Backup Power

August 9th, 2018
power, preparedness
I've been thinking about power in situations when the grid is down. Some things we'd be sad to miss:
  • Heat: we have forced hot water heating, which needs electricity both for the boiler (electronics and fan) and for the circulator pumps (three pumps).

  • Fans: it pretty much never gets hot enough in Boston to cause harm as long as you can have a fan pointed directly at you, but without that it can be unpleasant to dangerous. Being able to run our whole house fan overnight would be pretty helpful.

  • Sump pump: our pump is only needed once a year, in early spring when there's a lot of rain but the ground is still frozen (n=3), but without it we can have several inches of water in the basement.

  • Lighting, fridge, chest freezer, microwave, etc: not going to hurt us by their absence, but good to have.

What are good sources of power when the grid is down?

  • Solar: we're getting solar with an "SPS" socket providing best-effort power, up to 2kW, when the grid is down. This is only when the sun is shining on our panels (noon-6pm at best) and we'd like to have at least some power all day.

  • Generator: provides lots of power, requires storing fuel, noisy, not that expensive. Gas isn't ideal because it goes bad, but propane lasts more or less indefinitely and there are good propane options.

  • Portable power station: a bunch of lithium batteries with a charger and an inverter.

  • UPS: small lead-acid battery with a charger and an inverter. Designed for high current over a few minutes, and for switching over from grid power to battery without a blip, neither of which are what I want.

  • Battery + charger + inverter: a deep cycle lead-acid battery, probably AGM, a smart charger that can feed the battery lots of power when empty and then slow down to a trickle as it fills up, and an inverter to convert back to AC to power things. Basically, a do-it-yourself UPS.

One factor to consider with each of these is how clean the power is. There's a range from "pure sine" output, which is what electronics are expecting, to "pure square" output, which is easiest to generate. Pretty much nothing puts out a square wave, so really the low end of this range is "modified sine", a very rough approximation of a sine wave. People talk about "total harmonic distortion" (THD) with "pure sine" being <3-5%. So many things these days involve electronics that we need some way to get pure-sine power (frying the computer in our boiler with dirty power would be pretty bad) but most of our power needs are for the fans which wouldn't care.

What are the costs involved? Some estimates:

  • Low-end generator: something like the Sportsman 2000 Watt Dual Fuel ($250). Peak 1800W, 1260W running, 12 hours per 20lb tank of propane at 630W (half load). Each 20lb tank is about $45 new, so marginal cost is $6/kWh. Doesn't say anything about THD or pure sine so probably modified sine. For picky things could use it to charge a battery and then invert from there.

  • Mid-range generator: something like the Champion 3800-Watt Dual Fuel ($475). Peak 4275W, 3240W running, 10.5hr per 20lb tank at 1620W (half load). Marginal cost is $2.5/kWh. An Amazon review reports the THD as 11.5%, and maybe this is ok for sensitive electronics?

  • High-end generator: the best generators don't seem to be dual fuel. For example, the three the Wirecutter reviews are all gas-only and about $1,000. It looks like they can be converted to propane: here's a place selling converted ones (~$1500) and here's another place selling conversion kits (~$160). Gives 2200W peak, 1800W running. They say 3.2hr at 1800W on 0.95 gallons of gas, so using the same ratios as with the Champion above (3.5gal gas ~= 20lb propane) it would run about 12hr at 1800W on a 20lb propane tank. Marginal cost is $2/kWh. Pure sine output.

  • Portable power station: these seem to be about $1 per watt-hour: EasyFocus 200Wh for $180, SUAOKI 222Wh for $190, Goal Zero Yeti 400Wh for $450. Could be recharged daily from solar. Cost is $1k/kWh. Pure sine.

  • UPS: the CyberPower 1500 ($200) has an 11min runtime at 450W (half load), which is $2.5k/kWh. Or the APC 1000VA Smart-UPS ($276) has 64min runtime at 100W, which is $2.5k. The only advantage of these over the portable power stations (above) is that they handle the cutover for you automatically, but that's not something I need here. (Though possibly you could swap out the battery for a higher capacity one of the same type?)

  • Battery + charger + inverter:

    • Battery: a 12V battery seems pretty standard. It needs to be a "deep-cycle" one, as opposed to the kind of battery you'd use to start a car. Reading about types, sealed absorbed glass mat (AGM) ones sound like what I want: good lifetime when used this way (4-8y), not worrying about fumes and leaks, generally reliable. To get watt-hours, you multiply volts (12) by amp-hours (say, 100Ah) then half that to represent not discharging the battery below 50% (important for extended life). Reasonable batteries look like Universal 100Ah ($170 for 600Wh), Renogy 100Ah ($200 for 600Wh), or Windy Nation 100Ah ($200 for 600Wh). These weigh about 66lbs, so any larger (200Ah) would be too heavy for easy use.

    • Charger: you want something that can put a lot of current into a a heavily discharged battery, then notice as the battery gets full and cut the current down. It would be possible to do this with two dumb chargers and a person watching a voltmeter, but ideally you have a smart charger that can do this for you. NOCO Genius smart chargers seem well reviewed, and they offer a wide range of sizes. It looks like the G7200 7.2A charger ($100) would charge a 100Ah battery in 7hr, while a G15000 15A charger ($160) takes half as long.

    • Inverter: as I wrote above I want something with pure sine output. Since we don't need that much current, one or two 300W inverters ($44) could be good. There's nothing here to stop you pulling too much out of the battery, so you'd need to monitor the voltage.

    A 0.6kWh system would be $200 + $100 + $44, or $344, so $600/kWh. That's about half the cost of a portable power station. This would be good savings if you were building something big, but at the low end there are larger fixed costs from buying things separately.

We can plot these different options, showing how much you'd need to spend to get a given amount of power during an outage. These are all assuming that what you start the outage with is what you have for the duration:

What I take away from this:

  • If you want more than about 1kWh you should get a generator.
  • The better fuel efficiency of a better generator does't make up for the higher price until around 100 kWh.
  • Google Sheets interpolation on log-log plots isn't in log-log space, so the lines are weirdly bendy.

What are things we might want to run during a power outage, and how much power do they take over the course of a day? It depends on the time of year; here's summer:

  • 5.4kWh: Box fans (3 fans, 75W each, 24hr)
  • 2.7kWh: Whole house fan (2x 170W, 8hr)
  • 2.2kWh: Fridge (92W)
  • 0.5kWh: Freezer (25W)

And here's winter:

  • 0.25kWh: Lights (3 lights, 14W each, 5pm-11pm)
  • 0.16kWh: Heat (6.6VA) [EDIT: this was mismeasured, it's much higher]

Even though it gets much farther from ideal human temperature here in winter than in summer, our main electrical demands are in summer. This is because the heat is almost entirely natural gas, with just a bit of electricity to keep it going. We do still use the fridge and freezer in the winter, but in an extended outage we could keep food in coolers outside, or make ice overnight and bring it in.

I'm not sure yet what I want to do. Here's what I'm considering:

  • Small portable power supply ($180). Run the boiler and nothing else. Recharge from solar. $180 for 0.2kWh.

  • Cheap generator ($250), a couple tanks of propane ($90), and a small pure sine portable power station ($180). Recharge the PPS off the generator as needed and run sensitive things (boiler) off the PPS. $520 for 7.8 kWh.

  • Mid-range generator ($475) and a tank of propane ($45). $520 for 17 kWh.

Talking about generator+tank with a kWh rating assumes that generator run time scales linearly with load, which it probably doesn't. For example, if I want to run the generator constantly for three days and it never uses much less power than it does at half load then both generator options go from needing one tank ($45) to needing seven ($315). Looking online it's hard to get figures on how much power a generator consumes at, say, 10% load.

Still not sure what I'll do, but here's where I am now. Thoughts?

Referenced in: Furnace Battery Backup

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