Working from home – Green energy off-grid

With Covid-19 lockdown measures in place I thought it a good time to review how environmentally friendly working from home could be.

Removing the carbon from the daily commute into the London office had already seen a net gain. But how much more electricity are we using working from home?

As it turns out power demand seems to have reduced from an average 30GW per day in pre lockdown 2019 to 25GW per day here in the UK in mid May 2020. Another carbon win.

An energy meter showed my home office consumes between 50 and 140 watts running a couple of laptops – PC for client work and MacBook Pro for business – along with screens network routers and lighting. So it got me thinking can I run it on solar and go off-grid?

The answer is yes. At an outlay of £180 for a couple of 100 watt mono-crystalline panels together with solar battery charger, a £100 12v car battery and a £200 pure sine wave inverter I have an uninterrupted green office power supply.

Here’s how you can go green and off-grid too and probably for less outlay.

If you don’t have solar panels already installed then you will need four main components. The PV solar panels that generate the low voltage DC, a solar charger to regulate the power from the panels, the battery to store the energy from the charger and provide power when the sun does not shine and finally the inverter to provide your standard AC mains supply.

Solar panels – I chose a solar kit that bundled two 100 watt panels with mounting kit, the low voltage cables and 20A solar charger in one package. You should be able to find similar for under £200. I found a UK based supplier on eBay that delivered in a few days with lockdown measures just going into place. I used a couple of old pallets to mount the panels together at an angle of about 30 degrees. If you have more time and space the angle could be improved for a few percent increase in output. You could get away with a single 100 watt panel that would give around 5 amps charge allowing a 50 to 75 watt demand.

Solar charger – a simple PWM (pulse width modulation) smart charger came bundled with the panels and kicks out a maximum of around 10 amps at 14.4v from the two panels. A quick calculation tells me I’m getting around 144 watts from the 200w panels which seems fair given conditions in the south east of England. It’s worth remembering like most specifications you are unlikely to get the actual rated power output so allow for this if you have higher demands.

Battery – Let’s start by admitting the battery storage technology chosen was not ideal. I picked a replacement car battery from Halfords as one of the few places I could source a battery from in lockdown. Halfords delivery was very fast and UPS courier hazardous batteries to the door. Lead acid technology is old and heavy. And a car battery is not a deep cycle cell – it’s intended to supply high current, some 800A, for short time, 30 seconds to start a car. At 85A/hr capacity though it would power my office for about 10 hours and far longer if it’s being charged by the solar panels. Running it flat, after a dozen or so cycles, the capacity will be impacted. But I don’t intend to do that so it will be fine – just like it runs your car electrical system time after time, as long as I keep it topped up from solar it should not let me down. And when I’m finished I can swap out my old car’s battery that’s seeing little use in lockdown.

If you have time it may well be worth investigating lithium batteries for storage. Tesla and others have invested millions in the technology and all laptops rely on lithium cells as their power to weight radio is hard to beat. The bundled charger automatically senses battery voltage and adapts the charging cycle to lithium so it’s an easy swap. I’m not keen on having very high power density lithium batteries inside my house due to risk of fire if a cell fails but I’m sure they will become more popular.

Inverter – this gizmo turns the DC low voltage, high current from the battery into a mains voltage AC power we expect from the grid. There are many different types of inverter and a few traps for the unwary. Don’t be tempted to pick the cheaper inverter. It’s best to choose a pure sine wave type as that’s what your grid will supply. Pure sine waves can power inductive and capacitive loads which most appliances need including laptop chargers, monitor screens, and fans and fridges (for the beer and wine).

I selected a 2500 watt pure sine wave inverter so in an emergency I could run an induction hob directly off-grid. Of course at that consumption rate my battery would not last long and I’d go overdrawn with only a 5 – 10% charging current from the solar source.

A 500 watt inverter would be ample for a small home office set up and cost less than £100.

Installation – You need about 1 square metre of solar panel to generate 200 watts at around 20% efficiency. I placed my two panels appropriately next to the greenhouse. Just for the lockdown. Shaded shelving inside the greenhouse houses the inverter above the battery that’s on the concrete floor. For safety, I installed an earth rod next to the glasshouse drain pipe so the inverter has a ground connection for our three pin plugs. It’s worth checking electrical regulations in your locale to make sure off-grid set up is safe and compliant.

I have to run a 20m extension cable from glasshouse to home office which is not ideal but is the easiest way to switch over from mains to off-grid with one power socket.

How does it work? – After over a month of testing and an above average amount of sunlight for April and early May I have to report it’s a welcome success! On average the battery receives around as much charge from the panels as I’m demanding from the office. It terms of a bank account – I’ve gone overdrawn just a couple of times when the cloud refused to let the sun shine through enough. And that was quickly replenished over the weekends when the home office was rarely in use.

Some facts and figures

Output power – Each panel is rated to generate 18V and 5.5A in ideal conditions. So far my rig has output a maximum of 10A at 14.4V in full sun. With some cloud that drops to less than 2A and full cloud below 1A at midday. So while it’s true these panels do generate power on a cloudy day you can only expect at best 10-20%.

Heat – In full sun at midday the top panel recorded just over 50 deg C and the lower panel around 48 deg C. Ambient was around 25C. As mono-crystalline panels have a temperature coefficient of about -0.5%/ deg C this means a reduction of 12.5% so it’s worth ensuring you keep the panels well ventilated.

Cooling – As an experiment I tried spaying both panels with fine water mist and waiting for the water to evaporate a couple of times. In just 5 minutes the temperature had reduced by 18-20C and output increased from 6.6A to 7.1A – not far off expected. Of course using water is not very green and the power needed to automatically spray the panels would likely cancel out the efficiency improvement but it’s worth thinking how heat recovery could improve efficiency for a large array.

How green is your grid? – If you are based in the UK you may be surprised and there are some really useful sites that provide real-time data on power consumption demand and the production sources. For example National Grid: Live Status presents concise pie charts and today is reporting over 50% renewable energy with solar making up over 25% of demand – at 7.65GW makes my setup look insignificant but every little helps.

Or for a gauge look try Gridwatch.co.uk which kindly provide the widgets below to give you an update every 5 minutes.

Link to UK National Electricity Grid Status

UK Grid Demand Solar Power and Wind Generation Now

Leave a comment

Your email address will not be published. Required fields are marked *