The Problem with Solar (and other Renewables)

Night is the Achilles Heel of solar and this leads onto other interesting problems with other domestic renewable options

The Problem with Solar (and other Renewables)

I love my solar panels on my boat. They sit there soaking up the rays and pumping up to 6 amps an hour into my 12v battery allowing me to bob around on anchor enjoying cold beers for a couple of days (oh and providing all the other electrical power on the boat).

But they have a big problem – nighttime. And clouds, and rain and anything else that obscures or dims the sunlight. Hence the 135amp/hr 12v battery.

The diagram above shows quite graphically how much of a problem this can be for a National Grid style Solar Generation. The UK has approximately 8GW of Solar PV capacity but the week shown clearly demonstrates that when the UK is covered in cloud capacity means nothing. The minimum during the week was just under 1GW and the maximum all week was on the Sunday at 3GW. If you look at any day individually the classic bell curve of generation distribution is very clearly visible with very little in the morning and evening and the peak around lunchtime.

This is only to be expected as it follows the sun's climb from horizon to zenith to horizon again. It does mean though that the is a limit to the maximum amount of generation that can be achieved on a daily basis.

I can already hear the howls of protest and yells of “storage, storage, storage” but this relies on there being an excess of electricity to charge the batteries to cover the hours of shortfall. Possible on a rooftop domestic system perhaps but of limited value to a 50GW demand National Grid if realistically max generation is only available for around 6hrs a day on cloudless days, 18hrs is a long time to fill a short fall from a battery (believe me I know from my experience on my boat).

Don't get me wrong mind. I do believe that rooftop solar (particularly on commercial roof tops) will play a massive part in the energy mix of the future even if it is only to charge our EV's while we wander round the supermarket doing the weekly groceries or to power said supermarket's freezers and lights. Yes with improved energy storage systems, not just chemical batteries, the fluctuating nature of solar generation will be smoothed out a great deal but there is always the night and our famously fickle British weather to contend with.

Just imagine the classic winter day sometime in the near future when it's foggy and there isn't a breath of wind (by this I mean less than 5mph). No wind turbines generating power and no solar. Excuse me while we fire up the Coal stations and bring the inefficient Open Cycle Gas Turbines online.

The other issue I have come across with solar PV is the payback time if I want to install some on my roof. Yes I have looked into it but frankly I'm totally unconvinced by any of the arguments when the £££££££'s don't add up.

I live in the Highlands (57ºN, 5ºW) with a significant sq meterage of south facing, unshaded roof. Perfect you'd think for solar PV. In the middle of summer we have 18hrs of daylight so a high potential but in mid winter day length is 6hrs if we are actually lucky to see the sun at all through the clouds. Using the Energy Saving Trust (EST) calculator ( and an estimate for a 3.9kw system from a friend I had a look at some numbers for a similar install on my roof.

After 25 years of FIT payments (first 20years) and estimated energy bill savings I'd be £800 short of paying back the install. OK no adjustment for inflation or energy bill inflation was made in the initial estimate so that needs to be factored in. The first year energy bill saving was estimated a £72. Is that all! I can probably save that by replacing the slightly old washing machine and tumble drier with modern A++ versions or replacing the last few CFL bulbs (and 4 incandescents) with LEDs.

One thing to note here is the estimates of our energy demand from the EST calculator were well above our observed energy demand for the year to date. Even factoring in Nov and Dec as 25% higher than our average monthly it was still hugely inaccurate. This may play a part in the low energy saving result. However the calculator also suggests a 3.5kw system would provide, on average, 7.6kWh/day. If this is true then it could provide all the power for a average day in my household thereby saving more (but only if the sun is shining brightly all day I presume). I played around with some inflation figures for increases in FIT payments (assumed 3%RPI) and increases in energy bills (assumed 6% energy inflation) and it would still take 18 years to pay back the system. This does not include maintenance costs or replacement of parts (such as a new inverter for instance) or the cost of a “powerwall”. Hardly a great investment me thinks and certainly one that doesn't sound all that appealing.

Ideally I'd like to have some annual figures for sunshine hours for where I live so I could factor that into a more accurate calculation as the EST calculator has to make an assumption. I live in one of the wettest parts of the country so we are generally cloud covered for much of the year which has got to limit the generation capacity up here.

This “lack” of savings got me wondering if I'd be better off looking at reducing the heating bill, in particular the hot water. Would solar water heating be a better way to go? After all we use heating oil as we are not on the gas main so not exactly the “greenest” of fuels. Using the EST information an average solar thermal system costs around £5000 and could save £45 a year over our A rated condensing boiler and save 350kg of CO2. Really is that all? How much CO2 was emitted during the construction and delivery of the thermal panels? Again I could probably save that much with a new A++ washing machine that heats the hot water itself against the tank supplied version I have now. Even reducing the timer by 30mins might save that much a year. Again not exactly a convincing argument even with the Renewable Heat Incentive.

So I took a look at Ground Source Heat Pumps purely because I'm going to be redeveloping a large area of garden next year. Imagine my surprise when the savings were negative against my A rated oil boiler. Yes that is correct it would actually cost more to run a GSHP than my current oil boiler, to the extent of £55 a year (estimated). Why would I even bother?

Now, if I were replacing an old, inefficient boiler or building a new house from scratch then any of these system probably would be viable options but there is no point spending anywhere between 5 to 15 thousand to save less than a hundred pounds a year when you have an efficient system already in place. Perhaps in 10 years time when my boiler has given up I will look at alternative heating systems again but by then electric boilers might be a simpler swap and be much more efficient themselves.

For now at least I think I am better off getting that new A++ washing machine and continuing to crawl around the loft improving the insulation.


Topics: Renewables
Tags: CO2 | coal | energy bill | energy bills | energy storage | Fire | gas | insulation | leds | renewables | solar | water | weather | wind turbines
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