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I think people aren't as aware of how power intensive showers are as maybe they should be. It takes roughly 10kw to provide a decent instant hot water shower. T

I think people aren't as aware of how power intensive showers are as maybe they should be. It takes roughly 10kw to provide a decent instant hot water shower. That's roughly the output of 40 domestic solar panels at 100% capacity. Of course you can buffer this with a battery bank but getting 10kw worth of inverter is non-trivial and the hit that a battery bank would take from providing that power would be challenging. Taken another way it's the same amount of energy as it required to propel a scooter or small motorcycle along at 50mph. How many people start their showers several minutes before the are actually ready to step into them?

icebraining 7y ago
> Of course you can buffer this with a battery bank but getting 10kw worth of inverter is non-trivial and the hit that a battery bank would take from providing that power would be challenging. Or you could just slowly heat water in a well insulated tank. And if you paint your tank black and put it in the sun, you may even be able to skip the PV panels :)
Filligree 7y ago
Is it common where you're from to use 10kW water heaters for a shower? We've got that at our cottage (gas-based, not electrical), but that's mainly because there's no grid connection. Everywhere else I've been, people just use tanks. It might take a few hours to heat up from nothing, but even a mid-sized tank can easily do 2-3 showers.
- VBprogrammer 7y ago
  At a wild guess I'd say 50% of UK homes would have at least one instant hot water electric shower. The other option being a power shower driven from the hot water loop on their gas central heating.
howard941 7y ago
You can heat water with DC and no inverter. Finding a DC water heater that's safe and inexpensive and voltage swing-tolerant might be a challenge.
- VBprogrammer 7y ago
  To be fair the most sensible way is as a resistive dump load (which as you mention doesn't care whether it's AC or DC) or as someone else mentioned skipping the electrical step altogether and going with a simple solar water heater.
  - cr0sh 7y ago
    I've often wondered why it wasn't more common to integrate solar water heating within the solar electric panels. My understanding is that as the cells heat up, they lose efficiency, so keeping them cool is important. So - mount them to a heatsink (waterblock) with a water (or more likely coolant) loop running thru them to transport that heat to a water tank for showers or other heating use. There's got to be a good reason why this isn't done that I don't know about (maybe it really isn't that efficient, or solar cells don't heat up that much, or something else)?
    - VBprogrammer 7y ago
      I suspect the reason is you want hot water stored at ~60C. If you pump water through it slowly enough / enough times to attain that temperature then you are actually increasing the temperature of the panels (assuming air temperature is less than that). You'd need to use a heat pump to get actual hot water from a small delta T in the coolant.
zeveb 7y ago
> It takes roughly 10kw to provide a decent instant hot water shower. Electricity is currently about 12¢/kwh, or 1¢/5kwm. If I'm doing the math right, that means that a 10-kwm shower costs all of tuppence. That's a small percentage even for folks in the world living on a couple dollars a day — it's basically free for those of us in the Western world. Did I get the math right?
- VBprogrammer 7y ago
  Yes, I think you did. Indeed it's almost embarrassingly cheap, about 25p for a 10 minute shower.
thaumasiotes 7y ago
> How many people start their showers several minutes before the are actually ready to step into them? The norm in Shanghai is that the bathroom contains a small water heater with a tank of water that is kept heated at all times. The shower then draws simultaneously from the heated water and from the regular supply of cold water. You're not supposed to draw exclusively from the hot water -- if you do, the tank will run out in a few minutes. When the preheated water runs out, your shower will go cold until you let the water heater refill and recharge.

icebraining 7y ago

> Of course you can buffer this with a battery bank but getting 10kw worth of inverter is non-trivial and the hit that a battery bank would take from providing that power would be challenging. Or you could just slowly heat water in a well insulated tank. And if you paint your tank black and put it in the sun, you may even be able to skip the PV panels :)

Filligree 7y ago

Is it common where you're from to use 10kW water heaters for a shower? We've got that at our cottage (gas-based, not electrical), but that's mainly because there's no grid connection. Everywhere else I've been, people just use tanks. It might take a few hours to heat up from nothing, but even a mid-sized tank can easily do 2-3 showers.

VBprogrammer 7y ago
At a wild guess I'd say 50% of UK homes would have at least one instant hot water electric shower. The other option being a power shower driven from the hot water loop on their gas central heating.

howard941 7y ago

You can heat water with DC and no inverter. Finding a DC water heater that's safe and inexpensive and voltage swing-tolerant might be a challenge.

VBprogrammer 7y ago
To be fair the most sensible way is as a resistive dump load (which as you mention doesn't care whether it's AC or DC) or as someone else mentioned skipping the electrical step altogether and going with a simple solar water heater.
- cr0sh 7y ago
  I've often wondered why it wasn't more common to integrate solar water heating within the solar electric panels. My understanding is that as the cells heat up, they lose efficiency, so keeping them cool is important. So - mount them to a heatsink (waterblock) with a water (or more likely coolant) loop running thru them to transport that heat to a water tank for showers or other heating use. There's got to be a good reason why this isn't done that I don't know about (maybe it really isn't that efficient, or solar cells don't heat up that much, or something else)?
  - VBprogrammer 7y ago
    I suspect the reason is you want hot water stored at ~60C. If you pump water through it slowly enough / enough times to attain that temperature then you are actually increasing the temperature of the panels (assuming air temperature is less than that). You'd need to use a heat pump to get actual hot water from a small delta T in the coolant.

zeveb 7y ago

> It takes roughly 10kw to provide a decent instant hot water shower. Electricity is currently about 12¢/kwh, or 1¢/5kwm. If I'm doing the math right, that means that a 10-kwm shower costs all of tuppence. That's a small percentage even for folks in the world living on a couple dollars a day — it's basically free for those of us in the Western world. Did I get the math right?

VBprogrammer 7y ago
Yes, I think you did. Indeed it's almost embarrassingly cheap, about 25p for a 10 minute shower.

thaumasiotes 7y ago

> How many people start their showers several minutes before the are actually ready to step into them? The norm in Shanghai is that the bathroom contains a small water heater with a tank of water that is kept heated at all times. The shower then draws simultaneously from the heated water and from the regular supply of cold water. You're not supposed to draw exclusively from the hot water -- if you do, the tank will run out in a few minutes. When the preheated water runs out, your shower will go cold until you let the water heater refill and recharge.