Confused by the logic of it all

[frayt]

I'm just trying to get a ballpark of what a solar system would cost to set up, so I start to put together a few basic devices but I come across a pretty confusing thing

Some electric shower that I'm looking at says it consumes 10kW of electricity. A 10kw solar panel costs about £8,000. What this says to me is that it costs £8k to be able to run JUST a shower, but I'm gonna go ahead and guess that I'm not understanding this correctly. Could someone clear this up? I realize this is probably general knowledge to you guys but I'm not experienced with anything electricity.

[sdold]

Where are you located, and do you have grid power available?

[frayt]

Where are you located, and do you have grid power available?

Ontario, Canada. No.

[prhamilton]

So you don't have any electricity at your house for lights or anything? When some asks are you 'on the grid' they mean are you able to buy electricity from a power company. Generating solar electricity while still buying power is much easier than generating electricity on your own('off grid').

When you say electric shower do you mean a water heater? A water heater with 10kW load is probably an on demand water heater. They use a lot of power for a short period of time. It would be difficult to power something like this using only solar electricity. It would require a system with a ton of batteries. I can't imagine it being a good idea.

If you are looking for hot water there are more efficient ways of making hot water than standard solar panels(PV) that make electricity. You should look at solar thermal system that use the sun to directly heat water. These systems cost less and are an order of magnitude more efficient than generating electricity and then heating the water.

The second thing to know is that a hot water storage system is going to be a better fit than an on demand system. A hot water storage system will let you slowly raise the temperature of a tank of water and then store until you need it(like early in the morning).

[frayt]

So you don't have any electricity at your house for lights or anything? When some asks are you 'on the grid' they mean are you able to buy electricity from a power company. Generating solar electricity while still buying power is much easier than generating electricity on your own('off grid').

When you say electric shower do you mean a water heater? A water heater with 10kW load is probably an on demand water heater. They use a lot of power for a short period of time. It would be difficult to power something like this using only solar electricity. It would require a system with a ton of batteries. I can't imagine it being a good idea.

If you are looking for hot water there are more efficient ways of making hot water than standard solar panels(PV) that make electricity. You should look at solar thermal system that use the sun to directly heat water. These systems cost less and are an order of magnitude more efficient than generating electricity and then heating the water.

The second thing to know is that a hot water storage system is going to be a better fit than an on demand system. A hot water storage system will let you slowly raise the temperature of a tank of water and then store until you need it(like early in the morning).

This really was just me hypothesising. I wasn't looking for an opinion on the efficiency of my system and possible other solutions, just some simple math.
Here:
If I had a 5kW solar panel and a (let's just say) infinite capacity battery bank, and I got 5 hours of sunlight on the solar panel. Would that charge the battery bank up to 25kWh?
If so, when I ran my 10kW electric shower (or on demand heater) for 15 minutes, that would use 2.5kWh out of my battery bank, leaving me with 22.5kWh. Is that right?

[foo1bar]

I'm just trying to get a ballpark of what a solar system would cost to set up, so I start to put together a few basic devices but I come across a pretty confusing thing

Some electric shower that I'm looking at says it consumes 10kW of electricity. A 10kw solar panel costs about £8,000. What this says to me is that it costs £8k to be able to run JUST a shower, but I'm gonna go ahead and guess that I'm not understanding this correctly. Could someone clear this up? I realize this is probably general knowledge to you guys but I'm not experienced with anything electricity.

So there are two things to think about here.
1: how much power am I using at that instant (watts/kW)
2: How much energy am I using (kilowatt-hours)

If the water heater uses 10kW, but your shower is only 6 minutes (1/10th of an hour) you used 1kWH.
If you have 1kW of panels and have them pointed perfectly at the sun, and have no losses in your conversion/storage in 1 hour you generate 1kWH of power.
(Realistically neither of those happen - but for simplifying things for general discussion, lets say they do)

So with a much smaller input power you've stored enough energy to supply a larger power demand over a shorter time.

Hopefully that explains one of the theoretical things that you need to understand for this.


Now for the more practical item in your post... 10kW == 42A of current at 240V. That's a LOT of power to be sucking out of batteries, which means more/bigger batteries if you're doing an off-grid setup. I'd guess there would be a lot better options for doing that. (propane powered? direct solar water heater? lower-power tank based water heater?)

Good luck.

[prhamilton]

Sure as a really rough approximation, you get the concepts. There are lots of variables in how much power 5kw of panels will make. So saying you get 5 hours of sunlight and 5kw in panels is a pretty gross simplification. The angle of the sun(changes throughout the year) and orientation of the panels will also influence how much power is generated. You will lose some power due to DC->AC conversion, voltage loss across the wires etc. Generally accepted around here to assume about 84% derate meaning you take your 5kw*5hours*0.84 to give you something like 21kwh. If you are truly using chemical batteries that derate factor drops, you might only be able to store 10kWh per day.

Checkout http://pvwatts.nrel.gov/pvwatts.php it will give you estimates of monthly production. Easier to estimate over monthly intervals to smooth out all weather related variance and since your battery is infinite that won't be a problem. I did a quick entry into PVwatts and says you could make around 6600kwh per year. Now look at what your power company charges per kwh.