Back-up plan

Back-up plan

Grid connected installations are a nice way to save some money. Kilowatts of power are generated and put back into the grid. Very nice of course but what happens during a power failure. When the grid fails the inverter stops feeding your solar power directly. Not a big issue if it’s a short time. But look at a hurricane, many households are without power for days. This would be a good time to use your PV installation as a stand-alone setup … right or wrong ?

The Problem:
As stated before, on-grid installations have no local storage for power and inverters stop working as soon as the carrier, the grid fails. Let’s assume it would keep working if the grid would fail. Your PV power would still be fed into the grid and be used by hunderds of people, maybe even more. This would be a large overload. Secondly it would be dangerous for the power company employees who try to fix the grid and your PV installations still puts a certain voltage on the grid.

So it would be nice to use our power in just our own house and swap between on-grid to off-grid if a situation occurs. This is however not easy and because of the complexity also not recommended. As mentioned before we also need some other ‘parts’ for an off-grid installation like storage (batteries), a charger controller and a DC/AC inverter. Beside those standard parts the grid needs to be disconnected and the house needs to be connected to the output of the DC/AC inverter. We won’t even discuss the difference between the steady power from the PV and the high fluctuation in poweruse by a house hold.

A Simple Solution:
We decided to make a simple solution that is easy for a project, cheaper than powering a whole house and still covers the most important items. Depending on where you live important items will differ. Personally I feel the refrigerator and freezer are of importance. If those appliances stop working food will go bad. Next would be heating and air, warm water and finally some lighting.

So what would be on the list ?

  • Full batterie(s)
  • DC / AC inverter
  • A charge controller
  • A low current charger (trickle)

trickle-chargeSince power failure mostly comes unexpected it’s a good idea to have some fully charged batteries, a small trickle charger will take care of that, simply have the batteries on charge 24/7 from the netgrid. When the grid fails the batterie(s) will be charged and by just using a DC/AC inverter it will provide power directly. Connect the lights, appliances or tools you need the most.


If the grid fails, a DC/AC inverter can use the power in the batterie(s) to make a ‘normal’ household voltage. The more you limit the amount of power consuming goods the longer the power of the battery will last.
To keep the batteries charged it’s time to connect you PV installation to the load-controller that is used to charge the batteries, do not confuse this with the trickle charger. When grid power returns simply re-connect the PV installation back to the netgrid controller.


This temporary solution is only to get by until power is restored, so the need for high power stand-alone equipment is not in order. Just run what you need, leave big power-users like an AC or heatpumps turned of. A few guidelines can be handy, so let’s see how much power we need and how much we can generate.

Monthly percentage on annual yield (Miami)

Every month has it’s own amount of sun hours and therefore solar power, a rough guideline for the south-east area of the US would be the following based on annual output:

  • Winter approx. 20.4%
  • Autumn approx. 26.8%
  • Summer approx. 29.7%
  • Spring approx. 23.1%


Depending on the geographic location and the season your netgrid fails you could be getting a lot less yield than expected from your PV installation.
Let’s calculate, a 3500Wp PV installation that brings 5000kWh annually.
Annual output = 5000kWh (average 13.7kW a day)
During the winter however it’s only 20.4% so 1020kWh, 340kWh per month, so just 11.2kWatts a day. This is just an average for this example, the output would be a lot less if it’s bad weather all day.

In the summer it would be 29.7% of 5000kWh = 1485kWh so that would be 16.3kWatts a day. This small example shows the difference in yield. The winter season would be worst scenario, so let’s use that output as maximum available and look next to what we need.

  • Refrigerator / freezer 20 to 25 cu : 550 – 600kWh annual (1.5 – 1.6kWh a day)
  • Lighting, 3 to 5 lights 60W a piece turned on a average of 5 hours (0.9 – 1.5kWh a day)

Heating & air and also hot water is to versatile to give good numbers but usually it’s a lot more than a fridge and lighting combined. No luxury was included here either, no tv or audio, no other appliances etc. Appliances that need to use electricity to heat up are the worst, think of a toaster iron or a coffee maker.

How about air conditioning ?
A small 5000BTU window model that is capable of covering approx. 150 sq ft will use 0.5kW (up to 12kWh/day) an hour easily on a hot day. Earlier we noticed the PV output in the summer when we need the AC most is over 16kW a day so it should be no problem. Every other season it becomes critical and should be kept to run the absolute minimum.