minutes. Yesterday I spent 5 hours on a burn and probably got a yield of 500 litres. My yields get better in summer when the wood dries out more, but I have to build up inventory because I'm selling the stuff and winter meant I went for over four months without making any.

Phil Stevens wrote: I'd love to be able to make a batch of biochar in ten minutes.  .

You'll need all that equipment.

Molten salt is corrosive and requires special alloys and seals and pumps.

  I know these from watching all those "potential" Thorium reactor videos.
  Happily India and China have those already running.

Not at all meant for a backyard operation unless you have a great big
backyard and even then you best take care.

Donna Lynn wrote:

David Baillie wrote:....hi donna, what is the model of the electric side by side?

It's a Hisun Motors Sector E1.  We had to drive several hours to pick it up because not many places were selling them.  Ours is a 2022.  My guy is partially disabled, and the Hisun made it possible for him to help with some homestead chores he just couldn't walk enough to do before.  He learned all about the batteries and how best to make them last, and that's how we use and charge it.  We also got the optional snow plow, and now that we've moved it's saved us a lot of money plowing our long hilly driveway.  I can't say it's paid for itself yet, but for our situation it's made a world of difference as far as "aging in place,"

Just to add a more legacy side-by-side that pops up on the used market occasionally-- The Polaris Ranger is a popular gas side-by-side but had a 48V electic version for many years as well.  I found a 2012 model a few years back priced around $5000 USD.  There are a few active and very helpful internet groups to assist for any troubleshooting and for lead-acid to lithium battery conversions.  Although we still use lead acid batteries in the machine and the stock 120V charger in mid-winter, a single 300W+ solar panel does fine for charging in appropriate months....for that, just wired in a port on the cart to accept the 50VDC+ output from the solar charge controller.  Some light particulars on the legacy units here:   https://www.polaris.com/en-ca/off-road/owner-resources/help-center/article/KA-01703/    but they are really pushing their new electic Polaris Kinetic ... which in Canada is likely to be more expensive than many of the coming import electric cars you might soon be seeing!  Hence the robust used market for the older Ranger EV.  As noted by Donna L. these vehicles can be a game-changer for homesteads....my wife just loves the thing for feeding animals and maintaining fences on the property.

I like the idea of cooling/heating the house; if you run your AC at 70, have it go down to 63 so it doesn’t run near as much at night while it gets back to 70.

If most people work while it’s producing it would probably take either timers or smart outlets but have your washer/dryer and or dishwasher if you use them preloaded to run at different intervals from late morning to mid afternoon.

Or if you store water run agitators and aeration.

We have grid-tied solar and were making extra for which the utility pays a pittance. The credit was paid in the beginning of the year when it's cool here and we'd have preferred to keep it for the summer months but were unable to do that
It was better for us to use all the power, so we had an electrician connect some of the appliances in the house next door directly and now we use it all.